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diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+           <p>December 18, 2016</p>
+           <p>This article was contributed by Paul E.&nbsp;McKenney</p>
+
+<h3>Introduction</h3>
+
+This document describes RCU's major data structures and their relationship
+to each other.
+
+<ol>
+<li>	<a href="#Data-Structure Relationships">
+	Data-Structure Relationships</a>
+<li>	<a href="#The rcu_state Structure">
+	The <tt>rcu_state</tt> Structure</a>
+<li>	<a href="#The rcu_node Structure">
+	The <tt>rcu_node</tt> Structure</a>
+<li>	<a href="#The rcu_segcblist Structure">
+	The <tt>rcu_segcblist</tt> Structure</a>
+<li>	<a href="#The rcu_data Structure">
+	The <tt>rcu_data</tt> Structure</a>
+<li>	<a href="#The rcu_dynticks Structure">
+	The <tt>rcu_dynticks</tt> Structure</a>
+<li>	<a href="#The rcu_head Structure">
+	The <tt>rcu_head</tt> Structure</a>
+<li>	<a href="#RCU-Specific Fields in the task_struct Structure">
+	RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
+<li>	<a href="#Accessor Functions">
+	Accessor Functions</a>
+</ol>
+
+<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
+
+<p>RCU is for all intents and purposes a large state machine, and its
+data structures maintain the state in such a way as to allow RCU readers
+to execute extremely quickly, while also processing the RCU grace periods
+requested by updaters in an efficient and extremely scalable fashion.
+The efficiency and scalability of RCU updaters is provided primarily
+by a combining tree, as shown below:
+
+</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
+
+</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
+containing a tree of <tt>rcu_node</tt> structures.
+Each leaf node of the <tt>rcu_node</tt> tree has up to 16
+<tt>rcu_data</tt> structures associated with it, so that there
+are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
+one for each possible CPU.
+This structure is adjusted at boot time, if needed, to handle the
+common case where <tt>nr_cpu_ids</tt> is much less than
+<tt>NR_CPUs</tt>.
+For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
+which results in a three-level <tt>rcu_node</tt> tree.
+If the actual hardware has only 16 CPUs, RCU will adjust itself
+at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
+
+</p><p>The purpose of this combining tree is to allow per-CPU events
+such as quiescent states, dyntick-idle transitions,
+and CPU hotplug operations to be processed efficiently
+and scalably.
+Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
+and other events are recorded by the leaf-level <tt>rcu_node</tt>
+structures.
+All of these events are combined at each level of the tree until finally
+grace periods are completed at the tree's root <tt>rcu_node</tt>
+structure.
+A grace period can be completed at the root once every CPU
+(or, in the case of <tt>CONFIG_PREEMPT_RCU</tt>, task)
+has passed through a quiescent state.
+Once a grace period has completed, record of that fact is propagated
+back down the tree.
+
+</p><p>As can be seen from the diagram, on a 64-bit system
+a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
+of 64 at the root and a fanout of 16 at the leaves.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why isn't the fanout at the leaves also 64?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because there are more types of events that affect the leaf-level
+	<tt>rcu_node</tt> structures than further up the tree.
+	Therefore, if the leaf <tt>rcu_node</tt> structures have fanout of
+	64, the contention on these structures' <tt>-&gt;structures</tt>
+	becomes excessive.
+	Experimentation on a wide variety of systems has shown that a fanout
+	of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
+	</font>
+
+	<p><font color="ffffff">Of course, further experience with
+	systems having hundreds or thousands of CPUs may demonstrate
+	that the fanout for the non-leaf <tt>rcu_node</tt> structures
+	must also be reduced.
+	Such reduction can be easily carried out when and if it proves
+	necessary.
+	In the meantime, if you are using such a system and running into
+	contention problems on the non-leaf <tt>rcu_node</tt> structures,
+	you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
+	parameter to reduce the non-leaf fanout as needed.
+	</font>
+
+	<p><font color="ffffff">Kernels built for systems with
+	strong NUMA characteristics might also need to adjust
+	<tt>CONFIG_RCU_FANOUT</tt> so that the domains of the
+	<tt>rcu_node</tt> structures align with hardware boundaries.
+	However, there has thus far been no need for this.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
+a 32-bit system), then RCU will automatically add more levels to the
+tree.
+For example, if you are crazy enough to build a 64-bit system with 65,536
+CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
+
+</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
+
+</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
+accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
+32-bit systems.
+On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
+as small as 2 if you wish, which would permit only 16 CPUs, which
+is useful for testing.
+
+</p><p>This multi-level combining tree allows us to get most of the
+performance and scalability
+benefits of partitioning, even though RCU grace-period detection is
+inherently a global operation.
+The trick here is that only the last CPU to report a quiescent state
+into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
+structure at the next level up the tree.
+This means that at the leaf-level <tt>rcu_node</tt> structure, only
+one access out of sixteen will progress up the tree.
+For the internal <tt>rcu_node</tt> structures, the situation is even
+more extreme:  Only one access out of sixty-four will progress up
+the tree.
+Because the vast majority of the CPUs do not progress up the tree,
+the lock contention remains roughly constant up the tree.
+No matter how many CPUs there are in the system, at most 64 quiescent-state
+reports per grace period will progress all the way to the root
+<tt>rcu_node</tt> structure, thus ensuring that the lock contention
+on that root <tt>rcu_node</tt> structure remains acceptably low.
+
+</p><p>In effect, the combining tree acts like a big shock absorber,
+keeping lock contention under control at all tree levels regardless
+of the level of loading on the system.
+
+</p><p>The Linux kernel actually supports multiple flavors of RCU
+running concurrently, so RCU builds separate data structures for each
+flavor.
+For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides
+rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%">
+
+</p><p>Energy efficiency is increasingly important, and for that
+reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which
+turns off the scheduling-clock interrupts on idle CPUs, which in
+turn allows those CPUs to attain deeper sleep states and to consume
+less energy.
+CPUs whose scheduling-clock interrupts have been turned off are
+said to be in <i>dyntick-idle mode</i>.
+RCU must handle dyntick-idle CPUs specially
+because RCU would otherwise wake up each CPU on every grace period,
+which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
+RCU uses the <tt>rcu_dynticks</tt> structure to track
+which CPUs are in dyntick idle mode, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
+
+</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
+for all flavors of RCU.
+Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
+CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
+that <tt>rcu_dynticks</tt>, as shown in the figure.
+
+</p><p>Kernels built with <tt>CONFIG_PREEMPT_RCU</tt> support
+rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%">
+
+</p><p>RCU updaters wait for normal grace periods by registering
+RCU callbacks, either directly via <tt>call_rcu()</tt> and
+friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
+there being a separate interface per flavor of RCU)
+or indirectly via <tt>synchronize_rcu()</tt> and friends.
+RCU callbacks are represented by <tt>rcu_head</tt> structures,
+which are queued on <tt>rcu_data</tt> structures while they are
+waiting for a grace period to elapse, as shown in the following figure:
+
+</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
+
+</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
+<tt>PREEMPT_RCU</tt>'s major data structures are related.
+Lesser data structures will be introduced with the algorithms that
+make use of them.
+
+</p><p>Note that each of the data structures in the above figure has
+its own synchronization:
+
+<p><ol>
+<li>	Each <tt>rcu_state</tt> structures has a lock and a mutex,
+	and some fields are protected by the corresponding root
+	<tt>rcu_node</tt> structure's lock.
+<li>	Each <tt>rcu_node</tt> structure has a spinlock.
+<li>	The fields in <tt>rcu_data</tt> are private to the corresponding
+	CPU, although a few can be read and written by other CPUs.
+<li>	Similarly, the fields in <tt>rcu_dynticks</tt> are private
+	to the corresponding CPU, although a few can be read by
+	other CPUs.
+</ol>
+
+<p>It is important to note that different data structures can have
+very different ideas about the state of RCU at any given time.
+For but one example, awareness of the start or end of a given RCU
+grace period propagates slowly through the data structures.
+This slow propagation is absolutely necessary for RCU to have good
+read-side performance.
+If this balkanized implementation seems foreign to you, one useful
+trick is to consider each instance of these data structures to be
+a different person, each having the usual slightly different
+view of reality.
+
+</p><p>The general role of each of these data structures is as
+follows:
+
+</p><ol>
+<li>	<tt>rcu_state</tt>:
+	This structure forms the interconnection between the
+	<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+	tracks grace periods, serves as short-term repository
+	for callbacks orphaned by CPU-hotplug events,
+	maintains <tt>rcu_barrier()</tt> state,
+	tracks expedited grace-period state,
+	and maintains state used to force quiescent states when
+	grace periods extend too long,
+<li>	<tt>rcu_node</tt>: This structure forms the combining
+	tree that propagates quiescent-state
+	information from the leaves to the root, and also propagates
+	grace-period information from the root to the leaves.
+	It provides local copies of the grace-period state in order
+	to allow this information to be accessed in a synchronized
+	manner without suffering the scalability limitations that
+	would otherwise be imposed by global locking.
+	In <tt>CONFIG_PREEMPT_RCU</tt> kernels, it manages the lists
+	of tasks that have blocked while in their current
+	RCU read-side critical section.
+	In <tt>CONFIG_PREEMPT_RCU</tt> with
+	<tt>CONFIG_RCU_BOOST</tt>, it manages the
+	per-<tt>rcu_node</tt> priority-boosting
+	kernel threads (kthreads) and state.
+	Finally, it records CPU-hotplug state in order to determine
+	which CPUs should be ignored during a given grace period.
+<li>	<tt>rcu_data</tt>: This per-CPU structure is the
+	focus of quiescent-state detection and RCU callback queuing.
+	It also tracks its relationship to the corresponding leaf
+	<tt>rcu_node</tt> structure to allow more-efficient
+	propagation of quiescent states up the <tt>rcu_node</tt>
+	combining tree.
+	Like the <tt>rcu_node</tt> structure, it provides a local
+	copy of the grace-period information to allow for-free
+	synchronized
+	access to this information from the corresponding CPU.
+	Finally, this structure records past dyntick-idle state
+	for the corresponding CPU and also tracks statistics.
+<li>	<tt>rcu_dynticks</tt>:
+	This per-CPU structure tracks the current dyntick-idle
+	state for the corresponding CPU.
+	Unlike the other three structures, the <tt>rcu_dynticks</tt>
+	structure is not replicated per RCU flavor.
+<li>	<tt>rcu_head</tt>:
+	This structure represents RCU callbacks, and is the
+	only structure allocated and managed by RCU users.
+	The <tt>rcu_head</tt> structure is normally embedded
+	within the RCU-protected data structure.
+</ol>
+
+<p>If all you wanted from this article was a general notion of how
+RCU's data structures are related, you are done.
+Otherwise, each of the following sections give more details on
+the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>,
+and <tt>rcu_dynticks</tt> data structures.
+
+<h3><a name="The rcu_state Structure">
+The <tt>rcu_state</tt> Structure</a></h3>
+
+<p>The <tt>rcu_state</tt> structure is the base structure that
+represents a flavor of RCU.
+This structure forms the interconnection between the
+<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+tracks grace periods, contains the lock used to
+synchronize with CPU-hotplug events,
+and maintains state used to force quiescent states when
+grace periods extend too long,
+
+</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+The more specialized fields are covered in the discussion of their
+use.
+
+<h5>Relationship to rcu_node and rcu_data Structures</h5>
+
+This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   struct rcu_node node[NUM_RCU_NODES];
+  2   struct rcu_node *level[NUM_RCU_LVLS + 1];
+  3   struct rcu_data __percpu *rda;
+</pre>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Wait a minute!
+	You said that the <tt>rcu_node</tt> structures formed a tree,
+	but they are declared as a flat array!
+	What gives?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	The tree is laid out in the array.
+	The first node In the array is the head, the next set of nodes in the
+	array are children of the head node, and so on until the last set of
+	nodes in the array are the leaves.
+	</font>
+
+	<p><font color="ffffff">See the following diagrams to see how
+	this works.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>The <tt>rcu_node</tt> tree is embedded into the
+<tt>-&gt;node[]</tt> array as shown in the following figure:
+
+</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
+
+</p><p>One interesting consequence of this mapping is that a
+breadth-first traversal of the tree is implemented as a simple
+linear scan of the array, which is in fact what the
+<tt>rcu_for_each_node_breadth_first()</tt> macro does.
+This macro is used at the beginning and ends of grace periods.
+
+</p><p>Each entry of the <tt>-&gt;level</tt> array references
+the first <tt>rcu_node</tt> structure on the corresponding level
+of the tree, for example, as shown below:
+
+</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
+
+</p><p>The zero<sup>th</sup> element of the array references the root
+<tt>rcu_node</tt> structure, the first element references the
+first child of the root <tt>rcu_node</tt>, and finally the second
+element references the first leaf <tt>rcu_node</tt> structure.
+
+</p><p>For whatever it is worth, if you draw the tree to be tree-shaped
+rather than array-shaped, it is easy to draw a planar representation:
+
+</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
+
+</p><p>Finally, the <tt>-&gt;rda</tt> field references a per-CPU
+pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
+
+</p><p>All of these fields are constant once initialization is complete,
+and therefore need no protection.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_seq;
+</pre>
+
+<p>RCU grace periods are numbered, and
+the <tt>-&gt;gp_seq</tt> field contains the current grace-period
+sequence number.
+The bottom two bits are the state of the current grace period,
+which can be zero for not yet started or one for in progress.
+In other words, if the bottom two bits of <tt>-&gt;gp_seq</tt> are
+zero, the corresponding flavor of RCU is idle.
+Any other value in the bottom two bits indicates that something is broken.
+This field is protected by the root <tt>rcu_node</tt> structure's
+<tt>-&gt;lock</tt> field.
+
+</p><p>There are <tt>-&gt;gp_seq</tt> fields
+in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
+as well.
+The fields in the <tt>rcu_state</tt> structure represent the
+most current value, and those of the other structures are compared
+in order to detect the beginnings and ends of grace periods in a distributed
+fashion.
+The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
+(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
+
+<h5>Miscellaneous</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_max;
+  2   char abbr;
+  3   char *name;
+</pre>
+
+<p>The <tt>-&gt;gp_max</tt> field tracks the duration of the longest
+grace period in jiffies.
+It is protected by the root <tt>rcu_node</tt>'s <tt>-&gt;lock</tt>.
+
+<p>The <tt>-&gt;name</tt> field points to the name of the RCU flavor
+(for example, &ldquo;rcu_sched&rdquo;), and is constant.
+The <tt>-&gt;abbr</tt> field contains a one-character abbreviation,
+for example, &ldquo;s&rdquo; for RCU-sched.
+
+<h3><a name="The rcu_node Structure">
+The <tt>rcu_node</tt> Structure</a></h3>
+
+<p>The <tt>rcu_node</tt> structures form the combining
+tree that propagates quiescent-state
+information from the leaves to the root and also that propagates
+grace-period information from the root down to the leaves.
+They provides local copies of the grace-period state in order
+to allow this information to be accessed in a synchronized
+manner without suffering the scalability limitations that
+would otherwise be imposed by global locking.
+In <tt>CONFIG_PREEMPT_RCU</tt> kernels, they manage the lists
+of tasks that have blocked while in their current
+RCU read-side critical section.
+In <tt>CONFIG_PREEMPT_RCU</tt> with
+<tt>CONFIG_RCU_BOOST</tt>, they manage the
+per-<tt>rcu_node</tt> priority-boosting
+kernel threads (kthreads) and state.
+Finally, they record CPU-hotplug state in order to determine
+which CPUs should be ignored during a given grace period.
+
+</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Combining Tree</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   struct rcu_node *parent;
+  2   u8 level;
+  3   u8 grpnum;
+  4   unsigned long grpmask;
+  5   int grplo;
+  6   int grphi;
+</pre>
+
+<p>The <tt>-&gt;parent</tt> pointer references the <tt>rcu_node</tt>
+one level up in the tree, and is <tt>NULL</tt> for the root
+<tt>rcu_node</tt>.
+The RCU implementation makes heavy use of this field to push quiescent
+states up the tree.
+The <tt>-&gt;level</tt> field gives the level in the tree, with
+the root being at level zero, its children at level one, and so on.
+The <tt>-&gt;grpnum</tt> field gives this node's position within
+the children of its parent, so this number can range between 0 and 31
+on 32-bit systems and between 0 and 63 on 64-bit systems.
+The <tt>-&gt;level</tt> and <tt>-&gt;grpnum</tt> fields are
+used only during initialization and for tracing.
+The <tt>-&gt;grpmask</tt> field is the bitmask counterpart of
+<tt>-&gt;grpnum</tt>, and therefore always has exactly one bit set.
+This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
+structure in its parent's bitmasks, which are described later.
+Finally, the <tt>-&gt;grplo</tt> and <tt>-&gt;grphi</tt> fields
+contain the lowest and highest numbered CPU served by this
+<tt>rcu_node</tt> structure, respectively.
+
+</p><p>All of these fields are constant, and thus do not require any
+synchronization.
+
+<h5>Synchronization</h5>
+
+<p>This field of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   raw_spinlock_t lock;
+</pre>
+
+<p>This field is used to protect the remaining fields in this structure,
+unless otherwise stated.
+That said, all of the fields in this structure can be accessed without
+locking for tracing purposes.
+Yes, this can result in confusing traces, but better some tracing confusion
+than to be heisenbugged out of existence.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_seq;
+  2   unsigned long gp_seq_needed;
+</pre>
+
+<p>The <tt>rcu_node</tt> structures' <tt>-&gt;gp_seq</tt> fields are
+the counterparts of the field of the same name in the <tt>rcu_state</tt>
+structure.
+They each may lag up to one step behind their <tt>rcu_state</tt>
+counterpart.
+If the bottom two bits of a given <tt>rcu_node</tt> structure's
+<tt>-&gt;gp_seq</tt> field is zero, then this <tt>rcu_node</tt>
+structure believes that RCU is idle.
+</p><p>The <tt>&gt;gp_seq</tt> field of each <tt>rcu_node</tt>
+structure is updated at the beginning and the end
+of each grace period.
+
+<p>The <tt>-&gt;gp_seq_needed</tt> fields record the
+furthest-in-the-future grace period request seen by the corresponding
+<tt>rcu_node</tt> structure.  The request is considered fulfilled when
+the value of the <tt>-&gt;gp_seq</tt> field equals or exceeds that of
+the <tt>-&gt;gp_seq_needed</tt> field.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Suppose that this <tt>rcu_node</tt> structure doesn't see
+	a request for a very long time.
+	Won't wrapping of the <tt>-&gt;gp_seq</tt> field cause
+	problems?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No, because if the <tt>-&gt;gp_seq_needed</tt> field lags behind the
+	<tt>-&gt;gp_seq</tt> field, the <tt>-&gt;gp_seq_needed</tt> field
+	will be updated at the end of the grace period.
+	Modulo-arithmetic comparisons therefore will always get the
+	correct answer, even with wrapping.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h5>Quiescent-State Tracking</h5>
+
+<p>These fields manage the propagation of quiescent states up the
+combining tree.
+
+</p><p>This portion of the <tt>rcu_node</tt> structure has fields
+as follows:
+
+<pre>
+  1   unsigned long qsmask;
+  2   unsigned long expmask;
+  3   unsigned long qsmaskinit;
+  4   unsigned long expmaskinit;
+</pre>
+
+<p>The <tt>-&gt;qsmask</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current normal grace period.
+Such children will have a value of 1 in their corresponding bit.
+Note that the leaf <tt>rcu_node</tt> structures should be
+thought of as having <tt>rcu_data</tt> structures as their
+children.
+Similarly, the <tt>-&gt;expmask</tt> field tracks which
+of this <tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current expedited grace period.
+An expedited grace period has
+the same conceptual properties as a normal grace period, but the
+expedited implementation accepts extreme CPU overhead to obtain
+much lower grace-period latency, for example, consuming a few
+tens of microseconds worth of CPU time to reduce grace-period
+duration from milliseconds to tens of microseconds.
+The <tt>-&gt;qsmaskinit</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children cover for at least
+one online CPU.
+This mask is used to initialize <tt>-&gt;qsmask</tt>,
+and <tt>-&gt;expmaskinit</tt> is used to initialize
+<tt>-&gt;expmask</tt> and the beginning of the
+normal and expedited grace periods, respectively.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why are these bitmasks protected by locking?
+	Come on, haven't you heard of atomic instructions???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Lockless grace-period computation!  Such a tantalizing possibility!
+	</font>
+
+	<p><font color="ffffff">But consider the following sequence of events:
+	</font>
+
+	<ol>
+	<li>	<font color="ffffff">CPU&nbsp;0 has been in dyntick-idle
+		mode for quite some time.
+		When it wakes up, it notices that the current RCU
+		grace period needs it to report in, so it sets a
+		flag where the scheduling clock interrupt will find it.
+		</font><p>
+	<li>	<font color="ffffff">Meanwhile, CPU&nbsp;1 is running
+		<tt>force_quiescent_state()</tt>,
+		and notices that CPU&nbsp;0 has been in dyntick idle mode,
+		which qualifies as an extended quiescent state.
+		</font><p>
+	<li>	<font color="ffffff">CPU&nbsp;0's scheduling clock
+		interrupt fires in the
+		middle of an RCU read-side critical section, and notices
+		that the RCU core needs something, so commences RCU softirq
+		processing.
+		</font>
+		<p>
+	<li>	<font color="ffffff">CPU&nbsp;0's softirq handler
+		executes and is just about ready
+		to report its quiescent state up the <tt>rcu_node</tt>
+		tree.
+		</font><p>
+	<li>	<font color="ffffff">But CPU&nbsp;1 beats it to the punch,
+		completing the current
+		grace period and starting a new one.
+		</font><p>
+	<li>	<font color="ffffff">CPU&nbsp;0 now reports its quiescent
+		state for the wrong
+		grace period.
+		That grace period might now end before the RCU read-side
+		critical section.
+		If that happens, disaster will ensue.
+		</font>
+	</ol>
+
+	<p><font color="ffffff">So the locking is absolutely required in
+	order to coordinate clearing of the bits with updating of the
+	grace-period sequence number in <tt>-&gt;gp_seq</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h5>Blocked-Task Management</h5>
+
+<p><tt>PREEMPT_RCU</tt> allows tasks to be preempted in the
+midst of their RCU read-side critical sections, and these tasks
+must be tracked explicitly.
+The details of exactly why and how they are tracked will be covered
+in a separate article on RCU read-side processing.
+For now, it is enough to know that the <tt>rcu_node</tt>
+structure tracks them.
+
+<pre>
+  1   struct list_head blkd_tasks;
+  2   struct list_head *gp_tasks;
+  3   struct list_head *exp_tasks;
+  4   bool wait_blkd_tasks;
+</pre>
+
+<p>The <tt>-&gt;blkd_tasks</tt> field is a list header for
+the list of blocked and preempted tasks.
+As tasks undergo context switches within RCU read-side critical
+sections, their <tt>task_struct</tt> structures are enqueued
+(via the <tt>task_struct</tt>'s <tt>-&gt;rcu_node_entry</tt>
+field) onto the head of the <tt>-&gt;blkd_tasks</tt> list for the
+leaf <tt>rcu_node</tt> structure corresponding to the CPU
+on which the outgoing context switch executed.
+As these tasks later exit their RCU read-side critical sections,
+they remove themselves from the list.
+This list is therefore in reverse time order, so that if one of the tasks
+is blocking the current grace period, all subsequent tasks must
+also be blocking that same grace period.
+Therefore, a single pointer into this list suffices to track
+all tasks blocking a given grace period.
+That pointer is stored in <tt>-&gt;gp_tasks</tt> for normal
+grace periods and in <tt>-&gt;exp_tasks</tt> for expedited
+grace periods.
+These last two fields are <tt>NULL</tt> if either there is
+no grace period in flight or if there are no blocked tasks
+preventing that grace period from completing.
+If either of these two pointers is referencing a task that
+removes itself from the <tt>-&gt;blkd_tasks</tt> list,
+then that task must advance the pointer to the next task on
+the list, or set the pointer to <tt>NULL</tt> if there
+are no subsequent tasks on the list.
+
+</p><p>For example, suppose that tasks&nbsp;T1, T2, and&nbsp;T3 are
+all hard-affinitied to the largest-numbered CPU in the system.
+Then if task&nbsp;T1 blocked in an RCU read-side
+critical section, then an expedited grace period started,
+then task&nbsp;T2 blocked in an RCU read-side critical section,
+then a normal grace period started, and finally task&nbsp;3 blocked
+in an RCU read-side critical section, then the state of the
+last leaf <tt>rcu_node</tt> structure's blocked-task list
+would be as shown below:
+
+</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
+
+</p><p>Task&nbsp;T1 is blocking both grace periods, task&nbsp;T2 is
+blocking only the normal grace period, and task&nbsp;T3 is blocking
+neither grace period.
+Note that these tasks will not remove themselves from this list
+immediately upon resuming execution.
+They will instead remain on the list until they execute the outermost
+<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
+section.
+
+<p>
+The <tt>-&gt;wait_blkd_tasks</tt> field indicates whether or not
+the current grace period is waiting on a blocked task.
+
+<h5>Sizing the <tt>rcu_node</tt> Array</h5>
+
+<p>The <tt>rcu_node</tt> array is sized via a series of
+C-preprocessor expressions as follows:
+
+<pre>
+ 1 #ifdef CONFIG_RCU_FANOUT
+ 2 #define RCU_FANOUT CONFIG_RCU_FANOUT
+ 3 #else
+ 4 # ifdef CONFIG_64BIT
+ 5 # define RCU_FANOUT 64
+ 6 # else
+ 7 # define RCU_FANOUT 32
+ 8 # endif
+ 9 #endif
+10
+11 #ifdef CONFIG_RCU_FANOUT_LEAF
+12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
+13 #else
+14 # ifdef CONFIG_64BIT
+15 # define RCU_FANOUT_LEAF 64
+16 # else
+17 # define RCU_FANOUT_LEAF 32
+18 # endif
+19 #endif
+20
+21 #define RCU_FANOUT_1        (RCU_FANOUT_LEAF)
+22 #define RCU_FANOUT_2        (RCU_FANOUT_1 * RCU_FANOUT)
+23 #define RCU_FANOUT_3        (RCU_FANOUT_2 * RCU_FANOUT)
+24 #define RCU_FANOUT_4        (RCU_FANOUT_3 * RCU_FANOUT)
+25
+26 #if NR_CPUS &lt;= RCU_FANOUT_1
+27 #  define RCU_NUM_LVLS        1
+28 #  define NUM_RCU_LVL_0        1
+29 #  define NUM_RCU_NODES        NUM_RCU_LVL_0
+30 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0 }
+31 #  define RCU_NODE_NAME_INIT  { "rcu_node_0" }
+32 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0" }
+33 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0" }
+34 #elif NR_CPUS &lt;= RCU_FANOUT_2
+35 #  define RCU_NUM_LVLS        2
+36 #  define NUM_RCU_LVL_0        1
+37 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+38 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
+39 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
+40 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1" }
+41 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1" }
+42 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1" }
+43 #elif NR_CPUS &lt;= RCU_FANOUT_3
+44 #  define RCU_NUM_LVLS        3
+45 #  define NUM_RCU_LVL_0        1
+46 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+47 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+48 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
+49 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
+50 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
+51 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
+52 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
+53 #elif NR_CPUS &lt;= RCU_FANOUT_4
+54 #  define RCU_NUM_LVLS        4
+55 #  define NUM_RCU_LVL_0        1
+56 #  define NUM_RCU_LVL_1        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
+57 #  define NUM_RCU_LVL_2        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+58 #  define NUM_RCU_LVL_3        DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+59 #  define NUM_RCU_NODES        (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
+60 #  define NUM_RCU_LVL_INIT    { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
+61 #  define RCU_NODE_NAME_INIT  { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
+62 #  define RCU_FQS_NAME_INIT   { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
+63 #  define RCU_EXP_NAME_INIT   { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
+64 #else
+65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
+66 #endif
+</pre>
+
+<p>The maximum number of levels in the <tt>rcu_node</tt> structure
+is currently limited to four, as specified by lines&nbsp;21-24
+and the structure of the subsequent &ldquo;if&rdquo; statement.
+For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
+should be sufficient for the next few years at least.
+For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
+should see us through the next decade or so.
+This four-level tree also allows kernels built with
+<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
+which might be useful in very large systems having eight CPUs per
+socket (but please note that no one has yet shown any measurable
+performance degradation due to misaligned socket and <tt>rcu_node</tt>
+boundaries).
+In addition, building kernels with a full four levels of <tt>rcu_node</tt>
+tree permits better testing of RCU's combining-tree code.
+
+</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
+are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
+If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
+it is set based on the word size of the system, which is also
+the Kconfig default.
+
+</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
+handled by each leaf <tt>rcu_node</tt> structure.
+Experience has shown that allowing a given leaf <tt>rcu_node</tt>
+structure to handle 64 CPUs, as permitted by the number of bits in
+the <tt>-&gt;qsmask</tt> field on a 64-bit system, results in
+excessive contention for the leaf <tt>rcu_node</tt> structures'
+<tt>-&gt;lock</tt> fields.
+The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
+limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
+If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
+selected is based on the word size of the system, just as for
+<tt>CONFIG_RCU_FANOUT</tt>.
+Lines&nbsp;11-19 perform this computation.
+
+</p><p>Lines&nbsp;21-24 compute the maximum number of CPUs supported by
+a single-level (which contains a single <tt>rcu_node</tt> structure),
+two-level, three-level, and four-level <tt>rcu_node</tt> tree,
+respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
+and <tt>RCU_FANOUT_LEAF</tt>.
+These numbers of CPUs are retained in the
+<tt>RCU_FANOUT_1</tt>,
+<tt>RCU_FANOUT_2</tt>,
+<tt>RCU_FANOUT_3</tt>, and
+<tt>RCU_FANOUT_4</tt>
+C-preprocessor variables, respectively.
+
+</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
+statement spanning lines&nbsp;26-66 that computes the number of
+<tt>rcu_node</tt> structures required for each level of the tree,
+as well as the number of levels required.
+The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
+C-preprocessor variable by lines&nbsp;27, 35, 44, and&nbsp;54.
+The number of <tt>rcu_node</tt> structures for the topmost level
+of the tree is always exactly one, and this value is unconditionally
+placed into <tt>NUM_RCU_LVL_0</tt> by lines&nbsp;28, 36, 45, and&nbsp;55.
+The rest of the levels (if any) of the <tt>rcu_node</tt> tree
+are computed by dividing the maximum number of CPUs by the
+fanout supported by the number of levels from the current level down,
+rounding up.  This computation is performed by lines&nbsp;37,
+46-47, and&nbsp;56-58.
+Lines&nbsp;31-33, 40-42, 50-52, and&nbsp;62-63 create initializers
+for lockdep lock-class names.
+Finally, lines&nbsp;64-66 produce an error if the maximum number of
+CPUs is too large for the specified fanout.
+
+<h3><a name="The rcu_segcblist Structure">
+The <tt>rcu_segcblist</tt> Structure</a></h3>
+
+The <tt>rcu_segcblist</tt> structure maintains a segmented list of
+callbacks as follows:
+
+<pre>
+ 1 #define RCU_DONE_TAIL        0
+ 2 #define RCU_WAIT_TAIL        1
+ 3 #define RCU_NEXT_READY_TAIL  2
+ 4 #define RCU_NEXT_TAIL        3
+ 5 #define RCU_CBLIST_NSEGS     4
+ 6
+ 7 struct rcu_segcblist {
+ 8   struct rcu_head *head;
+ 9   struct rcu_head **tails[RCU_CBLIST_NSEGS];
+10   unsigned long gp_seq[RCU_CBLIST_NSEGS];
+11   long len;
+12   long len_lazy;
+13 };
+</pre>
+
+<p>
+The segments are as follows:
+
+<ol>
+<li>	<tt>RCU_DONE_TAIL</tt>: Callbacks whose grace periods have elapsed.
+	These callbacks are ready to be invoked.
+<li>	<tt>RCU_WAIT_TAIL</tt>: Callbacks that are waiting for the
+	current grace period.
+	Note that different CPUs can have different ideas about which
+	grace period is current, hence the <tt>-&gt;gp_seq</tt> field.
+<li>	<tt>RCU_NEXT_READY_TAIL</tt>: Callbacks waiting for the next
+	grace period to start.
+<li>	<tt>RCU_NEXT_TAIL</tt>: Callbacks that have not yet been
+	associated with a grace period.
+</ol>
+
+<p>
+The <tt>-&gt;head</tt> pointer references the first callback or
+is <tt>NULL</tt> if the list contains no callbacks (which is
+<i>not</i> the same as being empty).
+Each element of the <tt>-&gt;tails[]</tt> array references the
+<tt>-&gt;next</tt> pointer of the last callback in the corresponding
+segment of the list, or the list's <tt>-&gt;head</tt> pointer if
+that segment and all previous segments are empty.
+If the corresponding segment is empty but some previous segment is
+not empty, then the array element is identical to its predecessor.
+Older callbacks are closer to the head of the list, and new callbacks
+are added at the tail.
+This relationship between the <tt>-&gt;head</tt> pointer, the
+<tt>-&gt;tails[]</tt> array, and the callbacks is shown in this
+diagram:
+
+</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
+
+</p><p>In this figure, the <tt>-&gt;head</tt> pointer references the
+first
+RCU callback in the list.
+The <tt>-&gt;tails[RCU_DONE_TAIL]</tt> array element references
+the <tt>-&gt;head</tt> pointer itself, indicating that none
+of the callbacks is ready to invoke.
+The <tt>-&gt;tails[RCU_WAIT_TAIL]</tt> array element references callback
+CB&nbsp;2's <tt>-&gt;next</tt> pointer, which indicates that
+CB&nbsp;1 and CB&nbsp;2 are both waiting on the current grace period,
+give or take possible disagreements about exactly which grace period
+is the current one.
+The <tt>-&gt;tails[RCU_NEXT_READY_TAIL]</tt> array element
+references the same RCU callback that <tt>-&gt;tails[RCU_WAIT_TAIL]</tt>
+does, which indicates that there are no callbacks waiting on the next
+RCU grace period.
+The <tt>-&gt;tails[RCU_NEXT_TAIL]</tt> array element references
+CB&nbsp;4's <tt>-&gt;next</tt> pointer, indicating that all the
+remaining RCU callbacks have not yet been assigned to an RCU grace
+period.
+Note that the <tt>-&gt;tails[RCU_NEXT_TAIL]</tt> array element
+always references the last RCU callback's <tt>-&gt;next</tt> pointer
+unless the callback list is empty, in which case it references
+the <tt>-&gt;head</tt> pointer.
+
+<p>
+There is one additional important special case for the
+<tt>-&gt;tails[RCU_NEXT_TAIL]</tt> array element: It can be <tt>NULL</tt>
+when this list is <i>disabled</i>.
+Lists are disabled when the corresponding CPU is offline or when
+the corresponding CPU's callbacks are offloaded to a kthread,
+both of which are described elsewhere.
+
+</p><p>CPUs advance their callbacks from the
+<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
+<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
+as grace periods advance.
+
+</p><p>The <tt>-&gt;gp_seq[]</tt> array records grace-period
+numbers corresponding to the list segments.
+This is what allows different CPUs to have different ideas as to
+which is the current grace period while still avoiding premature
+invocation of their callbacks.
+In particular, this allows CPUs that go idle for extended periods
+to determine which of their callbacks are ready to be invoked after
+reawakening.
+
+</p><p>The <tt>-&gt;len</tt> counter contains the number of
+callbacks in <tt>-&gt;head</tt>, and the
+<tt>-&gt;len_lazy</tt> contains the number of those callbacks that
+are known to only free memory, and whose invocation can therefore
+be safely deferred.
+
+<p><b>Important note</b>: It is the <tt>-&gt;len</tt> field that
+determines whether or not there are callbacks associated with
+this <tt>rcu_segcblist</tt> structure, <i>not</i> the <tt>-&gt;head</tt>
+pointer.
+The reason for this is that all the ready-to-invoke callbacks
+(that is, those in the <tt>RCU_DONE_TAIL</tt> segment) are extracted
+all at once at callback-invocation time.
+If callback invocation must be postponed, for example, because a
+high-priority process just woke up on this CPU, then the remaining
+callbacks are placed back on the <tt>RCU_DONE_TAIL</tt> segment.
+Either way, the <tt>-&gt;len</tt> and <tt>-&gt;len_lazy</tt> counts
+are adjusted after the corresponding callbacks have been invoked, and so
+again it is the <tt>-&gt;len</tt> count that accurately reflects whether
+or not there are callbacks associated with this <tt>rcu_segcblist</tt>
+structure.
+Of course, off-CPU sampling of the <tt>-&gt;len</tt> count requires
+the use of appropriate synchronization, for example, memory barriers.
+This synchronization can be a bit subtle, particularly in the case
+of <tt>rcu_barrier()</tt>.
+
+<h3><a name="The rcu_data Structure">
+The <tt>rcu_data</tt> Structure</a></h3>
+
+<p>The <tt>rcu_data</tt> maintains the per-CPU state for the
+corresponding flavor of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+This structure is the
+focus of quiescent-state detection and RCU callback queuing.
+It also tracks its relationship to the corresponding leaf
+<tt>rcu_node</tt> structure to allow more-efficient
+propagation of quiescent states up the <tt>rcu_node</tt>
+combining tree.
+Like the <tt>rcu_node</tt> structure, it provides a local
+copy of the grace-period information to allow for-free
+synchronized
+access to this information from the corresponding CPU.
+Finally, this structure records past dyntick-idle state
+for the corresponding CPU and also tracks statistics.
+
+</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Other Data Structures</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   int cpu;
+  2   struct rcu_state *rsp;
+  3   struct rcu_node *mynode;
+  4   struct rcu_dynticks *dynticks;
+  5   unsigned long grpmask;
+  6   bool beenonline;
+</pre>
+
+<p>The <tt>-&gt;cpu</tt> field contains the number of the
+corresponding CPU, the <tt>-&gt;rsp</tt> pointer references
+the corresponding <tt>rcu_state</tt> structure (and is most frequently
+used to locate the name of the corresponding flavor of RCU for tracing),
+and the <tt>-&gt;mynode</tt> field references the corresponding
+<tt>rcu_node</tt> structure.
+The <tt>-&gt;mynode</tt> is used to propagate quiescent states
+up the combining tree.
+<p>The <tt>-&gt;dynticks</tt> pointer references the
+<tt>rcu_dynticks</tt> structure corresponding to this
+CPU.
+Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt>
+structure is shared among all flavors of RCU.
+These first four fields are constant and therefore require not
+synchronization.
+
+</p><p>The <tt>-&gt;grpmask</tt> field indicates the bit in
+the <tt>-&gt;mynode-&gt;qsmask</tt> corresponding to this
+<tt>rcu_data</tt> structure, and is also used when propagating
+quiescent states.
+The <tt>-&gt;beenonline</tt> flag is set whenever the corresponding
+CPU comes online, which means that the debugfs tracing need not dump
+out any <tt>rcu_data</tt> structure for which this flag is not set.
+
+<h5>Quiescent-State and Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   unsigned long gp_seq;
+  2   unsigned long gp_seq_needed;
+  3   bool cpu_no_qs;
+  4   bool core_needs_qs;
+  5   bool gpwrap;
+  6   unsigned long rcu_qs_ctr_snap;
+</pre>
+
+<p>The <tt>-&gt;gp_seq</tt> and <tt>-&gt;gp_seq_needed</tt>
+fields are the counterparts of the fields of the same name
+in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
+They may each lag up to one behind their <tt>rcu_node</tt>
+counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
+<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
+arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
+will catch up upon exit from dyntick-idle mode).
+If the lower two bits of a given <tt>rcu_data</tt> structure's
+<tt>-&gt;gp_seq</tt> are zero, then this <tt>rcu_data</tt>
+structure believes that RCU is idle.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	All this replication of the grace period numbers can only cause
+	massive confusion.
+	Why not just keep a global sequence number and be done with it???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because if there was only a single global sequence
+	numbers, there would need to be a single global lock to allow
+	safely accessing and updating it.
+	And if we are not going to have a single global lock, we need
+	to carefully manage the numbers on a per-node basis.
+	Recall from the answer to a previous Quick Quiz that the consequences
+	of applying a previously sampled quiescent state to the wrong
+	grace period are quite severe.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>The <tt>-&gt;cpu_no_qs</tt> flag indicates that the
+CPU has not yet passed through a quiescent state,
+while the <tt>-&gt;core_needs_qs</tt> flag indicates that the
+RCU core needs a quiescent state from the corresponding CPU.
+The <tt>-&gt;gpwrap</tt> field indicates that the corresponding
+CPU has remained idle for so long that the
+<tt>gp_seq</tt> counter is in danger of overflow, which
+will cause the CPU to disregard the values of its counters on
+its next exit from idle.
+Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
+cases where a given operation has resulted in a quiescent state
+for all flavors of RCU, for example, <tt>cond_resched()</tt>
+when RCU has indicated a need for quiescent states.
+
+<h5>RCU Callback Handling</h5>
+
+<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
+the same CPU that registered them.
+This is strictly a cache-locality optimization: callbacks can and
+do get invoked on CPUs other than the one that registered them.
+After all, if the CPU that registered a given callback has gone
+offline before the callback can be invoked, there really is no other
+choice.
+
+</p><p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_segcblist cblist;
+ 2 long qlen_last_fqs_check;
+ 3 unsigned long n_cbs_invoked;
+ 4 unsigned long n_nocbs_invoked;
+ 5 unsigned long n_cbs_orphaned;
+ 6 unsigned long n_cbs_adopted;
+ 7 unsigned long n_force_qs_snap;
+ 8 long blimit;
+</pre>
+
+<p>The <tt>-&gt;cblist</tt> structure is the segmented callback list
+described earlier.
+The CPU advances the callbacks in its <tt>rcu_data</tt> structure
+whenever it notices that another RCU grace period has completed.
+The CPU detects the completion of an RCU grace period by noticing
+that the value of its <tt>rcu_data</tt> structure's
+<tt>-&gt;gp_seq</tt> field differs from that of its leaf
+<tt>rcu_node</tt> structure.
+Recall that each <tt>rcu_node</tt> structure's
+<tt>-&gt;gp_seq</tt> field is updated at the beginnings and ends of each
+grace period.
+
+<p>
+The <tt>-&gt;qlen_last_fqs_check</tt> and
+<tt>-&gt;n_force_qs_snap</tt> coordinate the forcing of quiescent
+states from <tt>call_rcu()</tt> and friends when callback
+lists grow excessively long.
+
+</p><p>The <tt>-&gt;n_cbs_invoked</tt>,
+<tt>-&gt;n_cbs_orphaned</tt>, and <tt>-&gt;n_cbs_adopted</tt>
+fields count the number of callbacks invoked,
+sent to other CPUs when this CPU goes offline,
+and received from other CPUs when those other CPUs go offline.
+The <tt>-&gt;n_nocbs_invoked</tt> is used when the CPU's callbacks
+are offloaded to a kthread.
+
+<p>
+Finally, the <tt>-&gt;blimit</tt> counter is the maximum number of
+RCU callbacks that may be invoked at a given time.
+
+<h5>Dyntick-Idle Handling</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+  1   int dynticks_snap;
+  2   unsigned long dynticks_fqs;
+</pre>
+
+The <tt>-&gt;dynticks_snap</tt> field is used to take a snapshot
+of the corresponding CPU's dyntick-idle state when forcing
+quiescent states, and is therefore accessed from other CPUs.
+Finally, the <tt>-&gt;dynticks_fqs</tt> field is used to
+count the number of times this CPU is determined to be in
+dyntick-idle state, and is used for tracing and debugging purposes.
+
+<h3><a name="The rcu_dynticks Structure">
+The <tt>rcu_dynticks</tt> Structure</a></h3>
+
+<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state
+for the corresponding CPU.
+Unlike the other structures, <tt>rcu_dynticks</tt> is not
+replicated over the different flavors of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+Its fields are as follows:
+
+<pre>
+  1   long dynticks_nesting;
+  2   long dynticks_nmi_nesting;
+  3   atomic_t dynticks;
+  4   bool rcu_need_heavy_qs;
+  5   unsigned long rcu_qs_ctr;
+  6   bool rcu_urgent_qs;
+</pre>
+
+<p>The <tt>-&gt;dynticks_nesting</tt> field counts the
+nesting depth of process execution, so that in normal circumstances
+this counter has value zero or one.
+NMIs, irqs, and tracers are counted by the <tt>-&gt;dynticks_nmi_nesting</tt>
+field.
+Because NMIs cannot be masked, changes to this variable have to be
+undertaken carefully using an algorithm provided by Andy Lutomirski.
+The initial transition from idle adds one, and nested transitions
+add two, so that a nesting level of five is represented by a
+<tt>-&gt;dynticks_nmi_nesting</tt> value of nine.
+This counter can therefore be thought of as counting the number
+of reasons why this CPU cannot be permitted to enter dyntick-idle
+mode, aside from process-level transitions.
+
+<p>However, it turns out that when running in non-idle kernel context,
+the Linux kernel is fully capable of entering interrupt handlers that
+never exit and perhaps also vice versa.
+Therefore, whenever the <tt>-&gt;dynticks_nesting</tt> field is
+incremented up from zero, the <tt>-&gt;dynticks_nmi_nesting</tt> field
+is set to a large positive number, and whenever the
+<tt>-&gt;dynticks_nesting</tt> field is decremented down to zero,
+the the <tt>-&gt;dynticks_nmi_nesting</tt> field is set to zero.
+Assuming that the number of misnested interrupts is not sufficient
+to overflow the counter, this approach corrects the
+<tt>-&gt;dynticks_nmi_nesting</tt> field every time the corresponding
+CPU enters the idle loop from process context.
+
+</p><p>The <tt>-&gt;dynticks</tt> field counts the corresponding
+CPU's transitions to and from dyntick-idle mode, so that this counter
+has an even value when the CPU is in dyntick-idle mode and an odd
+value otherwise.
+
+</p><p>The <tt>-&gt;rcu_need_heavy_qs</tt> field is used
+to record the fact that the RCU core code would really like to
+see a quiescent state from the corresponding CPU, so much so that
+it is willing to call for heavy-weight dyntick-counter operations.
+This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
+code, which provide a momentary idle sojourn in response.
+
+</p><p>The <tt>-&gt;rcu_qs_ctr</tt> field is used to record
+quiescent states from <tt>cond_resched()</tt>.
+Because <tt>cond_resched()</tt> can execute quite frequently, this
+must be quite lightweight, as in a non-atomic increment of this
+per-CPU field.
+
+</p><p>Finally, the <tt>-&gt;rcu_urgent_qs</tt> field is used to record
+the fact that the RCU core code would really like to see a quiescent
+state from the corresponding CPU, with the various other fields indicating
+just how badly RCU wants this quiescent state.
+This flag is checked by RCU's context-switch and <tt>cond_resched()</tt>
+code, which, if nothing else, non-atomically increment <tt>-&gt;rcu_qs_ctr</tt>
+in response.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why not simply combine the <tt>-&gt;dynticks_nesting</tt>
+	and <tt>-&gt;dynticks_nmi_nesting</tt> counters into a
+	single counter that just counts the number of reasons that
+	the corresponding CPU is non-idle?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because this would fail in the presence of interrupts whose
+	handlers never return and of handlers that manage to return
+	from a made-up interrupt.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>Additional fields are present for some special-purpose
+builds, and are discussed separately.
+
+<h3><a name="The rcu_head Structure">
+The <tt>rcu_head</tt> Structure</a></h3>
+
+<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
+These structures are normally embedded within RCU-protected data
+structures whose algorithms use asynchronous grace periods.
+In contrast, when using algorithms that block waiting for RCU grace periods,
+RCU users need not provide <tt>rcu_head</tt> structures.
+
+</p><p>The <tt>rcu_head</tt> structure has fields as follows:
+
+<pre>
+  1   struct rcu_head *next;
+  2   void (*func)(struct rcu_head *head);
+</pre>
+
+<p>The <tt>-&gt;next</tt> field is used
+to link the <tt>rcu_head</tt> structures together in the
+lists within the <tt>rcu_data</tt> structures.
+The <tt>-&gt;func</tt> field is a pointer to the function
+to be called when the callback is ready to be invoked, and
+this function is passed a pointer to the <tt>rcu_head</tt>
+structure.
+However, <tt>kfree_rcu()</tt> uses the <tt>-&gt;func</tt>
+field to record the offset of the <tt>rcu_head</tt>
+structure within the enclosing RCU-protected data structure.
+
+</p><p>Both of these fields are used internally by RCU.
+From the viewpoint of RCU users, this structure is an
+opaque &ldquo;cookie&rdquo;.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Given that the callback function <tt>-&gt;func</tt>
+	is passed a pointer to the <tt>rcu_head</tt> structure,
+	how is that function supposed to find the beginning of the
+	enclosing RCU-protected data structure?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In actual practice, there is a separate callback function per
+	type of RCU-protected data structure.
+	The callback function can therefore use the <tt>container_of()</tt>
+	macro in the Linux kernel (or other pointer-manipulation facilities
+	in other software environments) to find the beginning of the
+	enclosing structure.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="RCU-Specific Fields in the task_struct Structure">
+RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
+
+<p>The <tt>CONFIG_PREEMPT_RCU</tt> implementation uses some
+additional fields in the <tt>task_struct</tt> structure:
+
+<pre>
+ 1 #ifdef CONFIG_PREEMPT_RCU
+ 2   int rcu_read_lock_nesting;
+ 3   union rcu_special rcu_read_unlock_special;
+ 4   struct list_head rcu_node_entry;
+ 5   struct rcu_node *rcu_blocked_node;
+ 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
+ 7 #ifdef CONFIG_TASKS_RCU
+ 8   unsigned long rcu_tasks_nvcsw;
+ 9   bool rcu_tasks_holdout;
+10   struct list_head rcu_tasks_holdout_list;
+11   int rcu_tasks_idle_cpu;
+12 #endif /* #ifdef CONFIG_TASKS_RCU */
+</pre>
+
+<p>The <tt>-&gt;rcu_read_lock_nesting</tt> field records the
+nesting level for RCU read-side critical sections, and
+the <tt>-&gt;rcu_read_unlock_special</tt> field is a bitmask
+that records special conditions that require <tt>rcu_read_unlock()</tt>
+to do additional work.
+The <tt>-&gt;rcu_node_entry</tt> field is used to form lists of
+tasks that have blocked within preemptible-RCU read-side critical
+sections and the <tt>-&gt;rcu_blocked_node</tt> field references
+the <tt>rcu_node</tt> structure whose list this task is a member of,
+or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
+read-side critical section.
+
+<p>The <tt>-&gt;rcu_tasks_nvcsw</tt> field tracks the number of
+voluntary context switches that this task had undergone at the
+beginning of the current tasks-RCU grace period,
+<tt>-&gt;rcu_tasks_holdout</tt> is set if the current tasks-RCU
+grace period is waiting on this task, <tt>-&gt;rcu_tasks_holdout_list</tt>
+is a list element enqueuing this task on the holdout list,
+and <tt>-&gt;rcu_tasks_idle_cpu</tt> tracks which CPU this
+idle task is running, but only if the task is currently running,
+that is, if the CPU is currently idle.
+
+<h3><a name="Accessor Functions">
+Accessor Functions</a></h3>
+
+<p>The following listing shows the
+<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
+<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
+<tt>rcu_for_each_leaf_node()</tt> function and macros:
+
+<pre>
+  1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+  2 {
+  3   return &amp;rsp-&gt;node[0];
+  4 }
+  5
+  6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
+  7   for ((rnp) = &amp;(rsp)-&gt;node[0]; \
+  8        (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
+  9
+ 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
+ 11   for ((rnp) = &amp;(rsp)-&gt;node[0]; \
+ 12        (rnp) &lt; (rsp)-&gt;level[NUM_RCU_LVLS - 1]; (rnp)++)
+ 13
+ 14 #define rcu_for_each_leaf_node(rsp, rnp) \
+ 15   for ((rnp) = (rsp)-&gt;level[NUM_RCU_LVLS - 1]; \
+ 16        (rnp) &lt; &amp;(rsp)-&gt;node[NUM_RCU_NODES]; (rnp)++)
+</pre>
+
+<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
+first element of the specified <tt>rcu_state</tt> structure's
+<tt>-&gt;node[]</tt> array, which is the root <tt>rcu_node</tt>
+structure.
+
+</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
+macro takes advantage of the layout of the <tt>rcu_node</tt>
+structures in the <tt>rcu_state</tt> structure's
+<tt>-&gt;node[]</tt> array, performing a breadth-first traversal by
+simply traversing the array in order.
+The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
+similarly, but traverses only the first part of the array, thus excluding
+the leaf <tt>rcu_node</tt> structures.
+Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
+the last part of the array, thus traversing only the leaf
+<tt>rcu_node</tt> structures.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
+	<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
+	contains only a single node?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In the single-node case,
+	<tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
+	and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Summary">
+Summary</a></h3>
+
+So each flavor of RCU is represented by an <tt>rcu_state</tt> structure,
+which contains a combining tree of <tt>rcu_node</tt> and
+<tt>rcu_data</tt> structures.
+Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
+state is tracked by an <tt>rcu_dynticks</tt> structure.
+
+If you made it this far, you are well prepared to read the code
+walkthroughs in the other articles in this series.
+
+<h3><a name="Acknowledgments">
+Acknowledgments</a></h3>
+
+I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
+Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
+for helping me get this document into a more human-readable state.
+
+<h3><a name="Legal Statement">
+Legal Statement</a></h3>
+
+<p>This work represents the view of the author and does not necessarily
+represent the view of IBM.
+
+</p><p>Linux is a registered trademark of Linus Torvalds.
+
+</p><p>Other company, product, and service names may be trademarks or
+service marks of others.
+
+</body></html>
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diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
new file mode 100644
index 0000000..7394f03
--- /dev/null
+++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.html
@@ -0,0 +1,667 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through TREE_RCU's Expedited Grace Periods</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+<h2>Introduction</h2>
+
+This document describes RCU's expedited grace periods.
+Unlike RCU's normal grace periods, which accept long latencies to attain
+high efficiency and minimal disturbance, expedited grace periods accept
+lower efficiency and significant disturbance to attain shorter latencies.
+
+<p>
+There are three flavors of RCU (RCU-bh, RCU-preempt, and RCU-sched),
+but only two flavors of expedited grace periods because the RCU-bh
+expedited grace period maps onto the RCU-sched expedited grace period.
+Each of the remaining two implementations is covered in its own section.
+
+<ol>
+<li>	<a href="#Expedited Grace Period Design">
+	Expedited Grace Period Design</a>
+<li>	<a href="#RCU-preempt Expedited Grace Periods">
+	RCU-preempt Expedited Grace Periods</a>
+<li>	<a href="#RCU-sched Expedited Grace Periods">
+	RCU-sched Expedited Grace Periods</a>
+<li>	<a href="#Expedited Grace Period and CPU Hotplug">
+	Expedited Grace Period and CPU Hotplug</a>
+<li>	<a href="#Expedited Grace Period Refinements">
+	Expedited Grace Period Refinements</a>
+</ol>
+
+<h2><a name="Expedited Grace Period Design">
+Expedited Grace Period Design</a></h2>
+
+<p>
+The expedited RCU grace periods cannot be accused of being subtle,
+given that they for all intents and purposes hammer every CPU that
+has not yet provided a quiescent state for the current expedited
+grace period.
+The one saving grace is that the hammer has grown a bit smaller
+over time:  The old call to <tt>try_stop_cpus()</tt> has been
+replaced with a set of calls to <tt>smp_call_function_single()</tt>,
+each of which results in an IPI to the target CPU.
+The corresponding handler function checks the CPU's state, motivating
+a faster quiescent state where possible, and triggering a report
+of that quiescent state.
+As always for RCU, once everything has spent some time in a quiescent
+state, the expedited grace period has completed.
+
+<p>
+The details of the <tt>smp_call_function_single()</tt> handler's
+operation depend on the RCU flavor, as described in the following
+sections.
+
+<h2><a name="RCU-preempt Expedited Grace Periods">
+RCU-preempt Expedited Grace Periods</a></h2>
+
+<p>
+The overall flow of the handling of a given CPU by an RCU-preempt
+expedited grace period is shown in the following diagram:
+
+<p><img src="ExpRCUFlow.svg" alt="ExpRCUFlow.svg" width="55%">
+
+<p>
+The solid arrows denote direct action, for example, a function call.
+The dotted arrows denote indirect action, for example, an IPI
+or a state that is reached after some time.
+
+<p>
+If a given CPU is offline or idle, <tt>synchronize_rcu_expedited()</tt>
+will ignore it because idle and offline CPUs are already residing
+in quiescent states.
+Otherwise, the expedited grace period will use
+<tt>smp_call_function_single()</tt> to send the CPU an IPI, which
+is handled by <tt>sync_rcu_exp_handler()</tt>.
+
+<p>
+However, because this is preemptible RCU, <tt>sync_rcu_exp_handler()</tt>
+can check to see if the CPU is currently running in an RCU read-side
+critical section.
+If not, the handler can immediately report a quiescent state.
+Otherwise, it sets flags so that the outermost <tt>rcu_read_unlock()</tt>
+invocation will provide the needed quiescent-state report.
+This flag-setting avoids the previous forced preemption of all
+CPUs that might have RCU read-side critical sections.
+In addition, this flag-setting is done so as to avoid increasing
+the overhead of the common-case fastpath through the scheduler.
+
+<p>
+Again because this is preemptible RCU, an RCU read-side critical section
+can be preempted.
+When that happens, RCU will enqueue the task, which will the continue to
+block the current expedited grace period until it resumes and finds its
+outermost <tt>rcu_read_unlock()</tt>.
+The CPU will report a quiescent state just after enqueuing the task because
+the CPU is no longer blocking the grace period.
+It is instead the preempted task doing the blocking.
+The list of blocked tasks is managed by <tt>rcu_preempt_ctxt_queue()</tt>,
+which is called from <tt>rcu_preempt_note_context_switch()</tt>, which
+in turn is called from <tt>rcu_note_context_switch()</tt>, which in
+turn is called from the scheduler.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why not just have the expedited grace period check the
+	state of all the CPUs?
+	After all, that would avoid all those real-time-unfriendly IPIs.
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because we want the RCU read-side critical sections to run fast,
+	which means no memory barriers.
+	Therefore, it is not possible to safely check the state from some
+	other CPU.
+	And even if it was possible to safely check the state, it would
+	still be necessary to IPI the CPU to safely interact with the
+	upcoming <tt>rcu_read_unlock()</tt> invocation, which means that
+	the remote state testing would not help the worst-case
+	latency that real-time applications care about.
+
+	<p><font color="ffffff">One way to prevent your real-time
+	application from getting hit with these IPIs is to
+	build your kernel with <tt>CONFIG_NO_HZ_FULL=y</tt>.
+	RCU would then perceive the CPU running your application
+	as being idle, and it would be able to safely detect that
+	state without needing to IPI the CPU.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+Please note that this is just the overall flow:
+Additional complications can arise due to races with CPUs going idle
+or offline, among other things.
+
+<h2><a name="RCU-sched Expedited Grace Periods">
+RCU-sched Expedited Grace Periods</a></h2>
+
+<p>
+The overall flow of the handling of a given CPU by an RCU-sched
+expedited grace period is shown in the following diagram:
+
+<p><img src="ExpSchedFlow.svg" alt="ExpSchedFlow.svg" width="55%">
+
+<p>
+As with RCU-preempt's <tt>synchronize_rcu_expedited()</tt>,
+<tt>synchronize_sched_expedited()</tt> ignores offline and
+idle CPUs, again because they are in remotely detectable
+quiescent states.
+However, the <tt>synchronize_rcu_expedited()</tt> handler
+is <tt>sync_sched_exp_handler()</tt>, and because the
+<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
+leave no trace of their invocation, in general it is not possible to tell
+whether or not the current CPU is in an RCU read-side critical section.
+The best that <tt>sync_sched_exp_handler()</tt> can do is to check
+for idle, on the off-chance that the CPU went idle while the IPI
+was in flight.
+If the CPU is idle, then tt>sync_sched_exp_handler()</tt> reports
+the quiescent state.
+
+<p>
+Otherwise, the handler invokes <tt>resched_cpu()</tt>, which forces
+a future context switch.
+At the time of the context switch, the CPU reports the quiescent state.
+Should the CPU go offline first, it will report the quiescent state
+at that time.
+
+<h2><a name="Expedited Grace Period and CPU Hotplug">
+Expedited Grace Period and CPU Hotplug</a></h2>
+
+<p>
+The expedited nature of expedited grace periods require a much tighter
+interaction with CPU hotplug operations than is required for normal
+grace periods.
+In addition, attempting to IPI offline CPUs will result in splats, but
+failing to IPI online CPUs can result in too-short grace periods.
+Neither option is acceptable in production kernels.
+
+<p>
+The interaction between expedited grace periods and CPU hotplug operations
+is carried out at several levels:
+
+<ol>
+<li>	The number of CPUs that have ever been online is tracked
+	by the <tt>rcu_state</tt> structure's <tt>-&gt;ncpus</tt>
+	field.
+	The <tt>rcu_state</tt> structure's <tt>-&gt;ncpus_snap</tt>
+	field tracks the number of CPUs that have ever been online
+	at the beginning of an RCU expedited grace period.
+	Note that this number never decreases, at least in the absence
+	of a time machine.
+<li>	The identities of the CPUs that have ever been online is
+	tracked by the <tt>rcu_node</tt> structure's
+	<tt>-&gt;expmaskinitnext</tt> field.
+	The <tt>rcu_node</tt> structure's <tt>-&gt;expmaskinit</tt>
+	field tracks the identities of the CPUs that were online
+	at least once at the beginning of the most recent RCU
+	expedited grace period.
+	The <tt>rcu_state</tt> structure's <tt>-&gt;ncpus</tt> and
+	<tt>-&gt;ncpus_snap</tt> fields are used to detect when
+	new CPUs have come online for the first time, that is,
+	when the <tt>rcu_node</tt> structure's <tt>-&gt;expmaskinitnext</tt>
+	field has changed since the beginning of the last RCU
+	expedited grace period, which triggers an update of each
+	<tt>rcu_node</tt> structure's <tt>-&gt;expmaskinit</tt>
+	field from its <tt>-&gt;expmaskinitnext</tt> field.
+<li>	Each <tt>rcu_node</tt> structure's <tt>-&gt;expmaskinit</tt>
+	field is used to initialize that structure's
+	<tt>-&gt;expmask</tt> at the beginning of each RCU
+	expedited grace period.
+	This means that only those CPUs that have been online at least
+	once will be considered for a given grace period.
+<li>	Any CPU that goes offline will clear its bit in its leaf
+	<tt>rcu_node</tt> structure's <tt>-&gt;qsmaskinitnext</tt>
+	field, so any CPU with that bit clear can safely be ignored.
+	However, it is possible for a CPU coming online or going offline
+	to have this bit set for some time while <tt>cpu_online</tt>
+	returns <tt>false</tt>.
+<li>	For each non-idle CPU that RCU believes is currently online, the grace
+	period invokes <tt>smp_call_function_single()</tt>.
+	If this succeeds, the CPU was fully online.
+	Failure indicates that the CPU is in the process of coming online
+	or going offline, in which case it is necessary to wait for a
+	short time period and try again.
+	The purpose of this wait (or series of waits, as the case may be)
+	is to permit a concurrent CPU-hotplug operation to complete.
+<li>	In the case of RCU-sched, one of the last acts of an outgoing CPU
+	is to invoke <tt>rcu_report_dead()</tt>, which
+	reports a quiescent state for that CPU.
+	However, this is likely paranoia-induced redundancy. <!-- @@@ -->
+</ol>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why all the dancing around with multiple counters and masks
+	tracking CPUs that were once online?
+	Why not just have a single set of masks tracking the currently
+	online CPUs and be done with it?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Maintaining single set of masks tracking the online CPUs <i>sounds</i>
+	easier, at least until you try working out all the race conditions
+	between grace-period initialization and CPU-hotplug operations.
+	For example, suppose initialization is progressing down the
+	tree while a CPU-offline operation is progressing up the tree.
+	This situation can result in bits set at the top of the tree
+	that have no counterparts at the bottom of the tree.
+	Those bits will never be cleared, which will result in
+	grace-period hangs.
+	In short, that way lies madness, to say nothing of a great many
+	bugs, hangs, and deadlocks.
+
+	<p><font color="ffffff">
+	In contrast, the current multi-mask multi-counter scheme ensures
+	that grace-period initialization will always see consistent masks
+	up and down the tree, which brings significant simplifications
+	over the single-mask method.
+
+	<p><font color="ffffff">
+	This is an instance of
+	<a href="http://www.cs.columbia.edu/~library/TR-repository/reports/reports-1992/cucs-039-92.ps.gz"><font color="ffffff">
+	deferring work in order to avoid synchronization</a>.
+	Lazily recording CPU-hotplug events at the beginning of the next
+	grace period greatly simplifies maintenance of the CPU-tracking
+	bitmasks in the <tt>rcu_node</tt> tree.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h2><a name="Expedited Grace Period Refinements">
+Expedited Grace Period Refinements</a></h2>
+
+<ol>
+<li>	<a href="#Idle-CPU Checks">Idle-CPU checks</a>.
+<li>	<a href="#Batching via Sequence Counter">
+	Batching via sequence counter</a>.
+<li>	<a href="#Funnel Locking and Wait/Wakeup">
+	Funnel locking and wait/wakeup</a>.
+<li>	<a href="#Use of Workqueues">Use of Workqueues</a>.
+<li>	<a href="#Stall Warnings">Stall warnings</a>.
+<li>	<a href="#Mid-Boot Operation">Mid-boot operation</a>.
+</ol>
+
+<h3><a name="Idle-CPU Checks">Idle-CPU Checks</a></h3>
+
+<p>
+Each expedited grace period checks for idle CPUs when initially forming
+the mask of CPUs to be IPIed and again just before IPIing a CPU
+(both checks are carried out by <tt>sync_rcu_exp_select_cpus()</tt>).
+If the CPU is idle at any time between those two times, the CPU will
+not be IPIed.
+Instead, the task pushing the grace period forward will include the
+idle CPUs in the mask passed to <tt>rcu_report_exp_cpu_mult()</tt>.
+
+<p>
+For RCU-sched, there is an additional check for idle in the IPI
+handler, <tt>sync_sched_exp_handler()</tt>.
+If the IPI has interrupted the idle loop, then
+<tt>sync_sched_exp_handler()</tt> invokes <tt>rcu_report_exp_rdp()</tt>
+to report the corresponding quiescent state.
+
+<p>
+For RCU-preempt, there is no specific check for idle in the
+IPI handler (<tt>sync_rcu_exp_handler()</tt>), but because
+RCU read-side critical sections are not permitted within the
+idle loop, if <tt>sync_rcu_exp_handler()</tt> sees that the CPU is within
+RCU read-side critical section, the CPU cannot possibly be idle.
+Otherwise, <tt>sync_rcu_exp_handler()</tt> invokes
+<tt>rcu_report_exp_rdp()</tt> to report the corresponding quiescent
+state, regardless of whether or not that quiescent state was due to
+the CPU being idle.
+
+<p>
+In summary, RCU expedited grace periods check for idle when building
+the bitmask of CPUs that must be IPIed, just before sending each IPI,
+and (either explicitly or implicitly) within the IPI handler.
+
+<h3><a name="Batching via Sequence Counter">
+Batching via Sequence Counter</a></h3>
+
+<p>
+If each grace-period request was carried out separately, expedited
+grace periods would have abysmal scalability and
+problematic high-load characteristics.
+Because each grace-period operation can serve an unlimited number of
+updates, it is important to <i>batch</i> requests, so that a single
+expedited grace-period operation will cover all requests in the
+corresponding batch.
+
+<p>
+This batching is controlled by a sequence counter named
+<tt>-&gt;expedited_sequence</tt> in the <tt>rcu_state</tt> structure.
+This counter has an odd value when there is an expedited grace period
+in progress and an even value otherwise, so that dividing the counter
+value by two gives the number of completed grace periods.
+During any given update request, the counter must transition from
+even to odd and then back to even, thus indicating that a grace
+period has elapsed.
+Therefore, if the initial value of the counter is <tt>s</tt>,
+the updater must wait until the counter reaches at least the
+value <tt>(s+3)&amp;~0x1</tt>.
+This counter is managed by the following access functions:
+
+<ol>
+<li>	<tt>rcu_exp_gp_seq_start()</tt>, which marks the start of
+	an expedited grace period.
+<li>	<tt>rcu_exp_gp_seq_end()</tt>, which marks the end of an
+	expedited grace period.
+<li>	<tt>rcu_exp_gp_seq_snap()</tt>, which obtains a snapshot of
+	the counter.
+<li>	<tt>rcu_exp_gp_seq_done()</tt>, which returns <tt>true</tt>
+	if a full expedited grace period has elapsed since the
+	corresponding call to <tt>rcu_exp_gp_seq_snap()</tt>.
+</ol>
+
+<p>
+Again, only one request in a given batch need actually carry out
+a grace-period operation, which means there must be an efficient
+way to identify which of many concurrent reqeusts will initiate
+the grace period, and that there be an efficient way for the
+remaining requests to wait for that grace period to complete.
+However, that is the topic of the next section.
+
+<h3><a name="Funnel Locking and Wait/Wakeup">
+Funnel Locking and Wait/Wakeup</a></h3>
+
+<p>
+The natural way to sort out which of a batch of updaters will initiate
+the expedited grace period is to use the <tt>rcu_node</tt> combining
+tree, as implemented by the <tt>exp_funnel_lock()</tt> function.
+The first updater corresponding to a given grace period arriving
+at a given <tt>rcu_node</tt> structure records its desired grace-period
+sequence number in the <tt>-&gt;exp_seq_rq</tt> field and moves up
+to the next level in the tree.
+Otherwise, if the <tt>-&gt;exp_seq_rq</tt> field already contains
+the sequence number for the desired grace period or some later one,
+the updater blocks on one of four wait queues in the
+<tt>-&gt;exp_wq[]</tt> array, using the second-from-bottom
+and third-from bottom bits as an index.
+An <tt>-&gt;exp_lock</tt> field in the <tt>rcu_node</tt> structure
+synchronizes access to these fields.
+
+<p>
+An empty <tt>rcu_node</tt> tree is shown in the following diagram,
+with the white cells representing the <tt>-&gt;exp_seq_rq</tt> field
+and the red cells representing the elements of the
+<tt>-&gt;exp_wq[]</tt> array.
+
+<p><img src="Funnel0.svg" alt="Funnel0.svg" width="75%">
+
+<p>
+The next diagram shows the situation after the arrival of Task&nbsp;A
+and Task&nbsp;B at the leftmost and rightmost leaf <tt>rcu_node</tt>
+structures, respectively.
+The current value of the <tt>rcu_state</tt> structure's
+<tt>-&gt;expedited_sequence</tt> field is zero, so adding three and
+clearing the bottom bit results in the value two, which both tasks
+record in the <tt>-&gt;exp_seq_rq</tt> field of their respective
+<tt>rcu_node</tt> structures:
+
+<p><img src="Funnel1.svg" alt="Funnel1.svg" width="75%">
+
+<p>
+Each of Tasks&nbsp;A and&nbsp;B will move up to the root
+<tt>rcu_node</tt> structure.
+Suppose that Task&nbsp;A wins, recording its desired grace-period sequence
+number and resulting in the state shown below:
+
+<p><img src="Funnel2.svg" alt="Funnel2.svg" width="75%">
+
+<p>
+Task&nbsp;A now advances to initiate a new grace period, while Task&nbsp;B
+moves up to the root <tt>rcu_node</tt> structure, and, seeing that
+its desired sequence number is already recorded, blocks on
+<tt>-&gt;exp_wq[1]</tt>.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why <tt>-&gt;exp_wq[1]</tt>?
+	Given that the value of these tasks' desired sequence number is
+	two, so shouldn't they instead block on <tt>-&gt;exp_wq[2]</tt>?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No.
+
+	<p><font color="ffffff">
+	Recall that the bottom bit of the desired sequence number indicates
+	whether or not a grace period is currently in progress.
+	It is therefore necessary to shift the sequence number right one
+	bit position to obtain the number of the grace period.
+	This results in <tt>-&gt;exp_wq[1]</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+If Tasks&nbsp;C and&nbsp;D also arrive at this point, they will compute the
+same desired grace-period sequence number, and see that both leaf
+<tt>rcu_node</tt> structures already have that value recorded.
+They will therefore block on their respective <tt>rcu_node</tt>
+structures' <tt>-&gt;exp_wq[1]</tt> fields, as shown below:
+
+<p><img src="Funnel3.svg" alt="Funnel3.svg" width="75%">
+
+<p>
+Task&nbsp;A now acquires the <tt>rcu_state</tt> structure's
+<tt>-&gt;exp_mutex</tt> and initiates the grace period, which
+increments <tt>-&gt;expedited_sequence</tt>.
+Therefore, if Tasks&nbsp;E and&nbsp;F arrive, they will compute
+a desired sequence number of 4 and will record this value as
+shown below:
+
+<p><img src="Funnel4.svg" alt="Funnel4.svg" width="75%">
+
+<p>
+Tasks&nbsp;E and&nbsp;F will propagate up the <tt>rcu_node</tt>
+combining tree, with Task&nbsp;F blocking on the root <tt>rcu_node</tt>
+structure and Task&nbsp;E wait for Task&nbsp;A to finish so that
+it can start the next grace period.
+The resulting state is as shown below:
+
+<p><img src="Funnel5.svg" alt="Funnel5.svg" width="75%">
+
+<p>
+Once the grace period completes, Task&nbsp;A
+starts waking up the tasks waiting for this grace period to complete,
+increments the <tt>-&gt;expedited_sequence</tt>,
+acquires the <tt>-&gt;exp_wake_mutex</tt> and then releases the
+<tt>-&gt;exp_mutex</tt>.
+This results in the following state:
+
+<p><img src="Funnel6.svg" alt="Funnel6.svg" width="75%">
+
+<p>
+Task&nbsp;E can then acquire <tt>-&gt;exp_mutex</tt> and increment
+<tt>-&gt;expedited_sequence</tt> to the value three.
+If new tasks&nbsp;G and&nbsp;H arrive and moves up the combining tree at the
+same time, the state will be as follows:
+
+<p><img src="Funnel7.svg" alt="Funnel7.svg" width="75%">
+
+<p>
+Note that three of the root <tt>rcu_node</tt> structure's
+waitqueues are now occupied.
+However, at some point, Task&nbsp;A will wake up the
+tasks blocked on the <tt>-&gt;exp_wq</tt> waitqueues, resulting
+in the following state:
+
+<p><img src="Funnel8.svg" alt="Funnel8.svg" width="75%">
+
+<p>
+Execution will continue with Tasks&nbsp;E and&nbsp;H completing
+their grace periods and carrying out their wakeups.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	What happens if Task&nbsp;A takes so long to do its wakeups
+	that Task&nbsp;E's grace period completes?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Then Task&nbsp;E will block on the <tt>-&gt;exp_wake_mutex</tt>,
+	which will also prevent it from releasing <tt>-&gt;exp_mutex</tt>,
+	which in turn will prevent the next grace period from starting.
+	This last is important in preventing overflow of the
+	<tt>-&gt;exp_wq[]</tt> array.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Use of Workqueues">Use of Workqueues</a></h3>
+
+<p>
+In earlier implementations, the task requesting the expedited
+grace period also drove it to completion.
+This straightforward approach had the disadvantage of needing to
+account for POSIX signals sent to user tasks,
+so more recent implemementations use the Linux kernel's
+<a href="https://www.kernel.org/doc/Documentation/core-api/workqueue.rst">workqueues</a>.
+
+<p>
+The requesting task still does counter snapshotting and funnel-lock
+processing, but the task reaching the top of the funnel lock
+does a <tt>schedule_work()</tt> (from <tt>_synchronize_rcu_expedited()</tt>
+so that a workqueue kthread does the actual grace-period processing.
+Because workqueue kthreads do not accept POSIX signals, grace-period-wait
+processing need not allow for POSIX signals.
+
+In addition, this approach allows wakeups for the previous expedited
+grace period to be overlapped with processing for the next expedited
+grace period.
+Because there are only four sets of waitqueues, it is necessary to
+ensure that the previous grace period's wakeups complete before the
+next grace period's wakeups start.
+This is handled by having the <tt>-&gt;exp_mutex</tt>
+guard expedited grace-period processing and the
+<tt>-&gt;exp_wake_mutex</tt> guard wakeups.
+The key point is that the <tt>-&gt;exp_mutex</tt> is not released
+until the first wakeup is complete, which means that the
+<tt>-&gt;exp_wake_mutex</tt> has already been acquired at that point.
+This approach ensures that the previous grace period's wakeups can
+be carried out while the current grace period is in process, but
+that these wakeups will complete before the next grace period starts.
+This means that only three waitqueues are required, guaranteeing that
+the four that are provided are sufficient.
+
+<h3><a name="Stall Warnings">Stall Warnings</a></h3>
+
+<p>
+Expediting grace periods does nothing to speed things up when RCU
+readers take too long, and therefore expedited grace periods check
+for stalls just as normal grace periods do.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But why not just let the normal grace-period machinery
+	detect the stalls, given that a given reader must block
+	both normal and expedited grace periods?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because it is quite possible that at a given time there
+	is no normal grace period in progress, in which case the
+	normal grace period cannot emit a stall warning.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+The <tt>synchronize_sched_expedited_wait()</tt> function loops waiting
+for the expedited grace period to end, but with a timeout set to the
+current RCU CPU stall-warning time.
+If this time is exceeded, any CPUs or <tt>rcu_node</tt> structures
+blocking the current grace period are printed.
+Each stall warning results in another pass through the loop, but the
+second and subsequent passes use longer stall times.
+
+<h3><a name="Mid-Boot Operation">Mid-boot operation</a></h3>
+
+<p>
+The use of workqueues has the advantage that the expedited
+grace-period code need not worry about POSIX signals.
+Unfortunately, it has the
+corresponding disadvantage that workqueues cannot be used until
+they are initialized, which does not happen until some time after
+the scheduler spawns the first task.
+Given that there are parts of the kernel that really do want to
+execute grace periods during this mid-boot &ldquo;dead zone&rdquo;,
+expedited grace periods must do something else during thie time.
+
+<p>
+What they do is to fall back to the old practice of requiring that the
+requesting task drive the expedited grace period, as was the case
+before the use of workqueues.
+However, the requesting task is only required to drive the grace period
+during the mid-boot dead zone.
+Before mid-boot, a synchronous grace period is a no-op.
+Some time after mid-boot, workqueues are used.
+
+<p>
+Non-expedited non-SRCU synchronous grace periods must also operate
+normally during mid-boot.
+This is handled by causing non-expedited grace periods to take the
+expedited code path during mid-boot.
+
+<p>
+The current code assumes that there are no POSIX signals during
+the mid-boot dead zone.
+However, if an overwhelming need for POSIX signals somehow arises,
+appropriate adjustments can be made to the expedited stall-warning code.
+One such adjustment would reinstate the pre-workqueue stall-warning
+checks, but only during the mid-boot dead zone.
+
+<p>
+With this refinement, synchronous grace periods can now be used from
+task context pretty much any time during the life of the kernel.
+
+<h3><a name="Summary">
+Summary</a></h3>
+
+<p>
+Expedited grace periods use a sequence-number approach to promote
+batching, so that a single grace-period operation can serve numerous
+requests.
+A funnel lock is used to efficiently identify the one task out of
+a concurrent group that will request the grace period.
+All members of the group will block on waitqueues provided in
+the <tt>rcu_node</tt> structure.
+The actual grace-period processing is carried out by a workqueue.
+
+<p>
+CPU-hotplug operations are noted lazily in order to prevent the need
+for tight synchronization between expedited grace periods and
+CPU-hotplug operations.
+The dyntick-idle counters are used to avoid sending IPIs to idle CPUs,
+at least in the common case.
+RCU-preempt and RCU-sched use different IPI handlers and different
+code to respond to the state changes carried out by those handlers,
+but otherwise use common code.
+
+<p>
+Quiescent states are tracked using the <tt>rcu_node</tt> tree,
+and once all necessary quiescent states have been reported,
+all tasks waiting on this expedited grace period are awakened.
+A pair of mutexes are used to allow one grace period's wakeups
+to proceed concurrently with the next grace period's processing.
+
+<p>
+This combination of mechanisms allows expedited grace periods to
+run reasonably efficiently.
+However, for non-time-critical tasks, normal grace periods should be
+used instead because their longer duration permits much higher
+degrees of batching, and thus much lower per-request overheads.
+
+</body></html>
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diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Diagram.html b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Diagram.html
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+++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Diagram.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Diagram of TREE_RCU's Grace-Period Memory Ordering</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+<p><img src="TreeRCU-gp.svg" alt="TreeRCU-gp.svg">
+
+</body></html>
diff --git a/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html
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index 0000000..a346ce0
--- /dev/null
+++ b/Documentation/RCU/Design/Memory-Ordering/Tree-RCU-Memory-Ordering.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through TREE_RCU's Grace-Period Memory Ordering</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+           <p>August 8, 2017</p>
+           <p>This article was contributed by Paul E.&nbsp;McKenney</p>
+
+<h3>Introduction</h3>
+
+<p>This document gives a rough visual overview of how Tree RCU's
+grace-period memory ordering guarantee is provided.
+
+<ol>
+<li>	<a href="#What Is Tree RCU's Grace Period Memory Ordering Guarantee?">
+	What Is Tree RCU's Grace Period Memory Ordering Guarantee?</a>
+<li>	<a href="#Tree RCU Grace Period Memory Ordering Building Blocks">
+	Tree RCU Grace Period Memory Ordering Building Blocks</a>
+<li>	<a href="#Tree RCU Grace Period Memory Ordering Components">
+	Tree RCU Grace Period Memory Ordering Components</a>
+<li>	<a href="#Putting It All Together">Putting It All Together</a>
+</ol>
+
+<h3><a name="What Is Tree RCU's Grace Period Memory Ordering Guarantee?">
+What Is Tree RCU's Grace Period Memory Ordering Guarantee?</a></h3>
+
+<p>RCU grace periods provide extremely strong memory-ordering guarantees
+for non-idle non-offline code.
+Any code that happens after the end of a given RCU grace period is guaranteed
+to see the effects of all accesses prior to the beginning of that grace
+period that are within RCU read-side critical sections.
+Similarly, any code that happens before the beginning of a given RCU grace
+period is guaranteed to see the effects of all accesses following the end
+of that grace period that are within RCU read-side critical sections.
+
+<p>This guarantee is particularly pervasive for <tt>synchronize_sched()</tt>,
+for which RCU-sched read-side critical sections include any region
+of code for which preemption is disabled.
+Given that each individual machine instruction can be thought of as
+an extremely small region of preemption-disabled code, one can think of
+<tt>synchronize_sched()</tt> as <tt>smp_mb()</tt> on steroids.
+
+<p>RCU updaters use this guarantee by splitting their updates into
+two phases, one of which is executed before the grace period and
+the other of which is executed after the grace period.
+In the most common use case, phase one removes an element from
+a linked RCU-protected data structure, and phase two frees that element.
+For this to work, any readers that have witnessed state prior to the
+phase-one update (in the common case, removal) must not witness state
+following the phase-two update (in the common case, freeing).
+
+<p>The RCU implementation provides this guarantee using a network
+of lock-based critical sections, memory barriers, and per-CPU
+processing, as is described in the following sections.
+
+<h3><a name="Tree RCU Grace Period Memory Ordering Building Blocks">
+Tree RCU Grace Period Memory Ordering Building Blocks</a></h3>
+
+<p>The workhorse for RCU's grace-period memory ordering is the
+critical section for the <tt>rcu_node</tt> structure's
+<tt>-&gt;lock</tt>.
+These critical sections use helper functions for lock acquisition, including
+<tt>raw_spin_lock_rcu_node()</tt>,
+<tt>raw_spin_lock_irq_rcu_node()</tt>, and
+<tt>raw_spin_lock_irqsave_rcu_node()</tt>.
+Their lock-release counterparts are
+<tt>raw_spin_unlock_rcu_node()</tt>,
+<tt>raw_spin_unlock_irq_rcu_node()</tt>, and
+<tt>raw_spin_unlock_irqrestore_rcu_node()</tt>,
+respectively.
+For completeness, a
+<tt>raw_spin_trylock_rcu_node()</tt>
+is also provided.
+The key point is that the lock-acquisition functions, including
+<tt>raw_spin_trylock_rcu_node()</tt>, all invoke
+<tt>smp_mb__after_unlock_lock()</tt> immediately after successful
+acquisition of the lock.
+
+<p>Therefore, for any given <tt>rcu_node</tt> struction, any access
+happening before one of the above lock-release functions will be seen
+by all CPUs as happening before any access happening after a later
+one of the above lock-acquisition functions.
+Furthermore, any access happening before one of the
+above lock-release function on any given CPU will be seen by all
+CPUs as happening before any access happening after a later one
+of the above lock-acquisition functions executing on that same CPU,
+even if the lock-release and lock-acquisition functions are operating
+on different <tt>rcu_node</tt> structures.
+Tree RCU uses these two ordering guarantees to form an ordering
+network among all CPUs that were in any way involved in the grace
+period, including any CPUs that came online or went offline during
+the grace period in question.
+
+<p>The following litmus test exhibits the ordering effects of these
+lock-acquisition and lock-release functions:
+
+<pre>
+ 1 int x, y, z;
+ 2
+ 3 void task0(void)
+ 4 {
+ 5   raw_spin_lock_rcu_node(rnp);
+ 6   WRITE_ONCE(x, 1);
+ 7   r1 = READ_ONCE(y);
+ 8   raw_spin_unlock_rcu_node(rnp);
+ 9 }
+10
+11 void task1(void)
+12 {
+13   raw_spin_lock_rcu_node(rnp);
+14   WRITE_ONCE(y, 1);
+15   r2 = READ_ONCE(z);
+16   raw_spin_unlock_rcu_node(rnp);
+17 }
+18
+19 void task2(void)
+20 {
+21   WRITE_ONCE(z, 1);
+22   smp_mb();
+23   r3 = READ_ONCE(x);
+24 }
+25
+26 WARN_ON(r1 == 0 &amp;&amp; r2 == 0 &amp;&amp; r3 == 0);
+</pre>
+
+<p>The <tt>WARN_ON()</tt> is evaluated at &ldquo;the end of time&rdquo;,
+after all changes have propagated throughout the system.
+Without the <tt>smp_mb__after_unlock_lock()</tt> provided by the
+acquisition functions, this <tt>WARN_ON()</tt> could trigger, for example
+on PowerPC.
+The <tt>smp_mb__after_unlock_lock()</tt> invocations prevent this
+<tt>WARN_ON()</tt> from triggering.
+
+<p>This approach must be extended to include idle CPUs, which need
+RCU's grace-period memory ordering guarantee to extend to any
+RCU read-side critical sections preceding and following the current
+idle sojourn.
+This case is handled by calls to the strongly ordered
+<tt>atomic_add_return()</tt> read-modify-write atomic operation that
+is invoked within <tt>rcu_dynticks_eqs_enter()</tt> at idle-entry
+time and within <tt>rcu_dynticks_eqs_exit()</tt> at idle-exit time.
+The grace-period kthread invokes <tt>rcu_dynticks_snap()</tt> and
+<tt>rcu_dynticks_in_eqs_since()</tt> (both of which invoke
+an <tt>atomic_add_return()</tt> of zero) to detect idle CPUs.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But what about CPUs that remain offline for the entire
+	grace period?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Such CPUs will be offline at the beginning of the grace period,
+	so the grace period won't expect quiescent states from them.
+	Races between grace-period start and CPU-hotplug operations
+	are mediated by the CPU's leaf <tt>rcu_node</tt> structure's
+	<tt>-&gt;lock</tt> as described above.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>The approach must be extended to handle one final case, that
+of waking a task blocked in <tt>synchronize_rcu()</tt>.
+This task might be affinitied to a CPU that is not yet aware that
+the grace period has ended, and thus might not yet be subject to
+the grace period's memory ordering.
+Therefore, there is an <tt>smp_mb()</tt> after the return from
+<tt>wait_for_completion()</tt> in the <tt>synchronize_rcu()</tt>
+code path.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	What?  Where???
+	I don't see any <tt>smp_mb()</tt> after the return from
+	<tt>wait_for_completion()</tt>!!!
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	That would be because I spotted the need for that
+	<tt>smp_mb()</tt> during the creation of this documentation,
+	and it is therefore unlikely to hit mainline before v4.14.
+	Kudos to Lance Roy, Will Deacon, Peter Zijlstra, and
+	Jonathan Cameron for asking questions that sensitized me
+	to the rather elaborate sequence of events that demonstrate
+	the need for this memory barrier.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>Tree RCU's grace--period memory-ordering guarantees rely most
+heavily on the <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>
+field, so much so that it is necessary to abbreviate this pattern
+in the diagrams in the next section.
+For example, consider the <tt>rcu_prepare_for_idle()</tt> function
+shown below, which is one of several functions that enforce ordering
+of newly arrived RCU callbacks against future grace periods:
+
+<pre>
+ 1 static void rcu_prepare_for_idle(void)
+ 2 {
+ 3   bool needwake;
+ 4   struct rcu_data *rdp;
+ 5   struct rcu_dynticks *rdtp = this_cpu_ptr(&amp;rcu_dynticks);
+ 6   struct rcu_node *rnp;
+ 7   struct rcu_state *rsp;
+ 8   int tne;
+ 9
+10   if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_ALL) ||
+11       rcu_is_nocb_cpu(smp_processor_id()))
+12     return;
+13   tne = READ_ONCE(tick_nohz_active);
+14   if (tne != rdtp-&gt;tick_nohz_enabled_snap) {
+15     if (rcu_cpu_has_callbacks(NULL))
+16       invoke_rcu_core();
+17     rdtp-&gt;tick_nohz_enabled_snap = tne;
+18     return;
+19   }
+20   if (!tne)
+21     return;
+22   if (rdtp-&gt;all_lazy &amp;&amp;
+23       rdtp-&gt;nonlazy_posted != rdtp-&gt;nonlazy_posted_snap) {
+24     rdtp-&gt;all_lazy = false;
+25     rdtp-&gt;nonlazy_posted_snap = rdtp-&gt;nonlazy_posted;
+26     invoke_rcu_core();
+27     return;
+28   }
+29   if (rdtp-&gt;last_accelerate == jiffies)
+30     return;
+31   rdtp-&gt;last_accelerate = jiffies;
+32   for_each_rcu_flavor(rsp) {
+33     rdp = this_cpu_ptr(rsp-&gt;rda);
+34     if (rcu_segcblist_pend_cbs(&amp;rdp-&gt;cblist))
+35       continue;
+36     rnp = rdp-&gt;mynode;
+37     raw_spin_lock_rcu_node(rnp);
+38     needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
+39     raw_spin_unlock_rcu_node(rnp);
+40     if (needwake)
+41       rcu_gp_kthread_wake(rsp);
+42   }
+43 }
+</pre>
+
+<p>But the only part of <tt>rcu_prepare_for_idle()</tt> that really
+matters for this discussion are lines&nbsp;37&ndash;39.
+We will therefore abbreviate this function as follows:
+
+</p><p><img src="rcu_node-lock.svg" alt="rcu_node-lock.svg">
+
+<p>The box represents the <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>
+critical section, with the double line on top representing the additional
+<tt>smp_mb__after_unlock_lock()</tt>.
+
+<h3><a name="Tree RCU Grace Period Memory Ordering Components">
+Tree RCU Grace Period Memory Ordering Components</a></h3>
+
+<p>Tree RCU's grace-period memory-ordering guarantee is provided by
+a number of RCU components:
+
+<ol>
+<li>	<a href="#Callback Registry">Callback Registry</a>
+<li>	<a href="#Grace-Period Initialization">Grace-Period Initialization</a>
+<li>	<a href="#Self-Reported Quiescent States">
+	Self-Reported Quiescent States</a>
+<li>	<a href="#Dynamic Tick Interface">Dynamic Tick Interface</a>
+<li>	<a href="#CPU-Hotplug Interface">CPU-Hotplug Interface</a>
+<li>	<a href="Forcing Quiescent States">Forcing Quiescent States</a>
+<li>	<a href="Grace-Period Cleanup">Grace-Period Cleanup</a>
+<li>	<a href="Callback Invocation">Callback Invocation</a>
+</ol>
+
+<p>Each of the following section looks at the corresponding component
+in detail.
+
+<h4><a name="Callback Registry">Callback Registry</a></h4>
+
+<p>If RCU's grace-period guarantee is to mean anything at all, any
+access that happens before a given invocation of <tt>call_rcu()</tt>
+must also happen before the corresponding grace period.
+The implementation of this portion of RCU's grace period guarantee
+is shown in the following figure:
+
+</p><p><img src="TreeRCU-callback-registry.svg" alt="TreeRCU-callback-registry.svg">
+
+<p>Because <tt>call_rcu()</tt> normally acts only on CPU-local state,
+it provides no ordering guarantees, either for itself or for
+phase one of the update (which again will usually be removal of
+an element from an RCU-protected data structure).
+It simply enqueues the <tt>rcu_head</tt> structure on a per-CPU list,
+which cannot become associated with a grace period until a later
+call to <tt>rcu_accelerate_cbs()</tt>, as shown in the diagram above.
+
+<p>One set of code paths shown on the left invokes
+<tt>rcu_accelerate_cbs()</tt> via
+<tt>note_gp_changes()</tt>, either directly from <tt>call_rcu()</tt> (if
+the current CPU is inundated with queued <tt>rcu_head</tt> structures)
+or more likely from an <tt>RCU_SOFTIRQ</tt> handler.
+Another code path in the middle is taken only in kernels built with
+<tt>CONFIG_RCU_FAST_NO_HZ=y</tt>, which invokes
+<tt>rcu_accelerate_cbs()</tt> via <tt>rcu_prepare_for_idle()</tt>.
+The final code path on the right is taken only in kernels built with
+<tt>CONFIG_HOTPLUG_CPU=y</tt>, which invokes
+<tt>rcu_accelerate_cbs()</tt> via
+<tt>rcu_advance_cbs()</tt>, <tt>rcu_migrate_callbacks</tt>,
+<tt>rcutree_migrate_callbacks()</tt>, and <tt>takedown_cpu()</tt>,
+which in turn is invoked on a surviving CPU after the outgoing
+CPU has been completely offlined.
+
+<p>There are a few other code paths within grace-period processing
+that opportunistically invoke <tt>rcu_accelerate_cbs()</tt>.
+However, either way, all of the CPU's recently queued <tt>rcu_head</tt>
+structures are associated with a future grace-period number under
+the protection of the CPU's lead <tt>rcu_node</tt> structure's
+<tt>-&gt;lock</tt>.
+In all cases, there is full ordering against any prior critical section
+for that same <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>, and
+also full ordering against any of the current task's or CPU's prior critical
+sections for any <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>.
+
+<p>The next section will show how this ordering ensures that any
+accesses prior to the <tt>call_rcu()</tt> (particularly including phase
+one of the update)
+happen before the start of the corresponding grace period.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But what about <tt>synchronize_rcu()</tt>?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	The <tt>synchronize_rcu()</tt> passes <tt>call_rcu()</tt>
+	to <tt>wait_rcu_gp()</tt>, which invokes it.
+	So either way, it eventually comes down to <tt>call_rcu()</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h4><a name="Grace-Period Initialization">Grace-Period Initialization</a></h4>
+
+<p>Grace-period initialization is carried out by
+the grace-period kernel thread, which makes several passes over the
+<tt>rcu_node</tt> tree within the <tt>rcu_gp_init()</tt> function.
+This means that showing the full flow of ordering through the
+grace-period computation will require duplicating this tree.
+If you find this confusing, please note that the state of the
+<tt>rcu_node</tt> changes over time, just like Heraclitus's river.
+However, to keep the <tt>rcu_node</tt> river tractable, the
+grace-period kernel thread's traversals are presented in multiple
+parts, starting in this section with the various phases of
+grace-period initialization.
+
+<p>The first ordering-related grace-period initialization action is to
+advance the <tt>rcu_state</tt> structure's <tt>-&gt;gp_seq</tt>
+grace-period-number counter, as shown below:
+
+</p><p><img src="TreeRCU-gp-init-1.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
+
+<p>The actual increment is carried out using <tt>smp_store_release()</tt>,
+which helps reject false-positive RCU CPU stall detection.
+Note that only the root <tt>rcu_node</tt> structure is touched.
+
+<p>The first pass through the <tt>rcu_node</tt> tree updates bitmasks
+based on CPUs having come online or gone offline since the start of
+the previous grace period.
+In the common case where the number of online CPUs for this <tt>rcu_node</tt>
+structure has not transitioned to or from zero,
+this pass will scan only the leaf <tt>rcu_node</tt> structures.
+However, if the number of online CPUs for a given leaf <tt>rcu_node</tt>
+structure has transitioned from zero,
+<tt>rcu_init_new_rnp()</tt> will be invoked for the first incoming CPU.
+Similarly, if the number of online CPUs for a given leaf <tt>rcu_node</tt>
+structure has transitioned to zero,
+<tt>rcu_cleanup_dead_rnp()</tt> will be invoked for the last outgoing CPU.
+The diagram below shows the path of ordering if the leftmost
+<tt>rcu_node</tt> structure onlines its first CPU and if the next
+<tt>rcu_node</tt> structure has no online CPUs
+(or, alternatively if the leftmost <tt>rcu_node</tt> structure offlines
+its last CPU and if the next <tt>rcu_node</tt> structure has no online CPUs).
+
+</p><p><img src="TreeRCU-gp-init-2.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
+
+<p>The final <tt>rcu_gp_init()</tt> pass through the <tt>rcu_node</tt>
+tree traverses breadth-first, setting each <tt>rcu_node</tt> structure's
+<tt>-&gt;gp_seq</tt> field to the newly advanced value from the
+<tt>rcu_state</tt> structure, as shown in the following diagram.
+
+</p><p><img src="TreeRCU-gp-init-3.svg" alt="TreeRCU-gp-init-1.svg" width="75%">
+
+<p>This change will also cause each CPU's next call to
+<tt>__note_gp_changes()</tt>
+to notice that a new grace period has started, as described in the next
+section.
+But because the grace-period kthread started the grace period at the
+root (with the advancing of the <tt>rcu_state</tt> structure's
+<tt>-&gt;gp_seq</tt> field) before setting each leaf <tt>rcu_node</tt>
+structure's <tt>-&gt;gp_seq</tt> field, each CPU's observation of
+the start of the grace period will happen after the actual start
+of the grace period.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But what about the CPU that started the grace period?
+	Why wouldn't it see the start of the grace period right when
+	it started that grace period?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In some deep philosophical and overly anthromorphized
+	sense, yes, the CPU starting the grace period is immediately
+	aware of having done so.
+	However, if we instead assume that RCU is not self-aware,
+	then even the CPU starting the grace period does not really
+	become aware of the start of this grace period until its
+	first call to <tt>__note_gp_changes()</tt>.
+	On the other hand, this CPU potentially gets early notification
+	because it invokes <tt>__note_gp_changes()</tt> during its
+	last <tt>rcu_gp_init()</tt> pass through its leaf
+	<tt>rcu_node</tt> structure.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h4><a name="Self-Reported Quiescent States">
+Self-Reported Quiescent States</a></h4>
+
+<p>When all entities that might block the grace period have reported
+quiescent states (or as described in a later section, had quiescent
+states reported on their behalf), the grace period can end.
+Online non-idle CPUs report their own quiescent states, as shown
+in the following diagram:
+
+</p><p><img src="TreeRCU-qs.svg" alt="TreeRCU-qs.svg" width="75%">
+
+<p>This is for the last CPU to report a quiescent state, which signals
+the end of the grace period.
+Earlier quiescent states would push up the <tt>rcu_node</tt> tree
+only until they encountered an <tt>rcu_node</tt> structure that
+is waiting for additional quiescent states.
+However, ordering is nevertheless preserved because some later quiescent
+state will acquire that <tt>rcu_node</tt> structure's <tt>-&gt;lock</tt>.
+
+<p>Any number of events can lead up to a CPU invoking
+<tt>note_gp_changes</tt> (or alternatively, directly invoking
+<tt>__note_gp_changes()</tt>), at which point that CPU will notice
+the start of a new grace period while holding its leaf
+<tt>rcu_node</tt> lock.
+Therefore, all execution shown in this diagram happens after the
+start of the grace period.
+In addition, this CPU will consider any RCU read-side critical
+section that started before the invocation of <tt>__note_gp_changes()</tt>
+to have started before the grace period, and thus a critical
+section that the grace period must wait on.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But a RCU read-side critical section might have started
+	after the beginning of the grace period
+	(the advancing of <tt>-&gt;gp_seq</tt> from earlier), so why should
+	the grace period wait on such a critical section?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	It is indeed not necessary for the grace period to wait on such
+	a critical section.
+	However, it is permissible to wait on it.
+	And it is furthermore important to wait on it, as this
+	lazy approach is far more scalable than a &ldquo;big bang&rdquo;
+	all-at-once grace-period start could possibly be.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>If the CPU does a context switch, a quiescent state will be
+noted by <tt>rcu_node_context_switch()</tt> on the left.
+On the other hand, if the CPU takes a scheduler-clock interrupt
+while executing in usermode, a quiescent state will be noted by
+<tt>rcu_check_callbacks()</tt> on the right.
+Either way, the passage through a quiescent state will be noted
+in a per-CPU variable.
+
+<p>The next time an <tt>RCU_SOFTIRQ</tt> handler executes on
+this CPU (for example, after the next scheduler-clock
+interrupt), <tt>__rcu_process_callbacks()</tt> will invoke
+<tt>rcu_check_quiescent_state()</tt>, which will notice the
+recorded quiescent state, and invoke
+<tt>rcu_report_qs_rdp()</tt>.
+If <tt>rcu_report_qs_rdp()</tt> verifies that the quiescent state
+really does apply to the current grace period, it invokes
+<tt>rcu_report_rnp()</tt> which traverses up the <tt>rcu_node</tt>
+tree as shown at the bottom of the diagram, clearing bits from
+each <tt>rcu_node</tt> structure's <tt>-&gt;qsmask</tt> field,
+and propagating up the tree when the result is zero.
+
+<p>Note that traversal passes upwards out of a given <tt>rcu_node</tt>
+structure only if the current CPU is reporting the last quiescent
+state for the subtree headed by that <tt>rcu_node</tt> structure.
+A key point is that if a CPU's traversal stops at a given <tt>rcu_node</tt>
+structure, then there will be a later traversal by another CPU
+(or perhaps the same one) that proceeds upwards
+from that point, and the <tt>rcu_node</tt> <tt>-&gt;lock</tt>
+guarantees that the first CPU's quiescent state happens before the
+remainder of the second CPU's traversal.
+Applying this line of thought repeatedly shows that all CPUs'
+quiescent states happen before the last CPU traverses through
+the root <tt>rcu_node</tt> structure, the &ldquo;last CPU&rdquo;
+being the one that clears the last bit in the root <tt>rcu_node</tt>
+structure's <tt>-&gt;qsmask</tt> field.
+
+<h4><a name="Dynamic Tick Interface">Dynamic Tick Interface</a></h4>
+
+<p>Due to energy-efficiency considerations, RCU is forbidden from
+disturbing idle CPUs.
+CPUs are therefore required to notify RCU when entering or leaving idle
+state, which they do via fully ordered value-returning atomic operations
+on a per-CPU variable.
+The ordering effects are as shown below:
+
+</p><p><img src="TreeRCU-dyntick.svg" alt="TreeRCU-dyntick.svg" width="50%">
+
+<p>The RCU grace-period kernel thread samples the per-CPU idleness
+variable while holding the corresponding CPU's leaf <tt>rcu_node</tt>
+structure's <tt>-&gt;lock</tt>.
+This means that any RCU read-side critical sections that precede the
+idle period (the oval near the top of the diagram above) will happen
+before the end of the current grace period.
+Similarly, the beginning of the current grace period will happen before
+any RCU read-side critical sections that follow the
+idle period (the oval near the bottom of the diagram above).
+
+<p>Plumbing this into the full grace-period execution is described
+<a href="#Forcing Quiescent States">below</a>.
+
+<h4><a name="CPU-Hotplug Interface">CPU-Hotplug Interface</a></h4>
+
+<p>RCU is also forbidden from disturbing offline CPUs, which might well
+be powered off and removed from the system completely.
+CPUs are therefore required to notify RCU of their comings and goings
+as part of the corresponding CPU hotplug operations.
+The ordering effects are shown below:
+
+</p><p><img src="TreeRCU-hotplug.svg" alt="TreeRCU-hotplug.svg" width="50%">
+
+<p>Because CPU hotplug operations are much less frequent than idle transitions,
+they are heavier weight, and thus acquire the CPU's leaf <tt>rcu_node</tt>
+structure's <tt>-&gt;lock</tt> and update this structure's
+<tt>-&gt;qsmaskinitnext</tt>.
+The RCU grace-period kernel thread samples this mask to detect CPUs
+having gone offline since the beginning of this grace period.
+
+<p>Plumbing this into the full grace-period execution is described
+<a href="#Forcing Quiescent States">below</a>.
+
+<h4><a name="Forcing Quiescent States">Forcing Quiescent States</a></h4>
+
+<p>As noted above, idle and offline CPUs cannot report their own
+quiescent states, and therefore the grace-period kernel thread
+must do the reporting on their behalf.
+This process is called &ldquo;forcing quiescent states&rdquo;, it is
+repeated every few jiffies, and its ordering effects are shown below:
+
+</p><p><img src="TreeRCU-gp-fqs.svg" alt="TreeRCU-gp-fqs.svg" width="100%">
+
+<p>Each pass of quiescent state forcing is guaranteed to traverse the
+leaf <tt>rcu_node</tt> structures, and if there are no new quiescent
+states due to recently idled and/or offlined CPUs, then only the
+leaves are traversed.
+However, if there is a newly offlined CPU as illustrated on the left
+or a newly idled CPU as illustrated on the right, the corresponding
+quiescent state will be driven up towards the root.
+As with self-reported quiescent states, the upwards driving stops
+once it reaches an <tt>rcu_node</tt> structure that has quiescent
+states outstanding from other CPUs.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	The leftmost drive to root stopped before it reached
+	the root <tt>rcu_node</tt> structure, which means that
+	there are still CPUs subordinate to that structure on
+	which the current grace period is waiting.
+	Given that, how is it possible that the rightmost drive
+	to root ended the grace period?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Good analysis!
+	It is in fact impossible in the absence of bugs in RCU.
+	But this diagram is complex enough as it is, so simplicity
+	overrode accuracy.
+	You can think of it as poetic license, or you can think of
+	it as misdirection that is resolved in the
+	<a href="#Putting It All Together">stitched-together diagram</a>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h4><a name="Grace-Period Cleanup">Grace-Period Cleanup</a></h4>
+
+<p>Grace-period cleanup first scans the <tt>rcu_node</tt> tree
+breadth-first advancing all the <tt>-&gt;gp_seq</tt> fields, then it
+advances the <tt>rcu_state</tt> structure's <tt>-&gt;gp_seq</tt> field.
+The ordering effects are shown below:
+
+</p><p><img src="TreeRCU-gp-cleanup.svg" alt="TreeRCU-gp-cleanup.svg" width="75%">
+
+<p>As indicated by the oval at the bottom of the diagram, once
+grace-period cleanup is complete, the next grace period can begin.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But when precisely does the grace period end?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	There is no useful single point at which the grace period
+	can be said to end.
+	The earliest reasonable candidate is as soon as the last
+	CPU has reported its quiescent state, but it may be some
+	milliseconds before RCU becomes aware of this.
+	The latest reasonable candidate is once the <tt>rcu_state</tt>
+	structure's <tt>-&gt;gp_seq</tt> field has been updated,
+	but it is quite possible that some CPUs have already completed
+	phase two of their updates by that time.
+	In short, if you are going to work with RCU, you need to
+	learn to embrace uncertainty.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+
+<h4><a name="Callback Invocation">Callback Invocation</a></h4>
+
+<p>Once a given CPU's leaf <tt>rcu_node</tt> structure's
+<tt>-&gt;gp_seq</tt> field has been updated, that CPU can begin
+invoking its RCU callbacks that were waiting for this grace period
+to end.
+These callbacks are identified by <tt>rcu_advance_cbs()</tt>,
+which is usually invoked by <tt>__note_gp_changes()</tt>.
+As shown in the diagram below, this invocation can be triggered by
+the scheduling-clock interrupt (<tt>rcu_check_callbacks()</tt> on
+the left) or by idle entry (<tt>rcu_cleanup_after_idle()</tt> on
+the right, but only for kernels build with
+<tt>CONFIG_RCU_FAST_NO_HZ=y</tt>).
+Either way, <tt>RCU_SOFTIRQ</tt> is raised, which results in
+<tt>rcu_do_batch()</tt> invoking the callbacks, which in turn
+allows those callbacks to carry out (either directly or indirectly
+via wakeup) the needed phase-two processing for each update.
+
+</p><p><img src="TreeRCU-callback-invocation.svg" alt="TreeRCU-callback-invocation.svg" width="60%">
+
+<p>Please note that callback invocation can also be prompted by any
+number of corner-case code paths, for example, when a CPU notes that
+it has excessive numbers of callbacks queued.
+In all cases, the CPU acquires its leaf <tt>rcu_node</tt> structure's
+<tt>-&gt;lock</tt> before invoking callbacks, which preserves the
+required ordering against the newly completed grace period.
+
+<p>However, if the callback function communicates to other CPUs,
+for example, doing a wakeup, then it is that function's responsibility
+to maintain ordering.
+For example, if the callback function wakes up a task that runs on
+some other CPU, proper ordering must in place in both the callback
+function and the task being awakened.
+To see why this is important, consider the top half of the
+<a href="#Grace-Period Cleanup">grace-period cleanup</a> diagram.
+The callback might be running on a CPU corresponding to the leftmost
+leaf <tt>rcu_node</tt> structure, and awaken a task that is to run on
+a CPU corresponding to the rightmost leaf <tt>rcu_node</tt> structure,
+and the grace-period kernel thread might not yet have reached the
+rightmost leaf.
+In this case, the grace period's memory ordering might not yet have
+reached that CPU, so again the callback function and the awakened
+task must supply proper ordering.
+
+<h3><a name="Putting It All Together">Putting It All Together</a></h3>
+
+<p>A stitched-together diagram is
+<a href="Tree-RCU-Diagram.html">here</a>.
+
+<h3><a name="Legal Statement">
+Legal Statement</a></h3>
+
+<p>This work represents the view of the author and does not necessarily
+represent the view of IBM.
+
+</p><p>Linux is a registered trademark of Linus Torvalds.
+
+</p><p>Other company, product, and service names may be trademarks or
+service marks of others.
+
+</body></html>
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diff --git a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg
new file mode 100644
index 0000000..48cd162
--- /dev/null
+++ b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+       sodipodi:linespacing="125%"><tspan
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+         id="tspan4443"
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
+         id="tspan4451"
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
+         id="tspan4455"
+         x="396.10254"
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
+         id="tspan4459"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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+         y="251.53981">rcu_read_lock();</tspan></text>
+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
+         id="tspan4431-9-4"
+         x="396.10254"
+         y="353.91556">rcu_read_lock();</tspan></text>
+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
+         id="tspan4431-9-4-4"
+         x="396.10254"
+         y="587.40289">rcu_read_unlock();</tspan></text>
+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
+         id="tspan4431-9-4-4-6"
+         x="111.75929"
+         y="501.15311">rcu_read_unlock();</tspan></text>
+    <path
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+    <path
+       style="fill:none;stroke:#000000;stroke-width:1;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:4;stroke-opacity:1;stroke-dasharray:none;stroke-dashoffset:0"
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+    <path
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+       inkscape:connector-curvature="0"
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+       id="g4433-6"
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+         sodipodi:linespacing="125%"
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+         style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+           y="-261.66608"
+           x="440.12299"
+           id="tspan2995-1"
+           sodipodi:role="line">synchronize_rcu()</tspan></text>
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+         id="g4417-1"
+         transform="matrix(0,1,-1,0,730.90257,222.4928)">
+        <path
+           style="fill:none;stroke:#000000;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;marker-start:url(#Arrow2Lstart);marker-end:url(#Arrow2Lend)"
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+        <path
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+           inkscape:connector-curvature="0"
+           sodipodi:nodetypes="cc" />
+        <path
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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+         y="387.6217">r3 = READ_ONCE(d);</tspan></text>
+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through RCU's Requirements [LWN.net]</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">
+
+<h1>A Tour Through RCU's Requirements</h1>
+
+<p>Copyright IBM Corporation, 2015</p>
+<p>Author: Paul E.&nbsp;McKenney</p>
+<p><i>The initial version of this document appeared in the
+<a href="https://lwn.net/">LWN</a> articles
+<a href="https://lwn.net/Articles/652156/">here</a>,
+<a href="https://lwn.net/Articles/652677/">here</a>, and
+<a href="https://lwn.net/Articles/653326/">here</a>.</i></p>
+
+<h2>Introduction</h2>
+
+<p>
+Read-copy update (RCU) is a synchronization mechanism that is often
+used as a replacement for reader-writer locking.
+RCU is unusual in that updaters do not block readers,
+which means that RCU's read-side primitives can be exceedingly fast
+and scalable.
+In addition, updaters can make useful forward progress concurrently
+with readers.
+However, all this concurrency between RCU readers and updaters does raise
+the question of exactly what RCU readers are doing, which in turn
+raises the question of exactly what RCU's requirements are.
+
+<p>
+This document therefore summarizes RCU's requirements, and can be thought
+of as an informal, high-level specification for RCU.
+It is important to understand that RCU's specification is primarily
+empirical in nature;
+in fact, I learned about many of these requirements the hard way.
+This situation might cause some consternation, however, not only
+has this learning process been a lot of fun, but it has also been
+a great privilege to work with so many people willing to apply
+technologies in interesting new ways.
+
+<p>
+All that aside, here are the categories of currently known RCU requirements:
+</p>
+
+<ol>
+<li>	<a href="#Fundamental Requirements">
+	Fundamental Requirements</a>
+<li>	<a href="#Fundamental Non-Requirements">Fundamental Non-Requirements</a>
+<li>	<a href="#Parallelism Facts of Life">
+	Parallelism Facts of Life</a>
+<li>	<a href="#Quality-of-Implementation Requirements">
+	Quality-of-Implementation Requirements</a>
+<li>	<a href="#Linux Kernel Complications">
+	Linux Kernel Complications</a>
+<li>	<a href="#Software-Engineering Requirements">
+	Software-Engineering Requirements</a>
+<li>	<a href="#Other RCU Flavors">
+	Other RCU Flavors</a>
+<li>	<a href="#Possible Future Changes">
+	Possible Future Changes</a>
+</ol>
+
+<p>
+This is followed by a <a href="#Summary">summary</a>,
+however, the answers to each quick quiz immediately follows the quiz.
+Select the big white space with your mouse to see the answer.
+
+<h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2>
+
+<p>
+RCU's fundamental requirements are the closest thing RCU has to hard
+mathematical requirements.
+These are:
+
+<ol>
+<li>	<a href="#Grace-Period Guarantee">
+	Grace-Period Guarantee</a>
+<li>	<a href="#Publish-Subscribe Guarantee">
+	Publish-Subscribe Guarantee</a>
+<li>	<a href="#Memory-Barrier Guarantees">
+	Memory-Barrier Guarantees</a>
+<li>	<a href="#RCU Primitives Guaranteed to Execute Unconditionally">
+	RCU Primitives Guaranteed to Execute Unconditionally</a>
+<li>	<a href="#Guaranteed Read-to-Write Upgrade">
+	Guaranteed Read-to-Write Upgrade</a>
+</ol>
+
+<h3><a name="Grace-Period Guarantee">Grace-Period Guarantee</a></h3>
+
+<p>
+RCU's grace-period guarantee is unusual in being premeditated:
+Jack Slingwine and I had this guarantee firmly in mind when we started
+work on RCU (then called &ldquo;rclock&rdquo;) in the early 1990s.
+That said, the past two decades of experience with RCU have produced
+a much more detailed understanding of this guarantee.
+
+<p>
+RCU's grace-period guarantee allows updaters to wait for the completion
+of all pre-existing RCU read-side critical sections.
+An RCU read-side critical section
+begins with the marker <tt>rcu_read_lock()</tt> and ends with
+the marker <tt>rcu_read_unlock()</tt>.
+These markers may be nested, and RCU treats a nested set as one
+big RCU read-side critical section.
+Production-quality implementations of <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> are extremely lightweight, and in
+fact have exactly zero overhead in Linux kernels built for production
+use with <tt>CONFIG_PREEMPT=n</tt>.
+
+<p>
+This guarantee allows ordering to be enforced with extremely low
+overhead to readers, for example:
+
+<blockquote>
+<pre>
+ 1 int x, y;
+ 2
+ 3 void thread0(void)
+ 4 {
+ 5   rcu_read_lock();
+ 6   r1 = READ_ONCE(x);
+ 7   r2 = READ_ONCE(y);
+ 8   rcu_read_unlock();
+ 9 }
+10
+11 void thread1(void)
+12 {
+13   WRITE_ONCE(x, 1);
+14   synchronize_rcu();
+15   WRITE_ONCE(y, 1);
+16 }
+</pre>
+</blockquote>
+
+<p>
+Because the <tt>synchronize_rcu()</tt> on line&nbsp;14 waits for
+all pre-existing readers, any instance of <tt>thread0()</tt> that
+loads a value of zero from <tt>x</tt> must complete before
+<tt>thread1()</tt> stores to <tt>y</tt>, so that instance must
+also load a value of zero from <tt>y</tt>.
+Similarly, any instance of <tt>thread0()</tt> that loads a value of
+one from <tt>y</tt> must have started after the
+<tt>synchronize_rcu()</tt> started, and must therefore also load
+a value of one from <tt>x</tt>.
+Therefore, the outcome:
+<blockquote>
+<pre>
+(r1 == 0 &amp;&amp; r2 == 1)
+</pre>
+</blockquote>
+cannot happen.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Wait a minute!
+	You said that updaters can make useful forward progress concurrently
+	with readers, but pre-existing readers will block
+	<tt>synchronize_rcu()</tt>!!!
+	Just who are you trying to fool???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	First, if updaters do not wish to be blocked by readers, they can use
+	<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
+	be discussed later.
+	Second, even when using <tt>synchronize_rcu()</tt>, the other
+	update-side code does run concurrently with readers, whether
+	pre-existing or not.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+This scenario resembles one of the first uses of RCU in
+<a href="https://en.wikipedia.org/wiki/DYNIX">DYNIX/ptx</a>,
+which managed a distributed lock manager's transition into
+a state suitable for handling recovery from node failure,
+more or less as follows:
+
+<blockquote>
+<pre>
+ 1 #define STATE_NORMAL        0
+ 2 #define STATE_WANT_RECOVERY 1
+ 3 #define STATE_RECOVERING    2
+ 4 #define STATE_WANT_NORMAL   3
+ 5
+ 6 int state = STATE_NORMAL;
+ 7
+ 8 void do_something_dlm(void)
+ 9 {
+10   int state_snap;
+11
+12   rcu_read_lock();
+13   state_snap = READ_ONCE(state);
+14   if (state_snap == STATE_NORMAL)
+15     do_something();
+16   else
+17     do_something_carefully();
+18   rcu_read_unlock();
+19 }
+20
+21 void start_recovery(void)
+22 {
+23   WRITE_ONCE(state, STATE_WANT_RECOVERY);
+24   synchronize_rcu();
+25   WRITE_ONCE(state, STATE_RECOVERING);
+26   recovery();
+27   WRITE_ONCE(state, STATE_WANT_NORMAL);
+28   synchronize_rcu();
+29   WRITE_ONCE(state, STATE_NORMAL);
+30 }
+</pre>
+</blockquote>
+
+<p>
+The RCU read-side critical section in <tt>do_something_dlm()</tt>
+works with the <tt>synchronize_rcu()</tt> in <tt>start_recovery()</tt>
+to guarantee that <tt>do_something()</tt> never runs concurrently
+with <tt>recovery()</tt>, but with little or no synchronization
+overhead in <tt>do_something_dlm()</tt>.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Without that extra grace period, memory reordering could result in
+	<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
+	concurrently with the last bits of <tt>recovery()</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+In order to avoid fatal problems such as deadlocks,
+an RCU read-side critical section must not contain calls to
+<tt>synchronize_rcu()</tt>.
+Similarly, an RCU read-side critical section must not
+contain anything that waits, directly or indirectly, on completion of
+an invocation of <tt>synchronize_rcu()</tt>.
+
+<p>
+Although RCU's grace-period guarantee is useful in and of itself, with
+<a href="https://lwn.net/Articles/573497/">quite a few use cases</a>,
+it would be good to be able to use RCU to coordinate read-side
+access to linked data structures.
+For this, the grace-period guarantee is not sufficient, as can
+be seen in function <tt>add_gp_buggy()</tt> below.
+We will look at the reader's code later, but in the meantime, just think of
+the reader as locklessly picking up the <tt>gp</tt> pointer,
+and, if the value loaded is non-<tt>NULL</tt>, locklessly accessing the
+<tt>-&gt;a</tt> and <tt>-&gt;b</tt> fields.
+
+<blockquote>
+<pre>
+ 1 bool add_gp_buggy(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   p-&gt;a = a;
+12   p-&gt;b = a;
+13   gp = p; /* ORDERING BUG */
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+The problem is that both the compiler and weakly ordered CPUs are within
+their rights to reorder this code as follows:
+
+<blockquote>
+<pre>
+ 1 bool add_gp_buggy_optimized(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+<b>11   gp = p; /* ORDERING BUG */
+12   p-&gt;a = a;
+13   p-&gt;b = a;</b>
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+If an RCU reader fetches <tt>gp</tt> just after
+<tt>add_gp_buggy_optimized</tt> executes line&nbsp;11,
+it will see garbage in the <tt>-&gt;a</tt> and <tt>-&gt;b</tt>
+fields.
+And this is but one of many ways in which compiler and hardware optimizations
+could cause trouble.
+Therefore, we clearly need some way to prevent the compiler and the CPU from
+reordering in this manner, which brings us to the publish-subscribe
+guarantee discussed in the next section.
+
+<h3><a name="Publish-Subscribe Guarantee">Publish/Subscribe Guarantee</a></h3>
+
+<p>
+RCU's publish-subscribe guarantee allows data to be inserted
+into a linked data structure without disrupting RCU readers.
+The updater uses <tt>rcu_assign_pointer()</tt> to insert the
+new data, and readers use <tt>rcu_dereference()</tt> to
+access data, whether new or old.
+The following shows an example of insertion:
+
+<blockquote>
+<pre>
+ 1 bool add_gp(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   p-&gt;a = a;
+12   p-&gt;b = a;
+13   rcu_assign_pointer(gp, p);
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+The <tt>rcu_assign_pointer()</tt> on line&nbsp;13 is conceptually
+equivalent to a simple assignment statement, but also guarantees
+that its assignment will
+happen after the two assignments in lines&nbsp;11 and&nbsp;12,
+similar to the C11 <tt>memory_order_release</tt> store operation.
+It also prevents any number of &ldquo;interesting&rdquo; compiler
+optimizations, for example, the use of <tt>gp</tt> as a scratch
+location immediately preceding the assignment.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
+	two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
+	from being reordered.
+	Can't that also cause problems?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No, it cannot.
+	The readers cannot see either of these two fields until
+	the assignment to <tt>gp</tt>, by which time both fields are
+	fully initialized.
+	So reordering the assignments
+	to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
+	cause any problems.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+It is tempting to assume that the reader need not do anything special
+to control its accesses to the RCU-protected data,
+as shown in <tt>do_something_gp_buggy()</tt> below:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp_buggy(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   p = gp;  /* OPTIMIZATIONS GALORE!!! */
+ 5   if (p) {
+ 6     do_something(p-&gt;a, p-&gt;b);
+ 7     rcu_read_unlock();
+ 8     return true;
+ 9   }
+10   rcu_read_unlock();
+11   return false;
+12 }
+</pre>
+</blockquote>
+
+<p>
+However, this temptation must be resisted because there are a
+surprisingly large number of ways that the compiler
+(to say nothing of
+<a href="https://h71000.www7.hp.com/wizard/wiz_2637.html">DEC Alpha CPUs</a>)
+can trip this code up.
+For but one example, if the compiler were short of registers, it
+might choose to refetch from <tt>gp</tt> rather than keeping
+a separate copy in <tt>p</tt> as follows:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp_buggy_optimized(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   if (gp) { /* OPTIMIZATIONS GALORE!!! */
+<b> 5     do_something(gp-&gt;a, gp-&gt;b);</b>
+ 6     rcu_read_unlock();
+ 7     return true;
+ 8   }
+ 9   rcu_read_unlock();
+10   return false;
+11 }
+</pre>
+</blockquote>
+
+<p>
+If this function ran concurrently with a series of updates that
+replaced the current structure with a new one,
+the fetches of <tt>gp-&gt;a</tt>
+and <tt>gp-&gt;b</tt> might well come from two different structures,
+which could cause serious confusion.
+To prevent this (and much else besides), <tt>do_something_gp()</tt> uses
+<tt>rcu_dereference()</tt> to fetch from <tt>gp</tt>:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   p = rcu_dereference(gp);
+ 5   if (p) {
+ 6     do_something(p-&gt;a, p-&gt;b);
+ 7     rcu_read_unlock();
+ 8     return true;
+ 9   }
+10   rcu_read_unlock();
+11   return false;
+12 }
+</pre>
+</blockquote>
+
+<p>
+The <tt>rcu_dereference()</tt> uses volatile casts and (for DEC Alpha)
+memory barriers in the Linux kernel.
+Should a
+<a href="http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf">high-quality implementation of C11 <tt>memory_order_consume</tt> [PDF]</a>
+ever appear, then <tt>rcu_dereference()</tt> could be implemented
+as a <tt>memory_order_consume</tt> load.
+Regardless of the exact implementation, a pointer fetched by
+<tt>rcu_dereference()</tt> may not be used outside of the
+outermost RCU read-side critical section containing that
+<tt>rcu_dereference()</tt>, unless protection of
+the corresponding data element has been passed from RCU to some
+other synchronization mechanism, most commonly locking or
+<a href="https://www.kernel.org/doc/Documentation/RCU/rcuref.txt">reference counting</a>.
+
+<p>
+In short, updaters use <tt>rcu_assign_pointer()</tt> and readers
+use <tt>rcu_dereference()</tt>, and these two RCU API elements
+work together to ensure that readers have a consistent view of
+newly added data elements.
+
+<p>
+Of course, it is also necessary to remove elements from RCU-protected
+data structures, for example, using the following process:
+
+<ol>
+<li>	Remove the data element from the enclosing structure.
+<li>	Wait for all pre-existing RCU read-side critical sections
+	to complete (because only pre-existing readers can possibly have
+	a reference to the newly removed data element).
+<li>	At this point, only the updater has a reference to the
+	newly removed data element, so it can safely reclaim
+	the data element, for example, by passing it to <tt>kfree()</tt>.
+</ol>
+
+This process is implemented by <tt>remove_gp_synchronous()</tt>:
+
+<blockquote>
+<pre>
+ 1 bool remove_gp_synchronous(void)
+ 2 {
+ 3   struct foo *p;
+ 4
+ 5   spin_lock(&amp;gp_lock);
+ 6   p = rcu_access_pointer(gp);
+ 7   if (!p) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   rcu_assign_pointer(gp, NULL);
+12   spin_unlock(&amp;gp_lock);
+13   synchronize_rcu();
+14   kfree(p);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+This function is straightforward, with line&nbsp;13 waiting for a grace
+period before line&nbsp;14 frees the old data element.
+This waiting ensures that readers will reach line&nbsp;7 of
+<tt>do_something_gp()</tt> before the data element referenced by
+<tt>p</tt> is freed.
+The <tt>rcu_access_pointer()</tt> on line&nbsp;6 is similar to
+<tt>rcu_dereference()</tt>, except that:
+
+<ol>
+<li>	The value returned by <tt>rcu_access_pointer()</tt>
+	cannot be dereferenced.
+	If you want to access the value pointed to as well as
+	the pointer itself, use <tt>rcu_dereference()</tt>
+	instead of <tt>rcu_access_pointer()</tt>.
+<li>	The call to <tt>rcu_access_pointer()</tt> need not be
+	protected.
+	In contrast, <tt>rcu_dereference()</tt> must either be
+	within an RCU read-side critical section or in a code
+	segment where the pointer cannot change, for example, in
+	code protected by the corresponding update-side lock.
+</ol>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Without the <tt>rcu_dereference()</tt> or the
+	<tt>rcu_access_pointer()</tt>, what destructive optimizations
+	might the compiler make use of?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Let's start with what happens to <tt>do_something_gp()</tt>
+	if it fails to use <tt>rcu_dereference()</tt>.
+	It could reuse a value formerly fetched from this same pointer.
+	It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
+	manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
+	mash-up of two distinct pointer values.
+	It might even use value-speculation optimizations, where it makes
+	a wrong guess, but by the time it gets around to checking the
+	value, an update has changed the pointer to match the wrong guess.
+	Too bad about any dereferences that returned pre-initialization garbage
+	in the meantime!
+	</font>
+
+	<p><font color="ffffff">
+	For <tt>remove_gp_synchronous()</tt>, as long as all modifications
+	to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
+	the above optimizations are harmless.
+	However, <tt>sparse</tt> will complain if you
+	define <tt>gp</tt> with <tt>__rcu</tt> and then
+	access it without using
+	either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+In short, RCU's publish-subscribe guarantee is provided by the combination
+of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>.
+This guarantee allows data elements to be safely added to RCU-protected
+linked data structures without disrupting RCU readers.
+This guarantee can be used in combination with the grace-period
+guarantee to also allow data elements to be removed from RCU-protected
+linked data structures, again without disrupting RCU readers.
+
+<p>
+This guarantee was only partially premeditated.
+DYNIX/ptx used an explicit memory barrier for publication, but had nothing
+resembling <tt>rcu_dereference()</tt> for subscription, nor did it
+have anything resembling the <tt>smp_read_barrier_depends()</tt>
+that was later subsumed into <tt>rcu_dereference()</tt> and later
+still into <tt>READ_ONCE()</tt>.
+The need for these operations made itself known quite suddenly at a
+late-1990s meeting with the DEC Alpha architects, back in the days when
+DEC was still a free-standing company.
+It took the Alpha architects a good hour to convince me that any sort
+of barrier would ever be needed, and it then took me a good <i>two</i> hours
+to convince them that their documentation did not make this point clear.
+More recent work with the C and C++ standards committees have provided
+much education on tricks and traps from the compiler.
+In short, compilers were much less tricky in the early 1990s, but in
+2015, don't even think about omitting <tt>rcu_dereference()</tt>!
+
+<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3>
+
+<p>
+The previous section's simple linked-data-structure scenario clearly
+demonstrates the need for RCU's stringent memory-ordering guarantees on
+systems with more than one CPU:
+
+<ol>
+<li>	Each CPU that has an RCU read-side critical section that
+	begins before <tt>synchronize_rcu()</tt> starts is
+	guaranteed to execute a full memory barrier between the time
+	that the RCU read-side critical section ends and the time that
+	<tt>synchronize_rcu()</tt> returns.
+	Without this guarantee, a pre-existing RCU read-side critical section
+	might hold a reference to the newly removed <tt>struct foo</tt>
+	after the <tt>kfree()</tt> on line&nbsp;14 of
+	<tt>remove_gp_synchronous()</tt>.
+<li>	Each CPU that has an RCU read-side critical section that ends
+	after <tt>synchronize_rcu()</tt> returns is guaranteed
+	to execute a full memory barrier between the time that
+	<tt>synchronize_rcu()</tt> begins and the time that the RCU
+	read-side critical section begins.
+	Without this guarantee, a later RCU read-side critical section
+	running after the <tt>kfree()</tt> on line&nbsp;14 of
+	<tt>remove_gp_synchronous()</tt> might
+	later run <tt>do_something_gp()</tt> and find the
+	newly deleted <tt>struct foo</tt>.
+<li>	If the task invoking <tt>synchronize_rcu()</tt> remains
+	on a given CPU, then that CPU is guaranteed to execute a full
+	memory barrier sometime during the execution of
+	<tt>synchronize_rcu()</tt>.
+	This guarantee ensures that the <tt>kfree()</tt> on
+	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
+	execute after the removal on line&nbsp;11.
+<li>	If the task invoking <tt>synchronize_rcu()</tt> migrates
+	among a group of CPUs during that invocation, then each of the
+	CPUs in that group is guaranteed to execute a full memory barrier
+	sometime during the execution of <tt>synchronize_rcu()</tt>.
+	This guarantee also ensures that the <tt>kfree()</tt> on
+	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
+	execute after the removal on
+	line&nbsp;11, but also in the case where the thread executing the
+	<tt>synchronize_rcu()</tt> migrates in the meantime.
+</ol>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Given that multiple CPUs can start RCU read-side critical sections
+	at any time without any ordering whatsoever, how can RCU possibly
+	tell whether or not a given RCU read-side critical section starts
+	before a given instance of <tt>synchronize_rcu()</tt>?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	If RCU cannot tell whether or not a given
+	RCU read-side critical section starts before a
+	given instance of <tt>synchronize_rcu()</tt>,
+	then it must assume that the RCU read-side critical section
+	started first.
+	In other words, a given instance of <tt>synchronize_rcu()</tt>
+	can avoid waiting on a given RCU read-side critical section only
+	if it can prove that <tt>synchronize_rcu()</tt> started first.
+	</font>
+
+	<p><font color="ffffff">
+	A related question is &ldquo;When <tt>rcu_read_lock()</tt>
+	doesn't generate any code, why does it matter how it relates
+	to a grace period?&rdquo;
+	The answer is that it is not the relationship of
+	<tt>rcu_read_lock()</tt> itself that is important, but rather
+	the relationship of the code within the enclosed RCU read-side
+	critical section to the code preceding and following the
+	grace period.
+	If we take this viewpoint, then a given RCU read-side critical
+	section begins before a given grace period when some access
+	preceding the grace period observes the effect of some access
+	within the critical section, in which case none of the accesses
+	within the critical section may observe the effects of any
+	access following the grace period.
+	</font>
+
+	<p><font color="ffffff">
+	As of late 2016, mathematical models of RCU take this
+	viewpoint, for example, see slides&nbsp;62 and&nbsp;63
+	of the
+	<a href="http://www2.rdrop.com/users/paulmck/scalability/paper/LinuxMM.2016.10.04c.LCE.pdf">2016 LinuxCon EU</a>
+	presentation.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	The first and second guarantees require unbelievably strict ordering!
+	Are all these memory barriers <i> really</i> required?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Yes, they really are required.
+	To see why the first guarantee is required, consider the following
+	sequence of events:
+	</font>
+
+	<ol>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>rcu_read_lock()</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>q = rcu_dereference(gp);
+		/* Very likely to return p. */</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>list_del_rcu(p);</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>synchronize_rcu()</tt> starts.
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>do_something_with(q-&gt;a);
+		/* No smp_mb(), so might happen after kfree(). */</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>rcu_read_unlock()</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>synchronize_rcu()</tt> returns.
+		</font>
+	<li>	<font color="ffffff">
+		CPU 0: <tt>kfree(p);</tt>
+		</font>
+	</ol>
+
+	<p><font color="ffffff">
+	Therefore, there absolutely must be a full memory barrier between the
+	end of the RCU read-side critical section and the end of the
+	grace period.
+	</font>
+
+	<p><font color="ffffff">
+	The sequence of events demonstrating the necessity of the second rule
+	is roughly similar:
+	</font>
+
+	<ol>
+	<li>	<font color="ffffff">CPU 0: <tt>list_del_rcu(p);</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> starts.
+		</font>
+	<li>	<font color="ffffff">CPU 1: <tt>rcu_read_lock()</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 1: <tt>q = rcu_dereference(gp);
+		/* Might return p if no memory barrier. */</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 0: <tt>synchronize_rcu()</tt> returns.
+		</font>
+	<li>	<font color="ffffff">CPU 0: <tt>kfree(p);</tt>
+		</font>
+	<li>	<font color="ffffff">
+		CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
+		</font>
+	<li>	<font color="ffffff">CPU 1: <tt>rcu_read_unlock()</tt>
+		</font>
+	</ol>
+
+	<p><font color="ffffff">
+	And similarly, without a memory barrier between the beginning of the
+	grace period and the beginning of the RCU read-side critical section,
+	CPU&nbsp;1 might end up accessing the freelist.
+	</font>
+
+	<p><font color="ffffff">
+	The &ldquo;as if&rdquo; rule of course applies, so that any
+	implementation that acts as if the appropriate memory barriers
+	were in place is a correct implementation.
+	That said, it is much easier to fool yourself into believing
+	that you have adhered to the as-if rule than it is to actually
+	adhere to it!
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+	generate absolutely no code in some kernel builds.
+	This means that the compiler might arbitrarily rearrange consecutive
+	RCU read-side critical sections.
+	Given such rearrangement, if a given RCU read-side critical section
+	is done, how can you be sure that all prior RCU read-side critical
+	sections are done?
+	Won't the compiler rearrangements make that impossible to determine?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In cases where <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+	generate absolutely no code, RCU infers quiescent states only at
+	special locations, for example, within the scheduler.
+	Because calls to <tt>schedule()</tt> had better prevent calling-code
+	accesses to shared variables from being rearranged across the call to
+	<tt>schedule()</tt>, if RCU detects the end of a given RCU read-side
+	critical section, it will necessarily detect the end of all prior
+	RCU read-side critical sections, no matter how aggressively the
+	compiler scrambles the code.
+	</font>
+
+	<p><font color="ffffff">
+	Again, this all assumes that the compiler cannot scramble code across
+	calls to the scheduler, out of interrupt handlers, into the idle loop,
+	into user-mode code, and so on.
+	But if your kernel build allows that sort of scrambling, you have broken
+	far more than just RCU!
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+Note that these memory-barrier requirements do not replace the fundamental
+RCU requirement that a grace period wait for all pre-existing readers.
+On the contrary, the memory barriers called out in this section must operate in
+such a way as to <i>enforce</i> this fundamental requirement.
+Of course, different implementations enforce this requirement in different
+ways, but enforce it they must.
+
+<h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3>
+
+<p>
+The common-case RCU primitives are unconditional.
+They are invoked, they do their job, and they return, with no possibility
+of error, and no need to retry.
+This is a key RCU design philosophy.
+
+<p>
+However, this philosophy is pragmatic rather than pigheaded.
+If someone comes up with a good justification for a particular conditional
+RCU primitive, it might well be implemented and added.
+After all, this guarantee was reverse-engineered, not premeditated.
+The unconditional nature of the RCU primitives was initially an
+accident of implementation, and later experience with synchronization
+primitives with conditional primitives caused me to elevate this
+accident to a guarantee.
+Therefore, the justification for adding a conditional primitive to
+RCU would need to be based on detailed and compelling use cases.
+
+<h3><a name="Guaranteed Read-to-Write Upgrade">Guaranteed Read-to-Write Upgrade</a></h3>
+
+<p>
+As far as RCU is concerned, it is always possible to carry out an
+update within an RCU read-side critical section.
+For example, that RCU read-side critical section might search for
+a given data element, and then might acquire the update-side
+spinlock in order to update that element, all while remaining
+in that RCU read-side critical section.
+Of course, it is necessary to exit the RCU read-side critical section
+before invoking <tt>synchronize_rcu()</tt>, however, this
+inconvenience can be avoided through use of the
+<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
+described later in this document.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But how does the upgrade-to-write operation exclude other readers?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	It doesn't, just like normal RCU updates, which also do not exclude
+	RCU readers.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+This guarantee allows lookup code to be shared between read-side
+and update-side code, and was premeditated, appearing in the earliest
+DYNIX/ptx RCU documentation.
+
+<h2><a name="Fundamental Non-Requirements">Fundamental Non-Requirements</a></h2>
+
+<p>
+RCU provides extremely lightweight readers, and its read-side guarantees,
+though quite useful, are correspondingly lightweight.
+It is therefore all too easy to assume that RCU is guaranteeing more
+than it really is.
+Of course, the list of things that RCU does not guarantee is infinitely
+long, however, the following sections list a few non-guarantees that
+have caused confusion.
+Except where otherwise noted, these non-guarantees were premeditated.
+
+<ol>
+<li>	<a href="#Readers Impose Minimal Ordering">
+	Readers Impose Minimal Ordering</a>
+<li>	<a href="#Readers Do Not Exclude Updaters">
+	Readers Do Not Exclude Updaters</a>
+<li>	<a href="#Updaters Only Wait For Old Readers">
+	Updaters Only Wait For Old Readers</a>
+<li>	<a href="#Grace Periods Don't Partition Read-Side Critical Sections">
+	Grace Periods Don't Partition Read-Side Critical Sections</a>
+<li>	<a href="#Read-Side Critical Sections Don't Partition Grace Periods">
+	Read-Side Critical Sections Don't Partition Grace Periods</a>
+<li>	<a href="#Disabling Preemption Does Not Block Grace Periods">
+	Disabling Preemption Does Not Block Grace Periods</a>
+</ol>
+
+<h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3>
+
+<p>
+Reader-side markers such as <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> provide absolutely no ordering guarantees
+except through their interaction with the grace-period APIs such as
+<tt>synchronize_rcu()</tt>.
+To see this, consider the following pair of threads:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(x, 1);
+ 5   rcu_read_unlock();
+ 6   rcu_read_lock();
+ 7   WRITE_ONCE(y, 1);
+ 8   rcu_read_unlock();
+ 9 }
+10
+11 void thread1(void)
+12 {
+13   rcu_read_lock();
+14   r1 = READ_ONCE(y);
+15   rcu_read_unlock();
+16   rcu_read_lock();
+17   r2 = READ_ONCE(x);
+18   rcu_read_unlock();
+19 }
+</pre>
+</blockquote>
+
+<p>
+After <tt>thread0()</tt> and <tt>thread1()</tt> execute
+concurrently, it is quite possible to have
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 0)
+</pre>
+</blockquote>
+
+(that is, <tt>y</tt> appears to have been assigned before <tt>x</tt>),
+which would not be possible if <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> had much in the way of ordering
+properties.
+But they do not, so the CPU is within its rights
+to do significant reordering.
+This is by design:  Any significant ordering constraints would slow down
+these fast-path APIs.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Can't the compiler also reorder this code?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	No, the volatile casts in <tt>READ_ONCE()</tt> and
+	<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
+	this particular case.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
+
+<p>
+Neither <tt>rcu_read_lock()</tt> nor <tt>rcu_read_unlock()</tt>
+exclude updates.
+All they do is to prevent grace periods from ending.
+The following example illustrates this:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   r1 = READ_ONCE(y);
+ 5   if (r1) {
+ 6     do_something_with_nonzero_x();
+ 7     r2 = READ_ONCE(x);
+ 8     WARN_ON(!r2); /* BUG!!! */
+ 9   }
+10   rcu_read_unlock();
+11 }
+12
+13 void thread1(void)
+14 {
+15   spin_lock(&amp;my_lock);
+16   WRITE_ONCE(x, 1);
+17   WRITE_ONCE(y, 1);
+18   spin_unlock(&amp;my_lock);
+19 }
+</pre>
+</blockquote>
+
+<p>
+If the <tt>thread0()</tt> function's <tt>rcu_read_lock()</tt>
+excluded the <tt>thread1()</tt> function's update,
+the <tt>WARN_ON()</tt> could never fire.
+But the fact is that <tt>rcu_read_lock()</tt> does not exclude
+much of anything aside from subsequent grace periods, of which
+<tt>thread1()</tt> has none, so the
+<tt>WARN_ON()</tt> can and does fire.
+
+<h3><a name="Updaters Only Wait For Old Readers">Updaters Only Wait For Old Readers</a></h3>
+
+<p>
+It might be tempting to assume that after <tt>synchronize_rcu()</tt>
+completes, there are no readers executing.
+This temptation must be avoided because
+new readers can start immediately after <tt>synchronize_rcu()</tt>
+starts, and <tt>synchronize_rcu()</tt> is under no
+obligation to wait for these new readers.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Suppose that synchronize_rcu() did wait until <i>all</i>
+	readers had completed instead of waiting only on
+	pre-existing readers.
+	For how long would the updater be able to rely on there
+	being no readers?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	For no time at all.
+	Even if <tt>synchronize_rcu()</tt> were to wait until
+	all readers had completed, a new reader might start immediately after
+	<tt>synchronize_rcu()</tt> completed.
+	Therefore, the code following
+	<tt>synchronize_rcu()</tt> can <i>never</i> rely on there being
+	no readers.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
+Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
+
+<p>
+It is tempting to assume that if any part of one RCU read-side critical
+section precedes a given grace period, and if any part of another RCU
+read-side critical section follows that same grace period, then all of
+the first RCU read-side critical section must precede all of the second.
+However, this just isn't the case: A single grace period does not
+partition the set of RCU read-side critical sections.
+An example of this situation can be illustrated as follows, where
+<tt>x</tt>, <tt>y</tt>, and <tt>z</tt> are initially all zero:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   rcu_read_lock();
+19   r2 = READ_ONCE(b);
+20   r3 = READ_ONCE(c);
+21   rcu_read_unlock();
+22 }
+</pre>
+</blockquote>
+
+<p>
+It turns out that the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 0 &amp;&amp; r3 == 1)
+</pre>
+</blockquote>
+
+is entirely possible.
+The following figure show how this can happen, with each circled
+<tt>QS</tt> indicating the point at which RCU recorded a
+<i>quiescent state</i> for each thread, that is, a state in which
+RCU knows that the thread cannot be in the midst of an RCU read-side
+critical section that started before the current grace period:
+
+<p><img src="GPpartitionReaders1.svg" alt="GPpartitionReaders1.svg" width="60%"></p>
+
+<p>
+If it is necessary to partition RCU read-side critical sections in this
+manner, it is necessary to use two grace periods, where the first
+grace period is known to end before the second grace period starts:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   r2 = READ_ONCE(c);
+19   synchronize_rcu();
+20   WRITE_ONCE(d, 1);
+21 }
+22
+23 void thread3(void)
+24 {
+25   rcu_read_lock();
+26   r3 = READ_ONCE(b);
+27   r4 = READ_ONCE(d);
+28   rcu_read_unlock();
+29 }
+</pre>
+</blockquote>
+
+<p>
+Here, if <tt>(r1 == 1)</tt>, then
+<tt>thread0()</tt>'s write to <tt>b</tt> must happen
+before the end of <tt>thread1()</tt>'s grace period.
+If in addition <tt>(r4 == 1)</tt>, then
+<tt>thread3()</tt>'s read from <tt>b</tt> must happen
+after the beginning of <tt>thread2()</tt>'s grace period.
+If it is also the case that <tt>(r2 == 1)</tt>, then the
+end of <tt>thread1()</tt>'s grace period must precede the
+beginning of <tt>thread2()</tt>'s grace period.
+This mean that the two RCU read-side critical sections cannot overlap,
+guaranteeing that <tt>(r3 == 1)</tt>.
+As a result, the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 0 &amp;&amp; r4 == 1)
+</pre>
+</blockquote>
+
+cannot happen.
+
+<p>
+This non-requirement was also non-premeditated, but became apparent
+when studying RCU's interaction with memory ordering.
+
+<h3><a name="Read-Side Critical Sections Don't Partition Grace Periods">
+Read-Side Critical Sections Don't Partition Grace Periods</a></h3>
+
+<p>
+It is also tempting to assume that if an RCU read-side critical section
+happens between a pair of grace periods, then those grace periods cannot
+overlap.
+However, this temptation leads nowhere good, as can be illustrated by
+the following, with all variables initially zero:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   rcu_read_lock();
+19   WRITE_ONCE(d, 1);
+20   r2 = READ_ONCE(c);
+21   rcu_read_unlock();
+22 }
+23
+24 void thread3(void)
+25 {
+26   r3 = READ_ONCE(d);
+27   synchronize_rcu();
+28   WRITE_ONCE(e, 1);
+29 }
+30
+31 void thread4(void)
+32 {
+33   rcu_read_lock();
+34   r4 = READ_ONCE(b);
+35   r5 = READ_ONCE(e);
+36   rcu_read_unlock();
+37 }
+</pre>
+</blockquote>
+
+<p>
+In this case, the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 1 &amp;&amp; r4 == 0 &amp&amp; r5 == 1)
+</pre>
+</blockquote>
+
+is entirely possible, as illustrated below:
+
+<p><img src="ReadersPartitionGP1.svg" alt="ReadersPartitionGP1.svg" width="100%"></p>
+
+<p>
+Again, an RCU read-side critical section can overlap almost all of a
+given grace period, just so long as it does not overlap the entire
+grace period.
+As a result, an RCU read-side critical section cannot partition a pair
+of RCU grace periods.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	How long a sequence of grace periods, each separated by an RCU
+	read-side critical section, would be required to partition the RCU
+	read-side critical sections at the beginning and end of the chain?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	In theory, an infinite number.
+	In practice, an unknown number that is sensitive to both implementation
+	details and timing considerations.
+	Therefore, even in practice, RCU users must abide by the
+	theoretical rather than the practical answer.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Disabling Preemption Does Not Block Grace Periods">
+Disabling Preemption Does Not Block Grace Periods</a></h3>
+
+<p>
+There was a time when disabling preemption on any given CPU would block
+subsequent grace periods.
+However, this was an accident of implementation and is not a requirement.
+And in the current Linux-kernel implementation, disabling preemption
+on a given CPU in fact does not block grace periods, as Oleg Nesterov
+<a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>.
+
+<p>
+If you need a preempt-disable region to block grace periods, you need to add
+<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example
+as follows:
+
+<blockquote>
+<pre>
+ 1 preempt_disable();
+ 2 rcu_read_lock();
+ 3 do_something();
+ 4 rcu_read_unlock();
+ 5 preempt_enable();
+ 6
+ 7 /* Spinlocks implicitly disable preemption. */
+ 8 spin_lock(&amp;mylock);
+ 9 rcu_read_lock();
+10 do_something();
+11 rcu_read_unlock();
+12 spin_unlock(&amp;mylock);
+</pre>
+</blockquote>
+
+<p>
+In theory, you could enter the RCU read-side critical section first,
+but it is more efficient to keep the entire RCU read-side critical
+section contained in the preempt-disable region as shown above.
+Of course, RCU read-side critical sections that extend outside of
+preempt-disable regions will work correctly, but such critical sections
+can be preempted, which forces <tt>rcu_read_unlock()</tt> to do
+more work.
+And no, this is <i>not</i> an invitation to enclose all of your RCU
+read-side critical sections within preempt-disable regions, because
+doing so would degrade real-time response.
+
+<p>
+This non-requirement appeared with preemptible RCU.
+If you need a grace period that waits on non-preemptible code regions, use
+<a href="#Sched Flavor">RCU-sched</a>.
+
+<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
+
+<p>
+These parallelism facts of life are by no means specific to RCU, but
+the RCU implementation must abide by them.
+They therefore bear repeating:
+
+<ol>
+<li>	Any CPU or task may be delayed at any time,
+	and any attempts to avoid these delays by disabling
+	preemption, interrupts, or whatever are completely futile.
+	This is most obvious in preemptible user-level
+	environments and in virtualized environments (where
+	a given guest OS's VCPUs can be preempted at any time by
+	the underlying hypervisor), but can also happen in bare-metal
+	environments due to ECC errors, NMIs, and other hardware
+	events.
+	Although a delay of more than about 20 seconds can result
+	in splats, the RCU implementation is obligated to use
+	algorithms that can tolerate extremely long delays, but where
+	&ldquo;extremely long&rdquo; is not long enough to allow
+	wrap-around when incrementing a 64-bit counter.
+<li>	Both the compiler and the CPU can reorder memory accesses.
+	Where it matters, RCU must use compiler directives and
+	memory-barrier instructions to preserve ordering.
+<li>	Conflicting writes to memory locations in any given cache line
+	will result in expensive cache misses.
+	Greater numbers of concurrent writes and more-frequent
+	concurrent writes will result in more dramatic slowdowns.
+	RCU is therefore obligated to use algorithms that have
+	sufficient locality to avoid significant performance and
+	scalability problems.
+<li>	As a rough rule of thumb, only one CPU's worth of processing
+	may be carried out under the protection of any given exclusive
+	lock.
+	RCU must therefore use scalable locking designs.
+<li>	Counters are finite, especially on 32-bit systems.
+	RCU's use of counters must therefore tolerate counter wrap,
+	or be designed such that counter wrap would take way more
+	time than a single system is likely to run.
+	An uptime of ten years is quite possible, a runtime
+	of a century much less so.
+	As an example of the latter, RCU's dyntick-idle nesting counter
+	allows 54 bits for interrupt nesting level (this counter
+	is 64 bits even on a 32-bit system).
+	Overflowing this counter requires 2<sup>54</sup>
+	half-interrupts on a given CPU without that CPU ever going idle.
+	If a half-interrupt happened every microsecond, it would take
+	570 years of runtime to overflow this counter, which is currently
+	believed to be an acceptably long time.
+<li>	Linux systems can have thousands of CPUs running a single
+	Linux kernel in a single shared-memory environment.
+	RCU must therefore pay close attention to high-end scalability.
+</ol>
+
+<p>
+This last parallelism fact of life means that RCU must pay special
+attention to the preceding facts of life.
+The idea that Linux might scale to systems with thousands of CPUs would
+have been met with some skepticism in the 1990s, but these requirements
+would have otherwise have been unsurprising, even in the early 1990s.
+
+<h2><a name="Quality-of-Implementation Requirements">Quality-of-Implementation Requirements</a></h2>
+
+<p>
+These sections list quality-of-implementation requirements.
+Although an RCU implementation that ignores these requirements could
+still be used, it would likely be subject to limitations that would
+make it inappropriate for industrial-strength production use.
+Classes of quality-of-implementation requirements are as follows:
+
+<ol>
+<li>	<a href="#Specialization">Specialization</a>
+<li>	<a href="#Performance and Scalability">Performance and Scalability</a>
+<li>	<a href="#Composability">Composability</a>
+<li>	<a href="#Corner Cases">Corner Cases</a>
+</ol>
+
+<p>
+These classes is covered in the following sections.
+
+<h3><a name="Specialization">Specialization</a></h3>
+
+<p>
+RCU is and always has been intended primarily for read-mostly situations,
+which means that RCU's read-side primitives are optimized, often at the
+expense of its update-side primitives.
+Experience thus far is captured by the following list of situations:
+
+<ol>
+<li>	Read-mostly data, where stale and inconsistent data is not
+	a problem:   RCU works great!
+<li>	Read-mostly data, where data must be consistent:
+	RCU works well.
+<li>	Read-write data, where data must be consistent:
+	RCU <i>might</i> work OK.
+	Or not.
+<li>	Write-mostly data, where data must be consistent:
+	RCU is very unlikely to be the right tool for the job,
+	with the following exceptions, where RCU can provide:
+	<ol type=a>
+	<li>	Existence guarantees for update-friendly mechanisms.
+	<li>	Wait-free read-side primitives for real-time use.
+	</ol>
+</ol>
+
+<p>
+This focus on read-mostly situations means that RCU must interoperate
+with other synchronization primitives.
+For example, the <tt>add_gp()</tt> and <tt>remove_gp_synchronous()</tt>
+examples discussed earlier use RCU to protect readers and locking to
+coordinate updaters.
+However, the need extends much farther, requiring that a variety of
+synchronization primitives be legal within RCU read-side critical sections,
+including spinlocks, sequence locks, atomic operations, reference
+counters, and memory barriers.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	What about sleeping locks?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	These are forbidden within Linux-kernel RCU read-side critical
+	sections because it is not legal to place a quiescent state
+	(in this case, voluntary context switch) within an RCU read-side
+	critical section.
+	However, sleeping locks may be used within userspace RCU read-side
+	critical sections, and also within Linux-kernel sleepable RCU
+	<a href="#Sleepable RCU"><font color="ffffff">(SRCU)</font></a>
+	read-side critical sections.
+	In addition, the -rt patchset turns spinlocks into a
+	sleeping locks so that the corresponding critical sections
+	can be preempted, which also means that these sleeplockified
+	spinlocks (but not other sleeping locks!)  may be acquire within
+	-rt-Linux-kernel RCU read-side critical sections.
+	</font>
+
+	<p><font color="ffffff">
+	Note that it <i>is</i> legal for a normal RCU read-side
+	critical section to conditionally acquire a sleeping locks
+	(as in <tt>mutex_trylock()</tt>), but only as long as it does
+	not loop indefinitely attempting to conditionally acquire that
+	sleeping locks.
+	The key point is that things like <tt>mutex_trylock()</tt>
+	either return with the mutex held, or return an error indication if
+	the mutex was not immediately available.
+	Either way, <tt>mutex_trylock()</tt> returns immediately without
+	sleeping.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+It often comes as a surprise that many algorithms do not require a
+consistent view of data, but many can function in that mode,
+with network routing being the poster child.
+Internet routing algorithms take significant time to propagate
+updates, so that by the time an update arrives at a given system,
+that system has been sending network traffic the wrong way for
+a considerable length of time.
+Having a few threads continue to send traffic the wrong way for a
+few more milliseconds is clearly not a problem:  In the worst case,
+TCP retransmissions will eventually get the data where it needs to go.
+In general, when tracking the state of the universe outside of the
+computer, some level of inconsistency must be tolerated due to
+speed-of-light delays if nothing else.
+
+<p>
+Furthermore, uncertainty about external state is inherent in many cases.
+For example, a pair of veterinarians might use heartbeat to determine
+whether or not a given cat was alive.
+But how long should they wait after the last heartbeat to decide that
+the cat is in fact dead?
+Waiting less than 400 milliseconds makes no sense because this would
+mean that a relaxed cat would be considered to cycle between death
+and life more than 100 times per minute.
+Moreover, just as with human beings, a cat's heart might stop for
+some period of time, so the exact wait period is a judgment call.
+One of our pair of veterinarians might wait 30 seconds before pronouncing
+the cat dead, while the other might insist on waiting a full minute.
+The two veterinarians would then disagree on the state of the cat during
+the final 30 seconds of the minute following the last heartbeat.
+
+<p>
+Interestingly enough, this same situation applies to hardware.
+When push comes to shove, how do we tell whether or not some
+external server has failed?
+We send messages to it periodically, and declare it failed if we
+don't receive a response within a given period of time.
+Policy decisions can usually tolerate short
+periods of inconsistency.
+The policy was decided some time ago, and is only now being put into
+effect, so a few milliseconds of delay is normally inconsequential.
+
+<p>
+However, there are algorithms that absolutely must see consistent data.
+For example, the translation between a user-level SystemV semaphore
+ID to the corresponding in-kernel data structure is protected by RCU,
+but it is absolutely forbidden to update a semaphore that has just been
+removed.
+In the Linux kernel, this need for consistency is accommodated by acquiring
+spinlocks located in the in-kernel data structure from within
+the RCU read-side critical section, and this is indicated by the
+green box in the figure above.
+Many other techniques may be used, and are in fact used within the
+Linux kernel.
+
+<p>
+In short, RCU is not required to maintain consistency, and other
+mechanisms may be used in concert with RCU when consistency is required.
+RCU's specialization allows it to do its job extremely well, and its
+ability to interoperate with other synchronization mechanisms allows
+the right mix of synchronization tools to be used for a given job.
+
+<h3><a name="Performance and Scalability">Performance and Scalability</a></h3>
+
+<p>
+Energy efficiency is a critical component of performance today,
+and Linux-kernel RCU implementations must therefore avoid unnecessarily
+awakening idle CPUs.
+I cannot claim that this requirement was premeditated.
+In fact, I learned of it during a telephone conversation in which I
+was given &ldquo;frank and open&rdquo; feedback on the importance
+of energy efficiency in battery-powered systems and on specific
+energy-efficiency shortcomings of the Linux-kernel RCU implementation.
+In my experience, the battery-powered embedded community will consider
+any unnecessary wakeups to be extremely unfriendly acts.
+So much so that mere Linux-kernel-mailing-list posts are
+insufficient to vent their ire.
+
+<p>
+Memory consumption is not particularly important for in most
+situations, and has become decreasingly
+so as memory sizes have expanded and memory
+costs have plummeted.
+However, as I learned from Matt Mackall's
+<a href="http://elinux.org/Linux_Tiny-FAQ">bloatwatch</a>
+efforts, memory footprint is critically important on single-CPU systems with
+non-preemptible (<tt>CONFIG_PREEMPT=n</tt>) kernels, and thus
+<a href="https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com">tiny RCU</a>
+was born.
+Josh Triplett has since taken over the small-memory banner with his
+<a href="https://tiny.wiki.kernel.org/">Linux kernel tinification</a>
+project, which resulted in
+<a href="#Sleepable RCU">SRCU</a>
+becoming optional for those kernels not needing it.
+
+<p>
+The remaining performance requirements are, for the most part,
+unsurprising.
+For example, in keeping with RCU's read-side specialization,
+<tt>rcu_dereference()</tt> should have negligible overhead (for
+example, suppression of a few minor compiler optimizations).
+Similarly, in non-preemptible environments, <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> should have exactly zero overhead.
+
+<p>
+In preemptible environments, in the case where the RCU read-side
+critical section was not preempted (as will be the case for the
+highest-priority real-time process), <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> should have minimal overhead.
+In particular, they should not contain atomic read-modify-write
+operations, memory-barrier instructions, preemption disabling,
+interrupt disabling, or backwards branches.
+However, in the case where the RCU read-side critical section was preempted,
+<tt>rcu_read_unlock()</tt> may acquire spinlocks and disable interrupts.
+This is why it is better to nest an RCU read-side critical section
+within a preempt-disable region than vice versa, at least in cases
+where that critical section is short enough to avoid unduly degrading
+real-time latencies.
+
+<p>
+The <tt>synchronize_rcu()</tt> grace-period-wait primitive is
+optimized for throughput.
+It may therefore incur several milliseconds of latency in addition to
+the duration of the longest RCU read-side critical section.
+On the other hand, multiple concurrent invocations of
+<tt>synchronize_rcu()</tt> are required to use batching optimizations
+so that they can be satisfied by a single underlying grace-period-wait
+operation.
+For example, in the Linux kernel, it is not unusual for a single
+grace-period-wait operation to serve more than
+<a href="https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response">1,000 separate invocations</a>
+of <tt>synchronize_rcu()</tt>, thus amortizing the per-invocation
+overhead down to nearly zero.
+However, the grace-period optimization is also required to avoid
+measurable degradation of real-time scheduling and interrupt latencies.
+
+<p>
+In some cases, the multi-millisecond <tt>synchronize_rcu()</tt>
+latencies are unacceptable.
+In these cases, <tt>synchronize_rcu_expedited()</tt> may be used
+instead, reducing the grace-period latency down to a few tens of
+microseconds on small systems, at least in cases where the RCU read-side
+critical sections are short.
+There are currently no special latency requirements for
+<tt>synchronize_rcu_expedited()</tt> on large systems, but,
+consistent with the empirical nature of the RCU specification,
+that is subject to change.
+However, there most definitely are scalability requirements:
+A storm of <tt>synchronize_rcu_expedited()</tt> invocations on 4096
+CPUs should at least make reasonable forward progress.
+In return for its shorter latencies, <tt>synchronize_rcu_expedited()</tt>
+is permitted to impose modest degradation of real-time latency
+on non-idle online CPUs.
+Here, &ldquo;modest&rdquo; means roughly the same latency
+degradation as a scheduling-clock interrupt.
+
+<p>
+There are a number of situations where even
+<tt>synchronize_rcu_expedited()</tt>'s reduced grace-period
+latency is unacceptable.
+In these situations, the asynchronous <tt>call_rcu()</tt> can be
+used in place of <tt>synchronize_rcu()</tt> as follows:
+
+<blockquote>
+<pre>
+ 1 struct foo {
+ 2   int a;
+ 3   int b;
+ 4   struct rcu_head rh;
+ 5 };
+ 6
+ 7 static void remove_gp_cb(struct rcu_head *rhp)
+ 8 {
+ 9   struct foo *p = container_of(rhp, struct foo, rh);
+10
+11   kfree(p);
+12 }
+13
+14 bool remove_gp_asynchronous(void)
+15 {
+16   struct foo *p;
+17
+18   spin_lock(&amp;gp_lock);
+19   p = rcu_dereference(gp);
+20   if (!p) {
+21     spin_unlock(&amp;gp_lock);
+22     return false;
+23   }
+24   rcu_assign_pointer(gp, NULL);
+25   call_rcu(&amp;p-&gt;rh, remove_gp_cb);
+26   spin_unlock(&amp;gp_lock);
+27   return true;
+28 }
+</pre>
+</blockquote>
+
+<p>
+A definition of <tt>struct foo</tt> is finally needed, and appears
+on lines&nbsp;1-5.
+The function <tt>remove_gp_cb()</tt> is passed to <tt>call_rcu()</tt>
+on line&nbsp;25, and will be invoked after the end of a subsequent
+grace period.
+This gets the same effect as <tt>remove_gp_synchronous()</tt>,
+but without forcing the updater to wait for a grace period to elapse.
+The <tt>call_rcu()</tt> function may be used in a number of
+situations where neither <tt>synchronize_rcu()</tt> nor
+<tt>synchronize_rcu_expedited()</tt> would be legal,
+including within preempt-disable code, <tt>local_bh_disable()</tt> code,
+interrupt-disable code, and interrupt handlers.
+However, even <tt>call_rcu()</tt> is illegal within NMI handlers
+and from idle and offline CPUs.
+The callback function (<tt>remove_gp_cb()</tt> in this case) will be
+executed within softirq (software interrupt) environment within the
+Linux kernel,
+either within a real softirq handler or under the protection
+of <tt>local_bh_disable()</tt>.
+In both the Linux kernel and in userspace, it is bad practice to
+write an RCU callback function that takes too long.
+Long-running operations should be relegated to separate threads or
+(in the Linux kernel) workqueues.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
+	After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
+	structure, which would interact badly with concurrent insertions.
+	Doesn't this mean that <tt>rcu_dereference()</tt> is required?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
+	any changes, including any insertions that <tt>rcu_dereference()</tt>
+	would protect against.
+	Therefore, any insertions will be delayed until after
+	<tt>-&gt;gp_lock</tt>
+	is released on line&nbsp;25, which in turn means that
+	<tt>rcu_access_pointer()</tt> suffices.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+However, all that <tt>remove_gp_cb()</tt> is doing is
+invoking <tt>kfree()</tt> on the data element.
+This is a common idiom, and is supported by <tt>kfree_rcu()</tt>,
+which allows &ldquo;fire and forget&rdquo; operation as shown below:
+
+<blockquote>
+<pre>
+ 1 struct foo {
+ 2   int a;
+ 3   int b;
+ 4   struct rcu_head rh;
+ 5 };
+ 6
+ 7 bool remove_gp_faf(void)
+ 8 {
+ 9   struct foo *p;
+10
+11   spin_lock(&amp;gp_lock);
+12   p = rcu_dereference(gp);
+13   if (!p) {
+14     spin_unlock(&amp;gp_lock);
+15     return false;
+16   }
+17   rcu_assign_pointer(gp, NULL);
+18   kfree_rcu(p, rh);
+19   spin_unlock(&amp;gp_lock);
+20   return true;
+21 }
+</pre>
+</blockquote>
+
+<p>
+Note that <tt>remove_gp_faf()</tt> simply invokes
+<tt>kfree_rcu()</tt> and proceeds, without any need to pay any
+further attention to the subsequent grace period and <tt>kfree()</tt>.
+It is permissible to invoke <tt>kfree_rcu()</tt> from the same
+environments as for <tt>call_rcu()</tt>.
+Interestingly enough, DYNIX/ptx had the equivalents of
+<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>, but not
+<tt>synchronize_rcu()</tt>.
+This was due to the fact that RCU was not heavily used within DYNIX/ptx,
+so the very few places that needed something like
+<tt>synchronize_rcu()</tt> simply open-coded it.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Earlier it was claimed that <tt>call_rcu()</tt> and
+	<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
+	by readers.
+	But how can that be correct, given that the invocation of the callback
+	and the freeing of the memory (respectively) must still wait for
+	a grace period to elapse?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	We could define things this way, but keep in mind that this sort of
+	definition would say that updates in garbage-collected languages
+	cannot complete until the next time the garbage collector runs,
+	which does not seem at all reasonable.
+	The key point is that in most cases, an updater using either
+	<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
+	next update as soon as it has invoked <tt>call_rcu()</tt> or
+	<tt>kfree_rcu()</tt>, without having to wait for a subsequent
+	grace period.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+But what if the updater must wait for the completion of code to be
+executed after the end of the grace period, but has other tasks
+that can be carried out in the meantime?
+The polling-style <tt>get_state_synchronize_rcu()</tt> and
+<tt>cond_synchronize_rcu()</tt> functions may be used for this
+purpose, as shown below:
+
+<blockquote>
+<pre>
+ 1 bool remove_gp_poll(void)
+ 2 {
+ 3   struct foo *p;
+ 4   unsigned long s;
+ 5
+ 6   spin_lock(&amp;gp_lock);
+ 7   p = rcu_access_pointer(gp);
+ 8   if (!p) {
+ 9     spin_unlock(&amp;gp_lock);
+10     return false;
+11   }
+12   rcu_assign_pointer(gp, NULL);
+13   spin_unlock(&amp;gp_lock);
+14   s = get_state_synchronize_rcu();
+15   do_something_while_waiting();
+16   cond_synchronize_rcu(s);
+17   kfree(p);
+18   return true;
+19 }
+</pre>
+</blockquote>
+
+<p>
+On line&nbsp;14, <tt>get_state_synchronize_rcu()</tt> obtains a
+&ldquo;cookie&rdquo; from RCU,
+then line&nbsp;15 carries out other tasks,
+and finally, line&nbsp;16 returns immediately if a grace period has
+elapsed in the meantime, but otherwise waits as required.
+The need for <tt>get_state_synchronize_rcu</tt> and
+<tt>cond_synchronize_rcu()</tt> has appeared quite recently,
+so it is too early to tell whether they will stand the test of time.
+
+<p>
+RCU thus provides a range of tools to allow updaters to strike the
+required tradeoff between latency, flexibility and CPU overhead.
+
+<h3><a name="Composability">Composability</a></h3>
+
+<p>
+Composability has received much attention in recent years, perhaps in part
+due to the collision of multicore hardware with object-oriented techniques
+designed in single-threaded environments for single-threaded use.
+And in theory, RCU read-side critical sections may be composed, and in
+fact may be nested arbitrarily deeply.
+In practice, as with all real-world implementations of composable
+constructs, there are limitations.
+
+<p>
+Implementations of RCU for which <tt>rcu_read_lock()</tt>
+and <tt>rcu_read_unlock()</tt> generate no code, such as
+Linux-kernel RCU when <tt>CONFIG_PREEMPT=n</tt>, can be
+nested arbitrarily deeply.
+After all, there is no overhead.
+Except that if all these instances of <tt>rcu_read_lock()</tt>
+and <tt>rcu_read_unlock()</tt> are visible to the compiler,
+compilation will eventually fail due to exhausting memory,
+mass storage, or user patience, whichever comes first.
+If the nesting is not visible to the compiler, as is the case with
+mutually recursive functions each in its own translation unit,
+stack overflow will result.
+If the nesting takes the form of loops, perhaps in the guise of tail
+recursion, either the control variable
+will overflow or (in the Linux kernel) you will get an RCU CPU stall warning.
+Nevertheless, this class of RCU implementations is one
+of the most composable constructs in existence.
+
+<p>
+RCU implementations that explicitly track nesting depth
+are limited by the nesting-depth counter.
+For example, the Linux kernel's preemptible RCU limits nesting to
+<tt>INT_MAX</tt>.
+This should suffice for almost all practical purposes.
+That said, a consecutive pair of RCU read-side critical sections
+between which there is an operation that waits for a grace period
+cannot be enclosed in another RCU read-side critical section.
+This is because it is not legal to wait for a grace period within
+an RCU read-side critical section:  To do so would result either
+in deadlock or
+in RCU implicitly splitting the enclosing RCU read-side critical
+section, neither of which is conducive to a long-lived and prosperous
+kernel.
+
+<p>
+It is worth noting that RCU is not alone in limiting composability.
+For example, many transactional-memory implementations prohibit
+composing a pair of transactions separated by an irrevocable
+operation (for example, a network receive operation).
+For another example, lock-based critical sections can be composed
+surprisingly freely, but only if deadlock is avoided.
+
+<p>
+In short, although RCU read-side critical sections are highly composable,
+care is required in some situations, just as is the case for any other
+composable synchronization mechanism.
+
+<h3><a name="Corner Cases">Corner Cases</a></h3>
+
+<p>
+A given RCU workload might have an endless and intense stream of
+RCU read-side critical sections, perhaps even so intense that there
+was never a point in time during which there was not at least one
+RCU read-side critical section in flight.
+RCU cannot allow this situation to block grace periods:  As long as
+all the RCU read-side critical sections are finite, grace periods
+must also be finite.
+
+<p>
+That said, preemptible RCU implementations could potentially result
+in RCU read-side critical sections being preempted for long durations,
+which has the effect of creating a long-duration RCU read-side
+critical section.
+This situation can arise only in heavily loaded systems, but systems using
+real-time priorities are of course more vulnerable.
+Therefore, RCU priority boosting is provided to help deal with this
+case.
+That said, the exact requirements on RCU priority boosting will likely
+evolve as more experience accumulates.
+
+<p>
+Other workloads might have very high update rates.
+Although one can argue that such workloads should instead use
+something other than RCU, the fact remains that RCU must
+handle such workloads gracefully.
+This requirement is another factor driving batching of grace periods,
+but it is also the driving force behind the checks for large numbers
+of queued RCU callbacks in the <tt>call_rcu()</tt> code path.
+Finally, high update rates should not delay RCU read-side critical
+sections, although some small read-side delays can occur when using
+<tt>synchronize_rcu_expedited()</tt>, courtesy of this function's use
+of <tt>smp_call_function_single()</tt>.
+
+<p>
+Although all three of these corner cases were understood in the early
+1990s, a simple user-level test consisting of <tt>close(open(path))</tt>
+in a tight loop
+in the early 2000s suddenly provided a much deeper appreciation of the
+high-update-rate corner case.
+This test also motivated addition of some RCU code to react to high update
+rates, for example, if a given CPU finds itself with more than 10,000
+RCU callbacks queued, it will cause RCU to take evasive action by
+more aggressively starting grace periods and more aggressively forcing
+completion of grace-period processing.
+This evasive action causes the grace period to complete more quickly,
+but at the cost of restricting RCU's batching optimizations, thus
+increasing the CPU overhead incurred by that grace period.
+
+<h2><a name="Software-Engineering Requirements">
+Software-Engineering Requirements</a></h2>
+
+<p>
+Between Murphy's Law and &ldquo;To err is human&rdquo;, it is necessary to
+guard against mishaps and misuse:
+
+<ol>
+<li>	It is all too easy to forget to use <tt>rcu_read_lock()</tt>
+	everywhere that it is needed, so kernels built with
+	<tt>CONFIG_PROVE_RCU=y</tt> will splat if
+	<tt>rcu_dereference()</tt> is used outside of an
+	RCU read-side critical section.
+	Update-side code can use <tt>rcu_dereference_protected()</tt>,
+	which takes a
+	<a href="https://lwn.net/Articles/371986/">lockdep expression</a>
+	to indicate what is providing the protection.
+	If the indicated protection is not provided, a lockdep splat
+	is emitted.
+
+	<p>
+	Code shared between readers and updaters can use
+	<tt>rcu_dereference_check()</tt>, which also takes a
+	lockdep expression, and emits a lockdep splat if neither
+	<tt>rcu_read_lock()</tt> nor the indicated protection
+	is in place.
+	In addition, <tt>rcu_dereference_raw()</tt> is used in those
+	(hopefully rare) cases where the required protection cannot
+	be easily described.
+	Finally, <tt>rcu_read_lock_held()</tt> is provided to
+	allow a function to verify that it has been invoked within
+	an RCU read-side critical section.
+	I was made aware of this set of requirements shortly after Thomas
+	Gleixner audited a number of RCU uses.
+<li>	A given function might wish to check for RCU-related preconditions
+	upon entry, before using any other RCU API.
+	The <tt>rcu_lockdep_assert()</tt> does this job,
+	asserting the expression in kernels having lockdep enabled
+	and doing nothing otherwise.
+<li>	It is also easy to forget to use <tt>rcu_assign_pointer()</tt>
+	and <tt>rcu_dereference()</tt>, perhaps (incorrectly)
+	substituting a simple assignment.
+	To catch this sort of error, a given RCU-protected pointer may be
+	tagged with <tt>__rcu</tt>, after which sparse
+	will complain about simple-assignment accesses to that pointer.
+	Arnd Bergmann made me aware of this requirement, and also
+	supplied the needed
+	<a href="https://lwn.net/Articles/376011/">patch series</a>.
+<li>	Kernels built with <tt>CONFIG_DEBUG_OBJECTS_RCU_HEAD=y</tt>
+	will splat if a data element is passed to <tt>call_rcu()</tt>
+	twice in a row, without a grace period in between.
+	(This error is similar to a double free.)
+	The corresponding <tt>rcu_head</tt> structures that are
+	dynamically allocated are automatically tracked, but
+	<tt>rcu_head</tt> structures allocated on the stack
+	must be initialized with <tt>init_rcu_head_on_stack()</tt>
+	and cleaned up with <tt>destroy_rcu_head_on_stack()</tt>.
+	Similarly, statically allocated non-stack <tt>rcu_head</tt>
+	structures must be initialized with <tt>init_rcu_head()</tt>
+	and cleaned up with <tt>destroy_rcu_head()</tt>.
+	Mathieu Desnoyers made me aware of this requirement, and also
+	supplied the needed
+	<a href="https://lkml.kernel.org/g/20100319013024.GA28456@Krystal">patch</a>.
+<li>	An infinite loop in an RCU read-side critical section will
+	eventually trigger an RCU CPU stall warning splat, with
+	the duration of &ldquo;eventually&rdquo; being controlled by the
+	<tt>RCU_CPU_STALL_TIMEOUT</tt> <tt>Kconfig</tt> option, or,
+	alternatively, by the
+	<tt>rcupdate.rcu_cpu_stall_timeout</tt> boot/sysfs
+	parameter.
+	However, RCU is not obligated to produce this splat
+	unless there is a grace period waiting on that particular
+	RCU read-side critical section.
+	<p>
+	Some extreme workloads might intentionally delay
+	RCU grace periods, and systems running those workloads can
+	be booted with <tt>rcupdate.rcu_cpu_stall_suppress</tt>
+	to suppress the splats.
+	This kernel parameter may also be set via <tt>sysfs</tt>.
+	Furthermore, RCU CPU stall warnings are counter-productive
+	during sysrq dumps and during panics.
+	RCU therefore supplies the <tt>rcu_sysrq_start()</tt> and
+	<tt>rcu_sysrq_end()</tt> API members to be called before
+	and after long sysrq dumps.
+	RCU also supplies the <tt>rcu_panic()</tt> notifier that is
+	automatically invoked at the beginning of a panic to suppress
+	further RCU CPU stall warnings.
+
+	<p>
+	This requirement made itself known in the early 1990s, pretty
+	much the first time that it was necessary to debug a CPU stall.
+	That said, the initial implementation in DYNIX/ptx was quite
+	generic in comparison with that of Linux.
+<li>	Although it would be very good to detect pointers leaking out
+	of RCU read-side critical sections, there is currently no
+	good way of doing this.
+	One complication is the need to distinguish between pointers
+	leaking and pointers that have been handed off from RCU to
+	some other synchronization mechanism, for example, reference
+	counting.
+<li>	In kernels built with <tt>CONFIG_RCU_TRACE=y</tt>, RCU-related
+	information is provided via event tracing.
+<li>	Open-coded use of <tt>rcu_assign_pointer()</tt> and
+	<tt>rcu_dereference()</tt> to create typical linked
+	data structures can be surprisingly error-prone.
+	Therefore, RCU-protected
+	<a href="https://lwn.net/Articles/609973/#RCU List APIs">linked lists</a>
+	and, more recently, RCU-protected
+	<a href="https://lwn.net/Articles/612100/">hash tables</a>
+	are available.
+	Many other special-purpose RCU-protected data structures are
+	available in the Linux kernel and the userspace RCU library.
+<li>	Some linked structures are created at compile time, but still
+	require <tt>__rcu</tt> checking.
+	The <tt>RCU_POINTER_INITIALIZER()</tt> macro serves this
+	purpose.
+<li>	It is not necessary to use <tt>rcu_assign_pointer()</tt>
+	when creating linked structures that are to be published via
+	a single external pointer.
+	The <tt>RCU_INIT_POINTER()</tt> macro is provided for
+	this task and also for assigning <tt>NULL</tt> pointers
+	at runtime.
+</ol>
+
+<p>
+This not a hard-and-fast list:  RCU's diagnostic capabilities will
+continue to be guided by the number and type of usage bugs found
+in real-world RCU usage.
+
+<h2><a name="Linux Kernel Complications">Linux Kernel Complications</a></h2>
+
+<p>
+The Linux kernel provides an interesting environment for all kinds of
+software, including RCU.
+Some of the relevant points of interest are as follows:
+
+<ol>
+<li>	<a href="#Configuration">Configuration</a>.
+<li>	<a href="#Firmware Interface">Firmware Interface</a>.
+<li>	<a href="#Early Boot">Early Boot</a>.
+<li>	<a href="#Interrupts and NMIs">
+	Interrupts and non-maskable interrupts (NMIs)</a>.
+<li>	<a href="#Loadable Modules">Loadable Modules</a>.
+<li>	<a href="#Hotplug CPU">Hotplug CPU</a>.
+<li>	<a href="#Scheduler and RCU">Scheduler and RCU</a>.
+<li>	<a href="#Tracing and RCU">Tracing and RCU</a>.
+<li>	<a href="#Energy Efficiency">Energy Efficiency</a>.
+<li>	<a href="#Scheduling-Clock Interrupts and RCU">
+	Scheduling-Clock Interrupts and RCU</a>.
+<li>	<a href="#Memory Efficiency">Memory Efficiency</a>.
+<li>	<a href="#Performance, Scalability, Response Time, and Reliability">
+	Performance, Scalability, Response Time, and Reliability</a>.
+</ol>
+
+<p>
+This list is probably incomplete, but it does give a feel for the
+most notable Linux-kernel complications.
+Each of the following sections covers one of the above topics.
+
+<h3><a name="Configuration">Configuration</a></h3>
+
+<p>
+RCU's goal is automatic configuration, so that almost nobody
+needs to worry about RCU's <tt>Kconfig</tt> options.
+And for almost all users, RCU does in fact work well
+&ldquo;out of the box.&rdquo;
+
+<p>
+However, there are specialized use cases that are handled by
+kernel boot parameters and <tt>Kconfig</tt> options.
+Unfortunately, the <tt>Kconfig</tt> system will explicitly ask users
+about new <tt>Kconfig</tt> options, which requires almost all of them
+be hidden behind a <tt>CONFIG_RCU_EXPERT</tt> <tt>Kconfig</tt> option.
+
+<p>
+This all should be quite obvious, but the fact remains that
+Linus Torvalds recently had to
+<a href="https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com">remind</a>
+me of this requirement.
+
+<h3><a name="Firmware Interface">Firmware Interface</a></h3>
+
+<p>
+In many cases, kernel obtains information about the system from the
+firmware, and sometimes things are lost in translation.
+Or the translation is accurate, but the original message is bogus.
+
+<p>
+For example, some systems' firmware overreports the number of CPUs,
+sometimes by a large factor.
+If RCU naively believed the firmware, as it used to do,
+it would create too many per-CPU kthreads.
+Although the resulting system will still run correctly, the extra
+kthreads needlessly consume memory and can cause confusion
+when they show up in <tt>ps</tt> listings.
+
+<p>
+RCU must therefore wait for a given CPU to actually come online before
+it can allow itself to believe that the CPU actually exists.
+The resulting &ldquo;ghost CPUs&rdquo; (which are never going to
+come online) cause a number of
+<a href="https://paulmck.livejournal.com/37494.html">interesting complications</a>.
+
+<h3><a name="Early Boot">Early Boot</a></h3>
+
+<p>
+The Linux kernel's boot sequence is an interesting process,
+and RCU is used early, even before <tt>rcu_init()</tt>
+is invoked.
+In fact, a number of RCU's primitives can be used as soon as the
+initial task's <tt>task_struct</tt> is available and the
+boot CPU's per-CPU variables are set up.
+The read-side primitives (<tt>rcu_read_lock()</tt>,
+<tt>rcu_read_unlock()</tt>, <tt>rcu_dereference()</tt>,
+and <tt>rcu_access_pointer()</tt>) will operate normally very early on,
+as will <tt>rcu_assign_pointer()</tt>.
+
+<p>
+Although <tt>call_rcu()</tt> may be invoked at any
+time during boot, callbacks are not guaranteed to be invoked until after
+all of RCU's kthreads have been spawned, which occurs at
+<tt>early_initcall()</tt> time.
+This delay in callback invocation is due to the fact that RCU does not
+invoke callbacks until it is fully initialized, and this full initialization
+cannot occur until after the scheduler has initialized itself to the
+point where RCU can spawn and run its kthreads.
+In theory, it would be possible to invoke callbacks earlier,
+however, this is not a panacea because there would be severe restrictions
+on what operations those callbacks could invoke.
+
+<p>
+Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
+<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
+(<a href="#Bottom-Half Flavor">discussed below</a>),
+<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>,
+<tt>synchronize_rcu_expedited()</tt>,
+<tt>synchronize_rcu_bh_expedited()</tt>, and
+<tt>synchronize_sched_expedited()</tt>
+will all operate normally
+during very early boot, the reason being that there is only one CPU
+and preemption is disabled.
+This means that the call <tt>synchronize_rcu()</tt> (or friends)
+itself is a quiescent
+state and thus a grace period, so the early-boot implementation can
+be a no-op.
+
+<p>
+However, once the scheduler has spawned its first kthread, this early
+boot trick fails for <tt>synchronize_rcu()</tt> (as well as for
+<tt>synchronize_rcu_expedited()</tt>) in <tt>CONFIG_PREEMPT=y</tt>
+kernels.
+The reason is that an RCU read-side critical section might be preempted,
+which means that a subsequent <tt>synchronize_rcu()</tt> really does have
+to wait for something, as opposed to simply returning immediately.
+Unfortunately, <tt>synchronize_rcu()</tt> can't do this until all of
+its kthreads are spawned, which doesn't happen until some time during
+<tt>early_initcalls()</tt> time.
+But this is no excuse:  RCU is nevertheless required to correctly handle
+synchronous grace periods during this time period.
+Once all of its kthreads are up and running, RCU starts running
+normally.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	How can RCU possibly handle grace periods before all of its
+	kthreads have been spawned???
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Very carefully!
+	</font>
+
+	<p><font color="ffffff">
+	During the &ldquo;dead zone&rdquo; between the time that the
+	scheduler spawns the first task and the time that all of RCU's
+	kthreads have been spawned, all synchronous grace periods are
+	handled by the expedited grace-period mechanism.
+	At runtime, this expedited mechanism relies on workqueues, but
+	during the dead zone the requesting task itself drives the
+	desired expedited grace period.
+	Because dead-zone execution takes place within task context,
+	everything works.
+	Once the dead zone ends, expedited grace periods go back to
+	using workqueues, as is required to avoid problems that would
+	otherwise occur when a user task received a POSIX signal while
+	driving an expedited grace period.
+	</font>
+
+	<p><font color="ffffff">
+	And yes, this does mean that it is unhelpful to send POSIX
+	signals to random tasks between the time that the scheduler
+	spawns its first kthread and the time that RCU's kthreads
+	have all been spawned.
+	If there ever turns out to be a good reason for sending POSIX
+	signals during that time, appropriate adjustments will be made.
+	(If it turns out that POSIX signals are sent during this time for
+	no good reason, other adjustments will be made, appropriate
+	or otherwise.)
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+I learned of these boot-time requirements as a result of a series of
+system hangs.
+
+<h3><a name="Interrupts and NMIs">Interrupts and NMIs</a></h3>
+
+<p>
+The Linux kernel has interrupts, and RCU read-side critical sections are
+legal within interrupt handlers and within interrupt-disabled regions
+of code, as are invocations of <tt>call_rcu()</tt>.
+
+<p>
+Some Linux-kernel architectures can enter an interrupt handler from
+non-idle process context, and then just never leave it, instead stealthily
+transitioning back to process context.
+This trick is sometimes used to invoke system calls from inside the kernel.
+These &ldquo;half-interrupts&rdquo; mean that RCU has to be very careful
+about how it counts interrupt nesting levels.
+I learned of this requirement the hard way during a rewrite
+of RCU's dyntick-idle code.
+
+<p>
+The Linux kernel has non-maskable interrupts (NMIs), and
+RCU read-side critical sections are legal within NMI handlers.
+Thankfully, RCU update-side primitives, including
+<tt>call_rcu()</tt>, are prohibited within NMI handlers.
+
+<p>
+The name notwithstanding, some Linux-kernel architectures
+can have nested NMIs, which RCU must handle correctly.
+Andy Lutomirski
+<a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a>
+with this requirement;
+he also kindly surprised me with
+<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
+that meets this requirement.
+
+<h3><a name="Loadable Modules">Loadable Modules</a></h3>
+
+<p>
+The Linux kernel has loadable modules, and these modules can
+also be unloaded.
+After a given module has been unloaded, any attempt to call
+one of its functions results in a segmentation fault.
+The module-unload functions must therefore cancel any
+delayed calls to loadable-module functions, for example,
+any outstanding <tt>mod_timer()</tt> must be dealt with
+via <tt>del_timer_sync()</tt> or similar.
+
+<p>
+Unfortunately, there is no way to cancel an RCU callback;
+once you invoke <tt>call_rcu()</tt>, the callback function is
+going to eventually be invoked, unless the system goes down first.
+Because it is normally considered socially irresponsible to crash the system
+in response to a module unload request, we need some other way
+to deal with in-flight RCU callbacks.
+
+<p>
+RCU therefore provides
+<tt><a href="https://lwn.net/Articles/217484/">rcu_barrier()</a></tt>,
+which waits until all in-flight RCU callbacks have been invoked.
+If a module uses <tt>call_rcu()</tt>, its exit function should therefore
+prevent any future invocation of <tt>call_rcu()</tt>, then invoke
+<tt>rcu_barrier()</tt>.
+In theory, the underlying module-unload code could invoke
+<tt>rcu_barrier()</tt> unconditionally, but in practice this would
+incur unacceptable latencies.
+
+<p>
+Nikita Danilov noted this requirement for an analogous filesystem-unmount
+situation, and Dipankar Sarma incorporated <tt>rcu_barrier()</tt> into RCU.
+The need for <tt>rcu_barrier()</tt> for module unloading became
+apparent later.
+
+<p>
+<b>Important note</b>: The <tt>rcu_barrier()</tt> function is not,
+repeat, <i>not</i>, obligated to wait for a grace period.
+It is instead only required to wait for RCU callbacks that have
+already been posted.
+Therefore, if there are no RCU callbacks posted anywhere in the system,
+<tt>rcu_barrier()</tt> is within its rights to return immediately.
+Even if there are callbacks posted, <tt>rcu_barrier()</tt> does not
+necessarily need to wait for a grace period.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Wait a minute!
+	Each RCU callbacks must wait for a grace period to complete,
+	and <tt>rcu_barrier()</tt> must wait for each pre-existing
+	callback to be invoked.
+	Doesn't <tt>rcu_barrier()</tt> therefore need to wait for
+	a full grace period if there is even one callback posted anywhere
+	in the system?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Absolutely not!!!
+	</font>
+
+	<p><font color="ffffff">
+	Yes, each RCU callbacks must wait for a grace period to complete,
+	but it might well be partly (or even completely) finished waiting
+	by the time <tt>rcu_barrier()</tt> is invoked.
+	In that case, <tt>rcu_barrier()</tt> need only wait for the
+	remaining portion of the grace period to elapse.
+	So even if there are quite a few callbacks posted,
+	<tt>rcu_barrier()</tt> might well return quite quickly.
+	</font>
+
+	<p><font color="ffffff">
+	So if you need to wait for a grace period as well as for all
+	pre-existing callbacks, you will need to invoke both
+	<tt>synchronize_rcu()</tt> and <tt>rcu_barrier()</tt>.
+	If latency is a concern, you can always use workqueues
+	to invoke them concurrently.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<h3><a name="Hotplug CPU">Hotplug CPU</a></h3>
+
+<p>
+The Linux kernel supports CPU hotplug, which means that CPUs
+can come and go.
+It is of course illegal to use any RCU API member from an offline CPU,
+with the exception of <a href="#Sleepable RCU">SRCU</a> read-side
+critical sections.
+This requirement was present from day one in DYNIX/ptx, but
+on the other hand, the Linux kernel's CPU-hotplug implementation
+is &ldquo;interesting.&rdquo;
+
+<p>
+The Linux-kernel CPU-hotplug implementation has notifiers that
+are used to allow the various kernel subsystems (including RCU)
+to respond appropriately to a given CPU-hotplug operation.
+Most RCU operations may be invoked from CPU-hotplug notifiers,
+including even synchronous grace-period operations such as
+<tt>synchronize_rcu()</tt> and <tt>synchronize_rcu_expedited()</tt>.
+
+<p>
+However, all-callback-wait operations such as
+<tt>rcu_barrier()</tt> are also not supported, due to the
+fact that there are phases of CPU-hotplug operations where
+the outgoing CPU's callbacks will not be invoked until after
+the CPU-hotplug operation ends, which could also result in deadlock.
+Furthermore, <tt>rcu_barrier()</tt> blocks CPU-hotplug operations
+during its execution, which results in another type of deadlock
+when invoked from a CPU-hotplug notifier.
+
+<h3><a name="Scheduler and RCU">Scheduler and RCU</a></h3>
+
+<p>
+RCU depends on the scheduler, and the scheduler uses RCU to
+protect some of its data structures.
+This means the scheduler is forbidden from acquiring
+the runqueue locks and the priority-inheritance locks
+in the middle of an outermost RCU read-side critical section unless either
+(1)&nbsp;it releases them before exiting that same
+RCU read-side critical section, or
+(2)&nbsp;interrupts are disabled across
+that entire RCU read-side critical section.
+This same prohibition also applies (recursively!) to any lock that is acquired
+while holding any lock to which this prohibition applies.
+Adhering to this rule prevents preemptible RCU from invoking
+<tt>rcu_read_unlock_special()</tt> while either runqueue or
+priority-inheritance locks are held, thus avoiding deadlock.
+
+<p>
+Prior to v4.4, it was only necessary to disable preemption across
+RCU read-side critical sections that acquired scheduler locks.
+In v4.4, expedited grace periods started using IPIs, and these
+IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath.
+Therefore, this expedited-grace-period change required disabling of
+interrupts, not just preemption.
+
+<p>
+For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt>
+implementation must be written carefully to avoid similar deadlocks.
+In particular, <tt>rcu_read_unlock()</tt> must tolerate an
+interrupt where the interrupt handler invokes both
+<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
+This possibility requires <tt>rcu_read_unlock()</tt> to use
+negative nesting levels to avoid destructive recursion via
+interrupt handler's use of RCU.
+
+<p>
+This pair of mutual scheduler-RCU requirements came as a
+<a href="https://lwn.net/Articles/453002/">complete surprise</a>.
+
+<p>
+As noted above, RCU makes use of kthreads, and it is necessary to
+avoid excessive CPU-time accumulation by these kthreads.
+This requirement was no surprise, but RCU's violation of it
+when running context-switch-heavy workloads when built with
+<tt>CONFIG_NO_HZ_FULL=y</tt>
+<a href="http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf">did come as a surprise [PDF]</a>.
+RCU has made good progress towards meeting this requirement, even
+for context-switch-have <tt>CONFIG_NO_HZ_FULL=y</tt> workloads,
+but there is room for further improvement.
+
+<h3><a name="Tracing and RCU">Tracing and RCU</a></h3>
+
+<p>
+It is possible to use tracing on RCU code, but tracing itself
+uses RCU.
+For this reason, <tt>rcu_dereference_raw_notrace()</tt>
+is provided for use by tracing, which avoids the destructive
+recursion that could otherwise ensue.
+This API is also used by virtualization in some architectures,
+where RCU readers execute in environments in which tracing
+cannot be used.
+The tracing folks both located the requirement and provided the
+needed fix, so this surprise requirement was relatively painless.
+
+<h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
+
+<p>
+Interrupting idle CPUs is considered socially unacceptable,
+especially by people with battery-powered embedded systems.
+RCU therefore conserves energy by detecting which CPUs are
+idle, including tracking CPUs that have been interrupted from idle.
+This is a large part of the energy-efficiency requirement,
+so I learned of this via an irate phone call.
+
+<p>
+Because RCU avoids interrupting idle CPUs, it is illegal to
+execute an RCU read-side critical section on an idle CPU.
+(Kernels built with <tt>CONFIG_PROVE_RCU=y</tt> will splat
+if you try it.)
+The <tt>RCU_NONIDLE()</tt> macro and <tt>_rcuidle</tt>
+event tracing is provided to work around this restriction.
+In addition, <tt>rcu_is_watching()</tt> may be used to
+test whether or not it is currently legal to run RCU read-side
+critical sections on this CPU.
+I learned of the need for diagnostics on the one hand
+and <tt>RCU_NONIDLE()</tt> on the other while inspecting
+idle-loop code.
+Steven Rostedt supplied <tt>_rcuidle</tt> event tracing,
+which is used quite heavily in the idle loop.
+However, there are some restrictions on the code placed within
+<tt>RCU_NONIDLE()</tt>:
+
+<ol>
+<li>	Blocking is prohibited.
+	In practice, this is not a serious restriction given that idle
+	tasks are prohibited from blocking to begin with.
+<li>	Although nesting <tt>RCU_NONIDLE()</tt> is permitted, they cannot
+	nest indefinitely deeply.
+	However, given that they can be nested on the order of a million
+	deep, even on 32-bit systems, this should not be a serious
+	restriction.
+	This nesting limit would probably be reached long after the
+	compiler OOMed or the stack overflowed.
+<li>	Any code path that enters <tt>RCU_NONIDLE()</tt> must sequence
+	out of that same <tt>RCU_NONIDLE()</tt>.
+	For example, the following is grossly illegal:
+
+	<blockquote>
+	<pre>
+ 1     RCU_NONIDLE({
+ 2       do_something();
+ 3       goto bad_idea;  /* BUG!!! */
+ 4       do_something_else();});
+ 5   bad_idea:
+	</pre>
+	</blockquote>
+
+	<p>
+	It is just as illegal to transfer control into the middle of
+	<tt>RCU_NONIDLE()</tt>'s argument.
+	Yes, in theory, you could transfer in as long as you also
+	transferred out, but in practice you could also expect to get sharply
+	worded review comments.
+</ol>
+
+<p>
+It is similarly socially unacceptable to interrupt an
+<tt>nohz_full</tt> CPU running in userspace.
+RCU must therefore track <tt>nohz_full</tt> userspace
+execution.
+RCU must therefore be able to sample state at two points in
+time, and be able to determine whether or not some other CPU spent
+any time idle and/or executing in userspace.
+
+<p>
+These energy-efficiency requirements have proven quite difficult to
+understand and to meet, for example, there have been more than five
+clean-sheet rewrites of RCU's energy-efficiency code, the last of
+which was finally able to demonstrate
+<a href="http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf">real energy savings running on real hardware [PDF]</a>.
+As noted earlier,
+I learned of many of these requirements via angry phone calls:
+Flaming me on the Linux-kernel mailing list was apparently not
+sufficient to fully vent their ire at RCU's energy-efficiency bugs!
+
+<h3><a name="Scheduling-Clock Interrupts and RCU">
+Scheduling-Clock Interrupts and RCU</a></h3>
+
+<p>
+The kernel transitions between in-kernel non-idle execution, userspace
+execution, and the idle loop.
+Depending on kernel configuration, RCU handles these states differently:
+
+<table border=3>
+<tr><th><tt>HZ</tt> Kconfig</th>
+	<th>In-Kernel</th>
+		<th>Usermode</th>
+			<th>Idle</th></tr>
+<tr><th align="left"><tt>HZ_PERIODIC</tt></th>
+	<td>Can rely on scheduling-clock interrupt.</td>
+		<td>Can rely on scheduling-clock interrupt and its
+		    detection of interrupt from usermode.</td>
+			<td>Can rely on RCU's dyntick-idle detection.</td></tr>
+<tr><th align="left"><tt>NO_HZ_IDLE</tt></th>
+	<td>Can rely on scheduling-clock interrupt.</td>
+		<td>Can rely on scheduling-clock interrupt and its
+		    detection of interrupt from usermode.</td>
+			<td>Can rely on RCU's dyntick-idle detection.</td></tr>
+<tr><th align="left"><tt>NO_HZ_FULL</tt></th>
+	<td>Can only sometimes rely on scheduling-clock interrupt.
+	    In other cases, it is necessary to bound kernel execution
+	    times and/or use IPIs.</td>
+		<td>Can rely on RCU's dyntick-idle detection.</td>
+			<td>Can rely on RCU's dyntick-idle detection.</td></tr>
+</table>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	Why can't <tt>NO_HZ_FULL</tt> in-kernel execution rely on the
+	scheduling-clock interrupt, just like <tt>HZ_PERIODIC</tt>
+	and <tt>NO_HZ_IDLE</tt> do?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	Because, as a performance optimization, <tt>NO_HZ_FULL</tt>
+	does not necessarily re-enable the scheduling-clock interrupt
+	on entry to each and every system call.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+However, RCU must be reliably informed as to whether any given
+CPU is currently in the idle loop, and, for <tt>NO_HZ_FULL</tt>,
+also whether that CPU is executing in usermode, as discussed
+<a href="#Energy Efficiency">earlier</a>.
+It also requires that the scheduling-clock interrupt be enabled when
+RCU needs it to be:
+
+<ol>
+<li>	If a CPU is either idle or executing in usermode, and RCU believes
+	it is non-idle, the scheduling-clock tick had better be running.
+	Otherwise, you will get RCU CPU stall warnings.  Or at best,
+	very long (11-second) grace periods, with a pointless IPI waking
+	the CPU from time to time.
+<li>	If a CPU is in a portion of the kernel that executes RCU read-side
+	critical sections, and RCU believes this CPU to be idle, you will get
+	random memory corruption.  <b>DON'T DO THIS!!!</b>
+
+	<br>This is one reason to test with lockdep, which will complain
+	about this sort of thing.
+<li>	If a CPU is in a portion of the kernel that is absolutely
+	positively no-joking guaranteed to never execute any RCU read-side
+	critical sections, and RCU believes this CPU to to be idle,
+	no problem.  This sort of thing is used by some architectures
+	for light-weight exception handlers, which can then avoid the
+	overhead of <tt>rcu_irq_enter()</tt> and <tt>rcu_irq_exit()</tt>
+	at exception entry and exit, respectively.
+	Some go further and avoid the entireties of <tt>irq_enter()</tt>
+	and <tt>irq_exit()</tt>.
+
+	<br>Just make very sure you are running some of your tests with
+	<tt>CONFIG_PROVE_RCU=y</tt>, just in case one of your code paths
+	was in fact joking about not doing RCU read-side critical sections.
+<li>	If a CPU is executing in the kernel with the scheduling-clock
+	interrupt disabled and RCU believes this CPU to be non-idle,
+	and if the CPU goes idle (from an RCU perspective) every few
+	jiffies, no problem.  It is usually OK for there to be the
+	occasional gap between idle periods of up to a second or so.
+
+	<br>If the gap grows too long, you get RCU CPU stall warnings.
+<li>	If a CPU is either idle or executing in usermode, and RCU believes
+	it to be idle, of course no problem.
+<li>	If a CPU is executing in the kernel, the kernel code
+	path is passing through quiescent states at a reasonable
+	frequency (preferably about once per few jiffies, but the
+	occasional excursion to a second or so is usually OK) and the
+	scheduling-clock interrupt is enabled, of course no problem.
+
+	<br>If the gap between a successive pair of quiescent states grows
+	too long, you get RCU CPU stall warnings.
+</ol>
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But what if my driver has a hardware interrupt handler
+	that can run for many seconds?
+	I cannot invoke <tt>schedule()</tt> from an hardware
+	interrupt handler, after all!
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	One approach is to do <tt>rcu_irq_exit();rcu_irq_enter();</tt>
+	every so often.
+	But given that long-running interrupt handlers can cause
+	other problems, not least for response time, shouldn't you
+	work to keep your interrupt handler's runtime within reasonable
+	bounds?
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+But as long as RCU is properly informed of kernel state transitions between
+in-kernel execution, usermode execution, and idle, and as long as the
+scheduling-clock interrupt is enabled when RCU needs it to be, you
+can rest assured that the bugs you encounter will be in some other
+part of RCU or some other part of the kernel!
+
+<h3><a name="Memory Efficiency">Memory Efficiency</a></h3>
+
+<p>
+Although small-memory non-realtime systems can simply use Tiny RCU,
+code size is only one aspect of memory efficiency.
+Another aspect is the size of the <tt>rcu_head</tt> structure
+used by <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>.
+Although this structure contains nothing more than a pair of pointers,
+it does appear in many RCU-protected data structures, including
+some that are size critical.
+The <tt>page</tt> structure is a case in point, as evidenced by
+the many occurrences of the <tt>union</tt> keyword within that structure.
+
+<p>
+This need for memory efficiency is one reason that RCU uses hand-crafted
+singly linked lists to track the <tt>rcu_head</tt> structures that
+are waiting for a grace period to elapse.
+It is also the reason why <tt>rcu_head</tt> structures do not contain
+debug information, such as fields tracking the file and line of the
+<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> that posted them.
+Although this information might appear in debug-only kernel builds at some
+point, in the meantime, the <tt>-&gt;func</tt> field will often provide
+the needed debug information.
+
+<p>
+However, in some cases, the need for memory efficiency leads to even
+more extreme measures.
+Returning to the <tt>page</tt> structure, the <tt>rcu_head</tt> field
+shares storage with a great many other structures that are used at
+various points in the corresponding page's lifetime.
+In order to correctly resolve certain
+<a href="https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com">race conditions</a>,
+the Linux kernel's memory-management subsystem needs a particular bit
+to remain zero during all phases of grace-period processing,
+and that bit happens to map to the bottom bit of the
+<tt>rcu_head</tt> structure's <tt>-&gt;next</tt> field.
+RCU makes this guarantee as long as <tt>call_rcu()</tt>
+is used to post the callback, as opposed to <tt>kfree_rcu()</tt>
+or some future &ldquo;lazy&rdquo;
+variant of <tt>call_rcu()</tt> that might one day be created for
+energy-efficiency purposes.
+
+<p>
+That said, there are limits.
+RCU requires that the <tt>rcu_head</tt> structure be aligned to a
+two-byte boundary, and passing a misaligned <tt>rcu_head</tt>
+structure to one of the <tt>call_rcu()</tt> family of functions
+will result in a splat.
+It is therefore necessary to exercise caution when packing
+structures containing fields of type <tt>rcu_head</tt>.
+Why not a four-byte or even eight-byte alignment requirement?
+Because the m68k architecture provides only two-byte alignment,
+and thus acts as alignment's least common denominator.
+
+<p>
+The reason for reserving the bottom bit of pointers to
+<tt>rcu_head</tt> structures is to leave the door open to
+&ldquo;lazy&rdquo; callbacks whose invocations can safely be deferred.
+Deferring invocation could potentially have energy-efficiency
+benefits, but only if the rate of non-lazy callbacks decreases
+significantly for some important workload.
+In the meantime, reserving the bottom bit keeps this option open
+in case it one day becomes useful.
+
+<h3><a name="Performance, Scalability, Response Time, and Reliability">
+Performance, Scalability, Response Time, and Reliability</a></h3>
+
+<p>
+Expanding on the
+<a href="#Performance and Scalability">earlier discussion</a>,
+RCU is used heavily by hot code paths in performance-critical
+portions of the Linux kernel's networking, security, virtualization,
+and scheduling code paths.
+RCU must therefore use efficient implementations, especially in its
+read-side primitives.
+To that end, it would be good if preemptible RCU's implementation
+of <tt>rcu_read_lock()</tt> could be inlined, however, doing
+this requires resolving <tt>#include</tt> issues with the
+<tt>task_struct</tt> structure.
+
+<p>
+The Linux kernel supports hardware configurations with up to
+4096 CPUs, which means that RCU must be extremely scalable.
+Algorithms that involve frequent acquisitions of global locks or
+frequent atomic operations on global variables simply cannot be
+tolerated within the RCU implementation.
+RCU therefore makes heavy use of a combining tree based on the
+<tt>rcu_node</tt> structure.
+RCU is required to tolerate all CPUs continuously invoking any
+combination of RCU's runtime primitives with minimal per-operation
+overhead.
+In fact, in many cases, increasing load must <i>decrease</i> the
+per-operation overhead, witness the batching optimizations for
+<tt>synchronize_rcu()</tt>, <tt>call_rcu()</tt>,
+<tt>synchronize_rcu_expedited()</tt>, and <tt>rcu_barrier()</tt>.
+As a general rule, RCU must cheerfully accept whatever the
+rest of the Linux kernel decides to throw at it.
+
+<p>
+The Linux kernel is used for real-time workloads, especially
+in conjunction with the
+<a href="https://rt.wiki.kernel.org/index.php/Main_Page">-rt patchset</a>.
+The real-time-latency response requirements are such that the
+traditional approach of disabling preemption across RCU
+read-side critical sections is inappropriate.
+Kernels built with <tt>CONFIG_PREEMPT=y</tt> therefore
+use an RCU implementation that allows RCU read-side critical
+sections to be preempted.
+This requirement made its presence known after users made it
+clear that an earlier
+<a href="https://lwn.net/Articles/107930/">real-time patch</a>
+did not meet their needs, in conjunction with some
+<a href="https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com">RCU issues</a>
+encountered by a very early version of the -rt patchset.
+
+<p>
+In addition, RCU must make do with a sub-100-microsecond real-time latency
+budget.
+In fact, on smaller systems with the -rt patchset, the Linux kernel
+provides sub-20-microsecond real-time latencies for the whole kernel,
+including RCU.
+RCU's scalability and latency must therefore be sufficient for
+these sorts of configurations.
+To my surprise, the sub-100-microsecond real-time latency budget
+<a href="http://www.rdrop.com/users/paulmck/realtime/paper/bigrt.2013.01.31a.LCA.pdf">
+applies to even the largest systems [PDF]</a>,
+up to and including systems with 4096 CPUs.
+This real-time requirement motivated the grace-period kthread, which
+also simplified handling of a number of race conditions.
+
+<p>
+RCU must avoid degrading real-time response for CPU-bound threads, whether
+executing in usermode (which is one use case for
+<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel.
+That said, CPU-bound loops in the kernel must execute
+<tt>cond_resched()</tt> at least once per few tens of milliseconds
+in order to avoid receiving an IPI from RCU.
+
+<p>
+Finally, RCU's status as a synchronization primitive means that
+any RCU failure can result in arbitrary memory corruption that can be
+extremely difficult to debug.
+This means that RCU must be extremely reliable, which in
+practice also means that RCU must have an aggressive stress-test
+suite.
+This stress-test suite is called <tt>rcutorture</tt>.
+
+<p>
+Although the need for <tt>rcutorture</tt> was no surprise,
+the current immense popularity of the Linux kernel is posing
+interesting&mdash;and perhaps unprecedented&mdash;validation
+challenges.
+To see this, keep in mind that there are well over one billion
+instances of the Linux kernel running today, given Android
+smartphones, Linux-powered televisions, and servers.
+This number can be expected to increase sharply with the advent of
+the celebrated Internet of Things.
+
+<p>
+Suppose that RCU contains a race condition that manifests on average
+once per million years of runtime.
+This bug will be occurring about three times per <i>day</i> across
+the installed base.
+RCU could simply hide behind hardware error rates, given that no one
+should really expect their smartphone to last for a million years.
+However, anyone taking too much comfort from this thought should
+consider the fact that in most jurisdictions, a successful multi-year
+test of a given mechanism, which might include a Linux kernel,
+suffices for a number of types of safety-critical certifications.
+In fact, rumor has it that the Linux kernel is already being used
+in production for safety-critical applications.
+I don't know about you, but I would feel quite bad if a bug in RCU
+killed someone.
+Which might explain my recent focus on validation and verification.
+
+<h2><a name="Other RCU Flavors">Other RCU Flavors</a></h2>
+
+<p>
+One of the more surprising things about RCU is that there are now
+no fewer than five <i>flavors</i>, or API families.
+In addition, the primary flavor that has been the sole focus up to
+this point has two different implementations, non-preemptible and
+preemptible.
+The other four flavors are listed below, with requirements for each
+described in a separate section.
+
+<ol>
+<li>	<a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
+<li>	<a href="#Sched Flavor">Sched Flavor</a>
+<li>	<a href="#Sleepable RCU">Sleepable RCU</a>
+<li>	<a href="#Tasks RCU">Tasks RCU</a>
+<li>	<a href="#Waiting for Multiple Grace Periods">
+	Waiting for Multiple Grace Periods</a>
+</ol>
+
+<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
+
+<p>
+The softirq-disable (AKA &ldquo;bottom-half&rdquo;,
+hence the &ldquo;_bh&rdquo; abbreviations)
+flavor of RCU, or <i>RCU-bh</i>, was developed by
+Dipankar Sarma to provide a flavor of RCU that could withstand the
+network-based denial-of-service attacks researched by Robert
+Olsson.
+These attacks placed so much networking load on the system
+that some of the CPUs never exited softirq execution,
+which in turn prevented those CPUs from ever executing a context switch,
+which, in the RCU implementation of that time, prevented grace periods
+from ever ending.
+The result was an out-of-memory condition and a system hang.
+
+<p>
+The solution was the creation of RCU-bh, which does
+<tt>local_bh_disable()</tt>
+across its read-side critical sections, and which uses the transition
+from one type of softirq processing to another as a quiescent state
+in addition to context switch, idle, user mode, and offline.
+This means that RCU-bh grace periods can complete even when some of
+the CPUs execute in softirq indefinitely, thus allowing algorithms
+based on RCU-bh to withstand network-based denial-of-service attacks.
+
+<p>
+Because
+<tt>rcu_read_lock_bh()</tt> and <tt>rcu_read_unlock_bh()</tt>
+disable and re-enable softirq handlers, any attempt to start a softirq
+handlers during the
+RCU-bh read-side critical section will be deferred.
+In this case, <tt>rcu_read_unlock_bh()</tt>
+will invoke softirq processing, which can take considerable time.
+One can of course argue that this softirq overhead should be associated
+with the code following the RCU-bh read-side critical section rather
+than <tt>rcu_read_unlock_bh()</tt>, but the fact
+is that most profiling tools cannot be expected to make this sort
+of fine distinction.
+For example, suppose that a three-millisecond-long RCU-bh read-side
+critical section executes during a time of heavy networking load.
+There will very likely be an attempt to invoke at least one softirq
+handler during that three milliseconds, but any such invocation will
+be delayed until the time of the <tt>rcu_read_unlock_bh()</tt>.
+This can of course make it appear at first glance as if
+<tt>rcu_read_unlock_bh()</tt> was executing very slowly.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-bh API</a>
+includes
+<tt>rcu_read_lock_bh()</tt>,
+<tt>rcu_read_unlock_bh()</tt>,
+<tt>rcu_dereference_bh()</tt>,
+<tt>rcu_dereference_bh_check()</tt>,
+<tt>synchronize_rcu_bh()</tt>,
+<tt>synchronize_rcu_bh_expedited()</tt>,
+<tt>call_rcu_bh()</tt>,
+<tt>rcu_barrier_bh()</tt>, and
+<tt>rcu_read_lock_bh_held()</tt>.
+
+<h3><a name="Sched Flavor">Sched Flavor</a></h3>
+
+<p>
+Before preemptible RCU, waiting for an RCU grace period had the
+side effect of also waiting for all pre-existing interrupt
+and NMI handlers.
+However, there are legitimate preemptible-RCU implementations that
+do not have this property, given that any point in the code outside
+of an RCU read-side critical section can be a quiescent state.
+Therefore, <i>RCU-sched</i> was created, which follows &ldquo;classic&rdquo;
+RCU in that an RCU-sched grace period waits for for pre-existing
+interrupt and NMI handlers.
+In kernels built with <tt>CONFIG_PREEMPT=n</tt>, the RCU and RCU-sched
+APIs have identical implementations, while kernels built with
+<tt>CONFIG_PREEMPT=y</tt> provide a separate implementation for each.
+
+<p>
+Note well that in <tt>CONFIG_PREEMPT=y</tt> kernels,
+<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
+disable and re-enable preemption, respectively.
+This means that if there was a preemption attempt during the
+RCU-sched read-side critical section, <tt>rcu_read_unlock_sched()</tt>
+will enter the scheduler, with all the latency and overhead entailed.
+Just as with <tt>rcu_read_unlock_bh()</tt>, this can make it look
+as if <tt>rcu_read_unlock_sched()</tt> was executing very slowly.
+However, the highest-priority task won't be preempted, so that task
+will enjoy low-overhead <tt>rcu_read_unlock_sched()</tt> invocations.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-sched API</a>
+includes
+<tt>rcu_read_lock_sched()</tt>,
+<tt>rcu_read_unlock_sched()</tt>,
+<tt>rcu_read_lock_sched_notrace()</tt>,
+<tt>rcu_read_unlock_sched_notrace()</tt>,
+<tt>rcu_dereference_sched()</tt>,
+<tt>rcu_dereference_sched_check()</tt>,
+<tt>synchronize_sched()</tt>,
+<tt>synchronize_rcu_sched_expedited()</tt>,
+<tt>call_rcu_sched()</tt>,
+<tt>rcu_barrier_sched()</tt>, and
+<tt>rcu_read_lock_sched_held()</tt>.
+However, anything that disables preemption also marks an RCU-sched
+read-side critical section, including
+<tt>preempt_disable()</tt> and <tt>preempt_enable()</tt>,
+<tt>local_irq_save()</tt> and <tt>local_irq_restore()</tt>,
+and so on.
+
+<h3><a name="Sleepable RCU">Sleepable RCU</a></h3>
+
+<p>
+For well over a decade, someone saying &ldquo;I need to block within
+an RCU read-side critical section&rdquo; was a reliable indication
+that this someone did not understand RCU.
+After all, if you are always blocking in an RCU read-side critical
+section, you can probably afford to use a higher-overhead synchronization
+mechanism.
+However, that changed with the advent of the Linux kernel's notifiers,
+whose RCU read-side critical
+sections almost never sleep, but sometimes need to.
+This resulted in the introduction of
+<a href="https://lwn.net/Articles/202847/">sleepable RCU</a>,
+or <i>SRCU</i>.
+
+<p>
+SRCU allows different domains to be defined, with each such domain
+defined by an instance of an <tt>srcu_struct</tt> structure.
+A pointer to this structure must be passed in to each SRCU function,
+for example, <tt>synchronize_srcu(&amp;ss)</tt>, where
+<tt>ss</tt> is the <tt>srcu_struct</tt> structure.
+The key benefit of these domains is that a slow SRCU reader in one
+domain does not delay an SRCU grace period in some other domain.
+That said, one consequence of these domains is that read-side code
+must pass a &ldquo;cookie&rdquo; from <tt>srcu_read_lock()</tt>
+to <tt>srcu_read_unlock()</tt>, for example, as follows:
+
+<blockquote>
+<pre>
+ 1 int idx;
+ 2
+ 3 idx = srcu_read_lock(&amp;ss);
+ 4 do_something();
+ 5 srcu_read_unlock(&amp;ss, idx);
+</pre>
+</blockquote>
+
+<p>
+As noted above, it is legal to block within SRCU read-side critical sections,
+however, with great power comes great responsibility.
+If you block forever in one of a given domain's SRCU read-side critical
+sections, then that domain's grace periods will also be blocked forever.
+Of course, one good way to block forever is to deadlock, which can
+happen if any operation in a given domain's SRCU read-side critical
+section can block waiting, either directly or indirectly, for that domain's
+grace period to elapse.
+For example, this results in a self-deadlock:
+
+<blockquote>
+<pre>
+ 1 int idx;
+ 2
+ 3 idx = srcu_read_lock(&amp;ss);
+ 4 do_something();
+ 5 synchronize_srcu(&amp;ss);
+ 6 srcu_read_unlock(&amp;ss, idx);
+</pre>
+</blockquote>
+
+<p>
+However, if line&nbsp;5 acquired a mutex that was held across
+a <tt>synchronize_srcu()</tt> for domain <tt>ss</tt>,
+deadlock would still be possible.
+Furthermore, if line&nbsp;5 acquired a mutex that was held across
+a <tt>synchronize_srcu()</tt> for some other domain <tt>ss1</tt>,
+and if an <tt>ss1</tt>-domain SRCU read-side critical section
+acquired another mutex that was held across as <tt>ss</tt>-domain
+<tt>synchronize_srcu()</tt>,
+deadlock would again be possible.
+Such a deadlock cycle could extend across an arbitrarily large number
+of different SRCU domains.
+Again, with great power comes great responsibility.
+
+<p>
+Unlike the other RCU flavors, SRCU read-side critical sections can
+run on idle and even offline CPUs.
+This ability requires that <tt>srcu_read_lock()</tt> and
+<tt>srcu_read_unlock()</tt> contain memory barriers, which means
+that SRCU readers will run a bit slower than would RCU readers.
+It also motivates the <tt>smp_mb__after_srcu_read_unlock()</tt>
+API, which, in combination with <tt>srcu_read_unlock()</tt>,
+guarantees a full memory barrier.
+
+<p>
+Also unlike other RCU flavors, SRCU's callbacks-wait function
+<tt>srcu_barrier()</tt> may be invoked from CPU-hotplug notifiers,
+though this is not necessarily a good idea.
+The reason that this is possible is that SRCU is insensitive
+to whether or not a CPU is online, which means that <tt>srcu_barrier()</tt>
+need not exclude CPU-hotplug operations.
+
+<p>
+SRCU also differs from other RCU flavors in that SRCU's expedited and
+non-expedited grace periods are implemented by the same mechanism.
+This means that in the current SRCU implementation, expediting a
+future grace period has the side effect of expediting all prior
+grace periods that have not yet completed.
+(But please note that this is a property of the current implementation,
+not necessarily of future implementations.)
+In addition, if SRCU has been idle for longer than the interval
+specified by the <tt>srcutree.exp_holdoff</tt> kernel boot parameter
+(25&nbsp;microseconds by default),
+and if a <tt>synchronize_srcu()</tt> invocation ends this idle period,
+that invocation will be automatically expedited.
+
+<p>
+As of v4.12, SRCU's callbacks are maintained per-CPU, eliminating
+a locking bottleneck present in prior kernel versions.
+Although this will allow users to put much heavier stress on
+<tt>call_srcu()</tt>, it is important to note that SRCU does not
+yet take any special steps to deal with callback flooding.
+So if you are posting (say) 10,000 SRCU callbacks per second per CPU,
+you are probably totally OK, but if you intend to post (say) 1,000,000
+SRCU callbacks per second per CPU, please run some tests first.
+SRCU just might need a few adjustment to deal with that sort of load.
+Of course, your mileage may vary based on the speed of your CPUs and
+the size of your memory.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">SRCU API</a>
+includes
+<tt>srcu_read_lock()</tt>,
+<tt>srcu_read_unlock()</tt>,
+<tt>srcu_dereference()</tt>,
+<tt>srcu_dereference_check()</tt>,
+<tt>synchronize_srcu()</tt>,
+<tt>synchronize_srcu_expedited()</tt>,
+<tt>call_srcu()</tt>,
+<tt>srcu_barrier()</tt>, and
+<tt>srcu_read_lock_held()</tt>.
+It also includes
+<tt>DEFINE_SRCU()</tt>,
+<tt>DEFINE_STATIC_SRCU()</tt>, and
+<tt>init_srcu_struct()</tt>
+APIs for defining and initializing <tt>srcu_struct</tt> structures.
+
+<h3><a name="Tasks RCU">Tasks RCU</a></h3>
+
+<p>
+Some forms of tracing use &ldquo;trampolines&rdquo; to handle the
+binary rewriting required to install different types of probes.
+It would be good to be able to free old trampolines, which sounds
+like a job for some form of RCU.
+However, because it is necessary to be able to install a trace
+anywhere in the code, it is not possible to use read-side markers
+such as <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
+In addition, it does not work to have these markers in the trampoline
+itself, because there would need to be instructions following
+<tt>rcu_read_unlock()</tt>.
+Although <tt>synchronize_rcu()</tt> would guarantee that execution
+reached the <tt>rcu_read_unlock()</tt>, it would not be able to
+guarantee that execution had completely left the trampoline.
+
+<p>
+The solution, in the form of
+<a href="https://lwn.net/Articles/607117/"><i>Tasks RCU</i></a>,
+is to have implicit
+read-side critical sections that are delimited by voluntary context
+switches, that is, calls to <tt>schedule()</tt>,
+<tt>cond_resched()</tt>, and
+<tt>synchronize_rcu_tasks()</tt>.
+In addition, transitions to and from userspace execution also delimit
+tasks-RCU read-side critical sections.
+
+<p>
+The tasks-RCU API is quite compact, consisting only of
+<tt>call_rcu_tasks()</tt>,
+<tt>synchronize_rcu_tasks()</tt>, and
+<tt>rcu_barrier_tasks()</tt>.
+
+<h3><a name="Waiting for Multiple Grace Periods">
+Waiting for Multiple Grace Periods</a></h3>
+
+<p>
+Perhaps you have an RCU protected data structure that is accessed from
+RCU read-side critical sections, from softirq handlers, and from
+hardware interrupt handlers.
+That is three flavors of RCU, the normal flavor, the bottom-half flavor,
+and the sched flavor.
+How to wait for a compound grace period?
+
+<p>
+The best approach is usually to &ldquo;just say no!&rdquo; and
+insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+around each RCU read-side critical section, regardless of what
+environment it happens to be in.
+But suppose that some of the RCU read-side critical sections are
+on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
+is not a viable option, so that <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> are not free.
+What then?
+
+<p>
+You <i>could</i> wait on all three grace periods in succession, as follows:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu();
+ 2 synchronize_rcu_bh();
+ 3 synchronize_sched();
+</pre>
+</blockquote>
+
+<p>
+This works, but triples the update-side latency penalty.
+In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
+may be used to wait on all three flavors of grace period concurrently:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
+</pre>
+</blockquote>
+
+<p>
+But what if it is necessary to also wait on SRCU?
+This can be done as follows:
+
+<blockquote>
+<pre>
+ 1 static void call_my_srcu(struct rcu_head *head,
+ 2        void (*func)(struct rcu_head *head))
+ 3 {
+ 4   call_srcu(&amp;my_srcu, head, func);
+ 5 }
+ 6
+ 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
+</pre>
+</blockquote>
+
+<p>
+If you needed to wait on multiple different flavors of SRCU
+(but why???), you would need to create a wrapper function resembling
+<tt>call_my_srcu()</tt> for each SRCU flavor.
+
+<table>
+<tr><th>&nbsp;</th></tr>
+<tr><th align="left">Quick Quiz:</th></tr>
+<tr><td>
+	But what if I need to wait for multiple RCU flavors, but I also need
+	the grace periods to be expedited?
+</td></tr>
+<tr><th align="left">Answer:</th></tr>
+<tr><td bgcolor="#ffffff"><font color="ffffff">
+	If you are using expedited grace periods, there should be less penalty
+	for waiting on them in succession.
+	But if that is nevertheless a problem, you can use workqueues
+	or multiple kthreads to wait on the various expedited grace
+	periods concurrently.
+</font></td></tr>
+<tr><td>&nbsp;</td></tr>
+</table>
+
+<p>
+Again, it is usually better to adjust the RCU read-side critical sections
+to use a single flavor of RCU, but when this is not feasible, you can use
+<tt>synchronize_rcu_mult()</tt>.
+
+<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
+
+<p>
+One of the tricks that RCU uses to attain update-side scalability is
+to increase grace-period latency with increasing numbers of CPUs.
+If this becomes a serious problem, it will be necessary to rework the
+grace-period state machine so as to avoid the need for the additional
+latency.
+
+<p>
+Expedited grace periods scan the CPUs, so their latency and overhead
+increases with increasing numbers of CPUs.
+If this becomes a serious problem on large systems, it will be necessary
+to do some redesign to avoid this scalability problem.
+
+<p>
+RCU disables CPU hotplug in a few places, perhaps most notably in the
+<tt>rcu_barrier()</tt> operations.
+If there is a strong reason to use <tt>rcu_barrier()</tt> in CPU-hotplug
+notifiers, it will be necessary to avoid disabling CPU hotplug.
+This would introduce some complexity, so there had better be a <i>very</i>
+good reason.
+
+<p>
+The tradeoff between grace-period latency on the one hand and interruptions
+of other CPUs on the other hand may need to be re-examined.
+The desire is of course for zero grace-period latency as well as zero
+interprocessor interrupts undertaken during an expedited grace period
+operation.
+While this ideal is unlikely to be achievable, it is quite possible that
+further improvements can be made.
+
+<p>
+The multiprocessor implementations of RCU use a combining tree that
+groups CPUs so as to reduce lock contention and increase cache locality.
+However, this combining tree does not spread its memory across NUMA
+nodes nor does it align the CPU groups with hardware features such
+as sockets or cores.
+Such spreading and alignment is currently believed to be unnecessary
+because the hotpath read-side primitives do not access the combining
+tree, nor does <tt>call_rcu()</tt> in the common case.
+If you believe that your architecture needs such spreading and alignment,
+then your architecture should also benefit from the
+<tt>rcutree.rcu_fanout_leaf</tt> boot parameter, which can be set
+to the number of CPUs in a socket, NUMA node, or whatever.
+If the number of CPUs is too large, use a fraction of the number of
+CPUs.
+If the number of CPUs is a large prime number, well, that certainly
+is an &ldquo;interesting&rdquo; architectural choice!
+More flexible arrangements might be considered, but only if
+<tt>rcutree.rcu_fanout_leaf</tt> has proven inadequate, and only
+if the inadequacy has been demonstrated by a carefully run and
+realistic system-level workload.
+
+<p>
+Please note that arrangements that require RCU to remap CPU numbers will
+require extremely good demonstration of need and full exploration of
+alternatives.
+
+<p>
+There is an embarrassingly large number of flavors of RCU, and this
+number has been increasing over time.
+Perhaps it will be possible to combine some at some future date.
+
+<p>
+RCU's various kthreads are reasonably recent additions.
+It is quite likely that adjustments will be required to more gracefully
+handle extreme loads.
+It might also be necessary to be able to relate CPU utilization by
+RCU's kthreads and softirq handlers to the code that instigated this
+CPU utilization.
+For example, RCU callback overhead might be charged back to the
+originating <tt>call_rcu()</tt> instance, though probably not
+in production kernels.
+
+<h2><a name="Summary">Summary</a></h2>
+
+<p>
+This document has presented more than two decade's worth of RCU
+requirements.
+Given that the requirements keep changing, this will not be the last
+word on this subject, but at least it serves to get an important
+subset of the requirements set forth.
+
+<h2><a name="Acknowledgments">Acknowledgments</a></h2>
+
+I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar,
+Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and
+Andy Lutomirski for their help in rendering
+this article human readable, and to Michelle Rankin for her support
+of this effort.
+Other contributions are acknowledged in the Linux kernel's git archive.
+
+</body></html>