v4.19.13 snapshot.
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
new file mode 100644
index 0000000..3064c50
--- /dev/null
+++ b/kernel/locking/rwsem-xadd.c
@@ -0,0 +1,707 @@
+// SPDX-License-Identifier: GPL-2.0
+/* rwsem.c: R/W semaphores: contention handling functions
+ *
+ * Written by David Howells (dhowells@redhat.com).
+ * Derived from arch/i386/kernel/semaphore.c
+ *
+ * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
+ * and Michel Lespinasse <walken@google.com>
+ *
+ * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
+ * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
+ */
+#include <linux/rwsem.h>
+#include <linux/init.h>
+#include <linux/export.h>
+#include <linux/sched/signal.h>
+#include <linux/sched/rt.h>
+#include <linux/sched/wake_q.h>
+#include <linux/sched/debug.h>
+#include <linux/osq_lock.h>
+
+#include "rwsem.h"
+
+/*
+ * Guide to the rw_semaphore's count field for common values.
+ * (32-bit case illustrated, similar for 64-bit)
+ *
+ * 0x0000000X (1) X readers active or attempting lock, no writer waiting
+ * X = #active_readers + #readers attempting to lock
+ * (X*ACTIVE_BIAS)
+ *
+ * 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
+ * attempting to read lock or write lock.
+ *
+ * 0xffff000X (1) X readers active or attempting lock, with waiters for lock
+ * X = #active readers + # readers attempting lock
+ * (X*ACTIVE_BIAS + WAITING_BIAS)
+ * (2) 1 writer attempting lock, no waiters for lock
+ * X-1 = #active readers + #readers attempting lock
+ * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
+ * (3) 1 writer active, no waiters for lock
+ * X-1 = #active readers + #readers attempting lock
+ * ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
+ *
+ * 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
+ * (WAITING_BIAS + ACTIVE_BIAS)
+ * (2) 1 writer active or attempting lock, no waiters for lock
+ * (ACTIVE_WRITE_BIAS)
+ *
+ * 0xffff0000 (1) There are writers or readers queued but none active
+ * or in the process of attempting lock.
+ * (WAITING_BIAS)
+ * Note: writer can attempt to steal lock for this count by adding
+ * ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
+ *
+ * 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
+ * (ACTIVE_WRITE_BIAS + WAITING_BIAS)
+ *
+ * Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
+ * the count becomes more than 0 for successful lock acquisition,
+ * i.e. the case where there are only readers or nobody has lock.
+ * (1st and 2nd case above).
+ *
+ * Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
+ * checking the count becomes ACTIVE_WRITE_BIAS for successful lock
+ * acquisition (i.e. nobody else has lock or attempts lock). If
+ * unsuccessful, in rwsem_down_write_failed, we'll check to see if there
+ * are only waiters but none active (5th case above), and attempt to
+ * steal the lock.
+ *
+ */
+
+/*
+ * Initialize an rwsem:
+ */
+void __init_rwsem(struct rw_semaphore *sem, const char *name,
+ struct lock_class_key *key)
+{
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /*
+ * Make sure we are not reinitializing a held semaphore:
+ */
+ debug_check_no_locks_freed((void *)sem, sizeof(*sem));
+ lockdep_init_map(&sem->dep_map, name, key, 0);
+#endif
+ atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
+ raw_spin_lock_init(&sem->wait_lock);
+ INIT_LIST_HEAD(&sem->wait_list);
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+ sem->owner = NULL;
+ osq_lock_init(&sem->osq);
+#endif
+}
+
+EXPORT_SYMBOL(__init_rwsem);
+
+enum rwsem_waiter_type {
+ RWSEM_WAITING_FOR_WRITE,
+ RWSEM_WAITING_FOR_READ
+};
+
+struct rwsem_waiter {
+ struct list_head list;
+ struct task_struct *task;
+ enum rwsem_waiter_type type;
+};
+
+enum rwsem_wake_type {
+ RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
+ RWSEM_WAKE_READERS, /* Wake readers only */
+ RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
+};
+
+/*
+ * handle the lock release when processes blocked on it that can now run
+ * - if we come here from up_xxxx(), then:
+ * - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
+ * - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
+ * - there must be someone on the queue
+ * - the wait_lock must be held by the caller
+ * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
+ * to actually wakeup the blocked task(s) and drop the reference count,
+ * preferably when the wait_lock is released
+ * - woken process blocks are discarded from the list after having task zeroed
+ * - writers are only marked woken if downgrading is false
+ */
+static void __rwsem_mark_wake(struct rw_semaphore *sem,
+ enum rwsem_wake_type wake_type,
+ struct wake_q_head *wake_q)
+{
+ struct rwsem_waiter *waiter, *tmp;
+ long oldcount, woken = 0, adjustment = 0;
+
+ /*
+ * Take a peek at the queue head waiter such that we can determine
+ * the wakeup(s) to perform.
+ */
+ waiter = list_first_entry(&sem->wait_list, struct rwsem_waiter, list);
+
+ if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
+ if (wake_type == RWSEM_WAKE_ANY) {
+ /*
+ * Mark writer at the front of the queue for wakeup.
+ * Until the task is actually later awoken later by
+ * the caller, other writers are able to steal it.
+ * Readers, on the other hand, will block as they
+ * will notice the queued writer.
+ */
+ wake_q_add(wake_q, waiter->task);
+ }
+
+ return;
+ }
+
+ /*
+ * Writers might steal the lock before we grant it to the next reader.
+ * We prefer to do the first reader grant before counting readers
+ * so we can bail out early if a writer stole the lock.
+ */
+ if (wake_type != RWSEM_WAKE_READ_OWNED) {
+ adjustment = RWSEM_ACTIVE_READ_BIAS;
+ try_reader_grant:
+ oldcount = atomic_long_fetch_add(adjustment, &sem->count);
+ if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
+ /*
+ * If the count is still less than RWSEM_WAITING_BIAS
+ * after removing the adjustment, it is assumed that
+ * a writer has stolen the lock. We have to undo our
+ * reader grant.
+ */
+ if (atomic_long_add_return(-adjustment, &sem->count) <
+ RWSEM_WAITING_BIAS)
+ return;
+
+ /* Last active locker left. Retry waking readers. */
+ goto try_reader_grant;
+ }
+ /*
+ * It is not really necessary to set it to reader-owned here,
+ * but it gives the spinners an early indication that the
+ * readers now have the lock.
+ */
+ rwsem_set_reader_owned(sem);
+ }
+
+ /*
+ * Grant an infinite number of read locks to the readers at the front
+ * of the queue. We know that woken will be at least 1 as we accounted
+ * for above. Note we increment the 'active part' of the count by the
+ * number of readers before waking any processes up.
+ */
+ list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
+ struct task_struct *tsk;
+
+ if (waiter->type == RWSEM_WAITING_FOR_WRITE)
+ break;
+
+ woken++;
+ tsk = waiter->task;
+
+ wake_q_add(wake_q, tsk);
+ list_del(&waiter->list);
+ /*
+ * Ensure that the last operation is setting the reader
+ * waiter to nil such that rwsem_down_read_failed() cannot
+ * race with do_exit() by always holding a reference count
+ * to the task to wakeup.
+ */
+ smp_store_release(&waiter->task, NULL);
+ }
+
+ adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
+ if (list_empty(&sem->wait_list)) {
+ /* hit end of list above */
+ adjustment -= RWSEM_WAITING_BIAS;
+ }
+
+ if (adjustment)
+ atomic_long_add(adjustment, &sem->count);
+}
+
+/*
+ * Wait for the read lock to be granted
+ */
+static inline struct rw_semaphore __sched *
+__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
+{
+ long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
+ struct rwsem_waiter waiter;
+ DEFINE_WAKE_Q(wake_q);
+
+ waiter.task = current;
+ waiter.type = RWSEM_WAITING_FOR_READ;
+
+ raw_spin_lock_irq(&sem->wait_lock);
+ if (list_empty(&sem->wait_list))
+ adjustment += RWSEM_WAITING_BIAS;
+ list_add_tail(&waiter.list, &sem->wait_list);
+
+ /* we're now waiting on the lock, but no longer actively locking */
+ count = atomic_long_add_return(adjustment, &sem->count);
+
+ /*
+ * If there are no active locks, wake the front queued process(es).
+ *
+ * If there are no writers and we are first in the queue,
+ * wake our own waiter to join the existing active readers !
+ */
+ if (count == RWSEM_WAITING_BIAS ||
+ (count > RWSEM_WAITING_BIAS &&
+ adjustment != -RWSEM_ACTIVE_READ_BIAS))
+ __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
+
+ raw_spin_unlock_irq(&sem->wait_lock);
+ wake_up_q(&wake_q);
+
+ /* wait to be given the lock */
+ while (true) {
+ set_current_state(state);
+ if (!waiter.task)
+ break;
+ if (signal_pending_state(state, current)) {
+ raw_spin_lock_irq(&sem->wait_lock);
+ if (waiter.task)
+ goto out_nolock;
+ raw_spin_unlock_irq(&sem->wait_lock);
+ break;
+ }
+ schedule();
+ }
+
+ __set_current_state(TASK_RUNNING);
+ return sem;
+out_nolock:
+ list_del(&waiter.list);
+ if (list_empty(&sem->wait_list))
+ atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
+ raw_spin_unlock_irq(&sem->wait_lock);
+ __set_current_state(TASK_RUNNING);
+ return ERR_PTR(-EINTR);
+}
+
+__visible struct rw_semaphore * __sched
+rwsem_down_read_failed(struct rw_semaphore *sem)
+{
+ return __rwsem_down_read_failed_common(sem, TASK_UNINTERRUPTIBLE);
+}
+EXPORT_SYMBOL(rwsem_down_read_failed);
+
+__visible struct rw_semaphore * __sched
+rwsem_down_read_failed_killable(struct rw_semaphore *sem)
+{
+ return __rwsem_down_read_failed_common(sem, TASK_KILLABLE);
+}
+EXPORT_SYMBOL(rwsem_down_read_failed_killable);
+
+/*
+ * This function must be called with the sem->wait_lock held to prevent
+ * race conditions between checking the rwsem wait list and setting the
+ * sem->count accordingly.
+ */
+static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
+{
+ /*
+ * Avoid trying to acquire write lock if count isn't RWSEM_WAITING_BIAS.
+ */
+ if (count != RWSEM_WAITING_BIAS)
+ return false;
+
+ /*
+ * Acquire the lock by trying to set it to ACTIVE_WRITE_BIAS. If there
+ * are other tasks on the wait list, we need to add on WAITING_BIAS.
+ */
+ count = list_is_singular(&sem->wait_list) ?
+ RWSEM_ACTIVE_WRITE_BIAS :
+ RWSEM_ACTIVE_WRITE_BIAS + RWSEM_WAITING_BIAS;
+
+ if (atomic_long_cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS, count)
+ == RWSEM_WAITING_BIAS) {
+ rwsem_set_owner(sem);
+ return true;
+ }
+
+ return false;
+}
+
+#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
+/*
+ * Try to acquire write lock before the writer has been put on wait queue.
+ */
+static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
+{
+ long old, count = atomic_long_read(&sem->count);
+
+ while (true) {
+ if (!(count == 0 || count == RWSEM_WAITING_BIAS))
+ return false;
+
+ old = atomic_long_cmpxchg_acquire(&sem->count, count,
+ count + RWSEM_ACTIVE_WRITE_BIAS);
+ if (old == count) {
+ rwsem_set_owner(sem);
+ return true;
+ }
+
+ count = old;
+ }
+}
+
+static inline bool owner_on_cpu(struct task_struct *owner)
+{
+ /*
+ * As lock holder preemption issue, we both skip spinning if
+ * task is not on cpu or its cpu is preempted
+ */
+ return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
+}
+
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
+{
+ struct task_struct *owner;
+ bool ret = true;
+
+ BUILD_BUG_ON(!rwsem_has_anonymous_owner(RWSEM_OWNER_UNKNOWN));
+
+ if (need_resched())
+ return false;
+
+ rcu_read_lock();
+ owner = READ_ONCE(sem->owner);
+ if (owner) {
+ ret = is_rwsem_owner_spinnable(owner) &&
+ owner_on_cpu(owner);
+ }
+ rcu_read_unlock();
+ return ret;
+}
+
+/*
+ * Return true only if we can still spin on the owner field of the rwsem.
+ */
+static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
+{
+ struct task_struct *owner = READ_ONCE(sem->owner);
+
+ if (!is_rwsem_owner_spinnable(owner))
+ return false;
+
+ rcu_read_lock();
+ while (owner && (READ_ONCE(sem->owner) == owner)) {
+ /*
+ * Ensure we emit the owner->on_cpu, dereference _after_
+ * checking sem->owner still matches owner, if that fails,
+ * owner might point to free()d memory, if it still matches,
+ * the rcu_read_lock() ensures the memory stays valid.
+ */
+ barrier();
+
+ /*
+ * abort spinning when need_resched or owner is not running or
+ * owner's cpu is preempted.
+ */
+ if (need_resched() || !owner_on_cpu(owner)) {
+ rcu_read_unlock();
+ return false;
+ }
+
+ cpu_relax();
+ }
+ rcu_read_unlock();
+
+ /*
+ * If there is a new owner or the owner is not set, we continue
+ * spinning.
+ */
+ return is_rwsem_owner_spinnable(READ_ONCE(sem->owner));
+}
+
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+{
+ bool taken = false;
+
+ preempt_disable();
+
+ /* sem->wait_lock should not be held when doing optimistic spinning */
+ if (!rwsem_can_spin_on_owner(sem))
+ goto done;
+
+ if (!osq_lock(&sem->osq))
+ goto done;
+
+ /*
+ * Optimistically spin on the owner field and attempt to acquire the
+ * lock whenever the owner changes. Spinning will be stopped when:
+ * 1) the owning writer isn't running; or
+ * 2) readers own the lock as we can't determine if they are
+ * actively running or not.
+ */
+ while (rwsem_spin_on_owner(sem)) {
+ /*
+ * Try to acquire the lock
+ */
+ if (rwsem_try_write_lock_unqueued(sem)) {
+ taken = true;
+ break;
+ }
+
+ /*
+ * When there's no owner, we might have preempted between the
+ * owner acquiring the lock and setting the owner field. If
+ * we're an RT task that will live-lock because we won't let
+ * the owner complete.
+ */
+ if (!sem->owner && (need_resched() || rt_task(current)))
+ break;
+
+ /*
+ * The cpu_relax() call is a compiler barrier which forces
+ * everything in this loop to be re-loaded. We don't need
+ * memory barriers as we'll eventually observe the right
+ * values at the cost of a few extra spins.
+ */
+ cpu_relax();
+ }
+ osq_unlock(&sem->osq);
+done:
+ preempt_enable();
+ return taken;
+}
+
+/*
+ * Return true if the rwsem has active spinner
+ */
+static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
+{
+ return osq_is_locked(&sem->osq);
+}
+
+#else
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+{
+ return false;
+}
+
+static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
+{
+ return false;
+}
+#endif
+
+/*
+ * Wait until we successfully acquire the write lock
+ */
+static inline struct rw_semaphore *
+__rwsem_down_write_failed_common(struct rw_semaphore *sem, int state)
+{
+ long count;
+ bool waiting = true; /* any queued threads before us */
+ struct rwsem_waiter waiter;
+ struct rw_semaphore *ret = sem;
+ DEFINE_WAKE_Q(wake_q);
+
+ /* undo write bias from down_write operation, stop active locking */
+ count = atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS, &sem->count);
+
+ /* do optimistic spinning and steal lock if possible */
+ if (rwsem_optimistic_spin(sem))
+ return sem;
+
+ /*
+ * Optimistic spinning failed, proceed to the slowpath
+ * and block until we can acquire the sem.
+ */
+ waiter.task = current;
+ waiter.type = RWSEM_WAITING_FOR_WRITE;
+
+ raw_spin_lock_irq(&sem->wait_lock);
+
+ /* account for this before adding a new element to the list */
+ if (list_empty(&sem->wait_list))
+ waiting = false;
+
+ list_add_tail(&waiter.list, &sem->wait_list);
+
+ /* we're now waiting on the lock, but no longer actively locking */
+ if (waiting) {
+ count = atomic_long_read(&sem->count);
+
+ /*
+ * If there were already threads queued before us and there are
+ * no active writers, the lock must be read owned; so we try to
+ * wake any read locks that were queued ahead of us.
+ */
+ if (count > RWSEM_WAITING_BIAS) {
+ __rwsem_mark_wake(sem, RWSEM_WAKE_READERS, &wake_q);
+ /*
+ * The wakeup is normally called _after_ the wait_lock
+ * is released, but given that we are proactively waking
+ * readers we can deal with the wake_q overhead as it is
+ * similar to releasing and taking the wait_lock again
+ * for attempting rwsem_try_write_lock().
+ */
+ wake_up_q(&wake_q);
+
+ /*
+ * Reinitialize wake_q after use.
+ */
+ wake_q_init(&wake_q);
+ }
+
+ } else
+ count = atomic_long_add_return(RWSEM_WAITING_BIAS, &sem->count);
+
+ /* wait until we successfully acquire the lock */
+ set_current_state(state);
+ while (true) {
+ if (rwsem_try_write_lock(count, sem))
+ break;
+ raw_spin_unlock_irq(&sem->wait_lock);
+
+ /* Block until there are no active lockers. */
+ do {
+ if (signal_pending_state(state, current))
+ goto out_nolock;
+
+ schedule();
+ set_current_state(state);
+ } while ((count = atomic_long_read(&sem->count)) & RWSEM_ACTIVE_MASK);
+
+ raw_spin_lock_irq(&sem->wait_lock);
+ }
+ __set_current_state(TASK_RUNNING);
+ list_del(&waiter.list);
+ raw_spin_unlock_irq(&sem->wait_lock);
+
+ return ret;
+
+out_nolock:
+ __set_current_state(TASK_RUNNING);
+ raw_spin_lock_irq(&sem->wait_lock);
+ list_del(&waiter.list);
+ if (list_empty(&sem->wait_list))
+ atomic_long_add(-RWSEM_WAITING_BIAS, &sem->count);
+ else
+ __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
+ raw_spin_unlock_irq(&sem->wait_lock);
+ wake_up_q(&wake_q);
+
+ return ERR_PTR(-EINTR);
+}
+
+__visible struct rw_semaphore * __sched
+rwsem_down_write_failed(struct rw_semaphore *sem)
+{
+ return __rwsem_down_write_failed_common(sem, TASK_UNINTERRUPTIBLE);
+}
+EXPORT_SYMBOL(rwsem_down_write_failed);
+
+__visible struct rw_semaphore * __sched
+rwsem_down_write_failed_killable(struct rw_semaphore *sem)
+{
+ return __rwsem_down_write_failed_common(sem, TASK_KILLABLE);
+}
+EXPORT_SYMBOL(rwsem_down_write_failed_killable);
+
+/*
+ * handle waking up a waiter on the semaphore
+ * - up_read/up_write has decremented the active part of count if we come here
+ */
+__visible
+struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+ DEFINE_WAKE_Q(wake_q);
+
+ /*
+ * __rwsem_down_write_failed_common(sem)
+ * rwsem_optimistic_spin(sem)
+ * osq_unlock(sem->osq)
+ * ...
+ * atomic_long_add_return(&sem->count)
+ *
+ * - VS -
+ *
+ * __up_write()
+ * if (atomic_long_sub_return_release(&sem->count) < 0)
+ * rwsem_wake(sem)
+ * osq_is_locked(&sem->osq)
+ *
+ * And __up_write() must observe !osq_is_locked() when it observes the
+ * atomic_long_add_return() in order to not miss a wakeup.
+ *
+ * This boils down to:
+ *
+ * [S.rel] X = 1 [RmW] r0 = (Y += 0)
+ * MB RMB
+ * [RmW] Y += 1 [L] r1 = X
+ *
+ * exists (r0=1 /\ r1=0)
+ */
+ smp_rmb();
+
+ /*
+ * If a spinner is present, it is not necessary to do the wakeup.
+ * Try to do wakeup only if the trylock succeeds to minimize
+ * spinlock contention which may introduce too much delay in the
+ * unlock operation.
+ *
+ * spinning writer up_write/up_read caller
+ * --------------- -----------------------
+ * [S] osq_unlock() [L] osq
+ * MB RMB
+ * [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
+ *
+ * Here, it is important to make sure that there won't be a missed
+ * wakeup while the rwsem is free and the only spinning writer goes
+ * to sleep without taking the rwsem. Even when the spinning writer
+ * is just going to break out of the waiting loop, it will still do
+ * a trylock in rwsem_down_write_failed() before sleeping. IOW, if
+ * rwsem_has_spinner() is true, it will guarantee at least one
+ * trylock attempt on the rwsem later on.
+ */
+ if (rwsem_has_spinner(sem)) {
+ /*
+ * The smp_rmb() here is to make sure that the spinner
+ * state is consulted before reading the wait_lock.
+ */
+ smp_rmb();
+ if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
+ return sem;
+ goto locked;
+ }
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+locked:
+
+ if (!list_empty(&sem->wait_list))
+ __rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+ wake_up_q(&wake_q);
+
+ return sem;
+}
+EXPORT_SYMBOL(rwsem_wake);
+
+/*
+ * downgrade a write lock into a read lock
+ * - caller incremented waiting part of count and discovered it still negative
+ * - just wake up any readers at the front of the queue
+ */
+__visible
+struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
+{
+ unsigned long flags;
+ DEFINE_WAKE_Q(wake_q);
+
+ raw_spin_lock_irqsave(&sem->wait_lock, flags);
+
+ if (!list_empty(&sem->wait_list))
+ __rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
+
+ raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
+ wake_up_q(&wake_q);
+
+ return sem;
+}
+EXPORT_SYMBOL(rwsem_downgrade_wake);