v4.19.13 snapshot.
diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
new file mode 100644
index 0000000..883cf1b
--- /dev/null
+++ b/kernel/locking/percpu-rwsem.c
@@ -0,0 +1,192 @@
+#include <linux/atomic.h>
+#include <linux/rwsem.h>
+#include <linux/percpu.h>
+#include <linux/lockdep.h>
+#include <linux/percpu-rwsem.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/errno.h>
+
+int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
+ const char *name, struct lock_class_key *rwsem_key)
+{
+ sem->read_count = alloc_percpu(int);
+ if (unlikely(!sem->read_count))
+ return -ENOMEM;
+
+ /* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */
+ rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);
+ __init_rwsem(&sem->rw_sem, name, rwsem_key);
+ rcuwait_init(&sem->writer);
+ sem->readers_block = 0;
+ return 0;
+}
+EXPORT_SYMBOL_GPL(__percpu_init_rwsem);
+
+void percpu_free_rwsem(struct percpu_rw_semaphore *sem)
+{
+ /*
+ * XXX: temporary kludge. The error path in alloc_super()
+ * assumes that percpu_free_rwsem() is safe after kzalloc().
+ */
+ if (!sem->read_count)
+ return;
+
+ rcu_sync_dtor(&sem->rss);
+ free_percpu(sem->read_count);
+ sem->read_count = NULL; /* catch use after free bugs */
+}
+EXPORT_SYMBOL_GPL(percpu_free_rwsem);
+
+int __percpu_down_read(struct percpu_rw_semaphore *sem, int try)
+{
+ /*
+ * Due to having preemption disabled the decrement happens on
+ * the same CPU as the increment, avoiding the
+ * increment-on-one-CPU-and-decrement-on-another problem.
+ *
+ * If the reader misses the writer's assignment of readers_block, then
+ * the writer is guaranteed to see the reader's increment.
+ *
+ * Conversely, any readers that increment their sem->read_count after
+ * the writer looks are guaranteed to see the readers_block value,
+ * which in turn means that they are guaranteed to immediately
+ * decrement their sem->read_count, so that it doesn't matter that the
+ * writer missed them.
+ */
+
+ smp_mb(); /* A matches D */
+
+ /*
+ * If !readers_block the critical section starts here, matched by the
+ * release in percpu_up_write().
+ */
+ if (likely(!smp_load_acquire(&sem->readers_block)))
+ return 1;
+
+ /*
+ * Per the above comment; we still have preemption disabled and
+ * will thus decrement on the same CPU as we incremented.
+ */
+ __percpu_up_read(sem);
+
+ if (try)
+ return 0;
+
+ /*
+ * We either call schedule() in the wait, or we'll fall through
+ * and reschedule on the preempt_enable() in percpu_down_read().
+ */
+ preempt_enable_no_resched();
+
+ /*
+ * Avoid lockdep for the down/up_read() we already have them.
+ */
+ __down_read(&sem->rw_sem);
+ this_cpu_inc(*sem->read_count);
+ __up_read(&sem->rw_sem);
+
+ preempt_disable();
+ return 1;
+}
+EXPORT_SYMBOL_GPL(__percpu_down_read);
+
+void __percpu_up_read(struct percpu_rw_semaphore *sem)
+{
+ smp_mb(); /* B matches C */
+ /*
+ * In other words, if they see our decrement (presumably to aggregate
+ * zero, as that is the only time it matters) they will also see our
+ * critical section.
+ */
+ __this_cpu_dec(*sem->read_count);
+
+ /* Prod writer to recheck readers_active */
+ rcuwait_wake_up(&sem->writer);
+}
+EXPORT_SYMBOL_GPL(__percpu_up_read);
+
+#define per_cpu_sum(var) \
+({ \
+ typeof(var) __sum = 0; \
+ int cpu; \
+ compiletime_assert_atomic_type(__sum); \
+ for_each_possible_cpu(cpu) \
+ __sum += per_cpu(var, cpu); \
+ __sum; \
+})
+
+/*
+ * Return true if the modular sum of the sem->read_count per-CPU variable is
+ * zero. If this sum is zero, then it is stable due to the fact that if any
+ * newly arriving readers increment a given counter, they will immediately
+ * decrement that same counter.
+ */
+static bool readers_active_check(struct percpu_rw_semaphore *sem)
+{
+ if (per_cpu_sum(*sem->read_count) != 0)
+ return false;
+
+ /*
+ * If we observed the decrement; ensure we see the entire critical
+ * section.
+ */
+
+ smp_mb(); /* C matches B */
+
+ return true;
+}
+
+void percpu_down_write(struct percpu_rw_semaphore *sem)
+{
+ /* Notify readers to take the slow path. */
+ rcu_sync_enter(&sem->rss);
+
+ down_write(&sem->rw_sem);
+
+ /*
+ * Notify new readers to block; up until now, and thus throughout the
+ * longish rcu_sync_enter() above, new readers could still come in.
+ */
+ WRITE_ONCE(sem->readers_block, 1);
+
+ smp_mb(); /* D matches A */
+
+ /*
+ * If they don't see our writer of readers_block, then we are
+ * guaranteed to see their sem->read_count increment, and therefore
+ * will wait for them.
+ */
+
+ /* Wait for all now active readers to complete. */
+ rcuwait_wait_event(&sem->writer, readers_active_check(sem));
+}
+EXPORT_SYMBOL_GPL(percpu_down_write);
+
+void percpu_up_write(struct percpu_rw_semaphore *sem)
+{
+ /*
+ * Signal the writer is done, no fast path yet.
+ *
+ * One reason that we cannot just immediately flip to readers_fast is
+ * that new readers might fail to see the results of this writer's
+ * critical section.
+ *
+ * Therefore we force it through the slow path which guarantees an
+ * acquire and thereby guarantees the critical section's consistency.
+ */
+ smp_store_release(&sem->readers_block, 0);
+
+ /*
+ * Release the write lock, this will allow readers back in the game.
+ */
+ up_write(&sem->rw_sem);
+
+ /*
+ * Once this completes (at least one RCU-sched grace period hence) the
+ * reader fast path will be available again. Safe to use outside the
+ * exclusive write lock because its counting.
+ */
+ rcu_sync_exit(&sem->rss);
+}
+EXPORT_SYMBOL_GPL(percpu_up_write);