[PATCH 2/5] locking/rwsem: Document barrier need when waking tasks

[Davidlohr Bueso <dave@stgolabs.net>]

The need for the smp_mb in __rwsem_do_wake should be
properly documented. Applies to both xadd and spinlock
variants.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 kernel/locking/rwsem-spinlock.c | 7 +++++++
 kernel/locking/rwsem-xadd.c     | 7 +++++++
 2 files changed, 14 insertions(+)

diff --git a/kernel/locking/rwsem-spinlock.c b/kernel/locking/rwsem-spinlock.c
index 2555ae1..3a50485 100644
--- a/kernel/locking/rwsem-spinlock.c
+++ b/kernel/locking/rwsem-spinlock.c
@@ -85,6 +85,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wakewrite)
 
 		list_del(&waiter->list);
 		tsk = waiter->task;
+		/*
+		 * Make sure we do not wakeup the next reader before
+		 * setting the nil condition to grant the next reader;
+		 * otherwise we could miss the wakeup on the other
+		 * side and end up sleeping again. See the pairing
+		 * in rwsem_down_read_failed().
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2f7cc40..82aba46 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -186,6 +186,13 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
 		waiter = list_entry(next, struct rwsem_waiter, list);
 		next = waiter->list.next;
 		tsk = waiter->task;
+		/*
+		 * Make sure we do not wakeup the next reader before
+		 * setting the nil condition to grant the next reader;
+		 * otherwise we could miss the wakeup on the other
+		 * side and end up sleeping again. See the pairing
+		 * in rwsem_down_read_failed().
+		 */
 		smp_mb();
 		waiter->task = NULL;
 		wake_up_process(tsk);
-- 
2.1.4