locking/rwsem: Document barrier need when waking tasks

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Jason Low <jason.low2@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/1422609267-15102-3-git-send-email-dave@stgolabs.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 7 insertions, 0 deletions

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2f7cc4076f50..82aba467564a 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);