futex: Add memory barrier commentary to futex_wait_queue_me()

The memory barrier semantics of futex_wait_queue_me() are
non-obvious. Add some commentary to try and clarify it.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: John Stultz <johnstul@us.ibm.com>
LKML-Reference: <20090924185447.694.38948.stgit@Aeon>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 6 insertions, 0 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 463af2efa512..b911adceb2c4 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1656,6 +1656,12 @@ out:
 static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
                                 struct hrtimer_sleeper *timeout)
 {
+        /*
+         * The task state is guaranteed to be set before another task can
+         * wake it. set_current_state() is implemented using set_mb() and
+         * queue_me() calls spin_unlock() upon completion, both serializing
+         * access to the hash list and forcing another memory barrier.
+         */
         set_current_state(TASK_INTERRUPTIBLE);
         queue_me(q, hb);