[PATCH] futex: add memory barrier commentary to futex_wait_queue_me

[Darren Hart <dvhltc@us.ibm.com>]

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: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: Dinakar Guniguntala <dino@in.ibm.com>
CC: John Stultz <johnstul@us.ibm.com>
---

 kernel/futex.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 463af2e..b911adc 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);