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 <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Dinakar Guniguntala <[e-mail]>
Cc: John Stultz <[e-mail]>
LKML-Reference: <20090924185447.694.38948.stgit@Aeon>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/futex.c

···
 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);