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futex: Add memory barrier commentary to futex_wait_queue_me()

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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>
This commit is contained in:
Darren Hart 2009-09-24 11:54:47 -07:00 committed by Ingo Molnar
parent 0729e19614
commit 9beba3c54d
1 changed files with 6 additions and 0 deletions
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6
kernel/futex.c
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@ -1656,6 +1656,12 @@ out:
static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q, static void futex_wait_queue_me(struct futex_hash_bucket *hb, struct futex_q *q,
struct hrtimer_sleeper *timeout) 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); set_current_state(TASK_INTERRUPTIBLE);
queue_me(q, hb); queue_me(q, hb);