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authorDarren Hart <dvhltc@us.ibm.com>2009-09-24 11:54:47 -0700
committerIngo Molnar <mingo@elte.hu>2009-09-24 22:30:10 +0200
commit9beba3c54dd180a26a1da2027cfbe9edfaf9c40e (patch)
treeb7ede0a88beb7ee195adf40a7a03140e77ba43ee /kernel/futex.c
parent0729e196147692d84d4c099fcff056eba2ed61d8 (diff)
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>
Diffstat (limited to 'kernel/futex.c')
-rw-r--r--kernel/futex.c6
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);