2009-09-24 18:55:08

by Darren Hart

Subject: [PATCH] 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 <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
CC: Eric Dumazet <[email protected]>
CC: Dinakar Guniguntala <[email protected]>
CC: John Stultz <[email protected]>
---

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);

2009-09-25 06:20:54

by Darren Hart

Subject: [tip:core/urgent] futex: Add memory barrier commentary to futex_wait_queue_me()

Commit-ID: 9beba3c54dd180a26a1da2027cfbe9edfaf9c40e
Gitweb: http://git.kernel.org/tip/9beba3c54dd180a26a1da2027cfbe9edfaf9c40e
Author: Darren Hart <[email protected]>
AuthorDate: Thu, 24 Sep 2009 11:54:47 -0700
Committer: Ingo Molnar <[email protected]>
CommitDate: Thu, 24 Sep 2009 22:30:10 +0200

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


---
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);