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authorAndrew Morton <akpm@osdl.org>2005-12-23 19:54:46 -0800
committerLinus Torvalds <torvalds@g5.osdl.org>2005-12-24 12:13:27 -0800
commit8e31108b9f41069d55cb9b019ac8262c55fd2616 (patch)
tree4d3b74956402b978417ee1026fa18409773cd72e
parent6003a93e7bf6c02f33c02976ff364785d4273295 (diff)
[PATCH] Fix memory ordering problem in wake_futex()
Fix a memory ordering problem that occurs on IA64. The "store" to q->lock_ptr in wake_futex() can become visible before wake_up_all() clears the lock in the futex_q. Signed-off-by: Jack Steiner <steiner@sgi.com> Acked-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
-rw-r--r--kernel/futex.c6
1 files changed, 6 insertions, 0 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index 5872e3507f35..5e71a6bf6f6b 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -270,7 +270,13 @@ static void wake_futex(struct futex_q *q)
/*
* The waiting task can free the futex_q as soon as this is written,
* without taking any locks. This must come last.
+ *
+ * A memory barrier is required here to prevent the following store
+ * to lock_ptr from getting ahead of the wakeup. Clearing the lock
+ * at the end of wake_up_all() does not prevent this store from
+ * moving.
*/
+ wmb();
q->lock_ptr = NULL;
}

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