Messages in this thread Patch in this message | | | Date | Sat, 24 Dec 2005 07:45:23 -0600 | From | Jack Steiner <> | Subject | Re: [PATCH] - Fix memory ordering problem in wake_futex() |
| |
This patch is identical to the first patch except I used smp_wmb() instead of wmb(). Ordering doen't matter on non-SMP kernels.
Here is a fix for a ugly race condition that occurs in wake_futex() on IA64.
On IA64, locks are released using a "st.rel" instruction. This ensures that preceding "stores" are visible before the lock is released but does NOT prevent a "store" that follows the "st.rel" from becoming visible before the "st.rel". The result is that the task that owns the futex_q continues prematurely.
The failure I saw is the task that owned the futex_q resumed prematurely and was context-switch off of the cpu. The task's switch_stack occupied the same space of the futex_q. The store to q->lock_ptr overwrote the ar.bspstore in the switch_stack. When the task resumed, it ran with a corrupted ar.bspstore. Things went downhill from there.
Without the fix, the application fails roughly every 10 minutes. With the fix, it ran 16 hours without a failure.
---- 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>
Index: linux/kernel/futex.c =================================================================== --- linux.orig/kernel/futex.c 2005-12-22 15:05:43.821889257 -0600 +++ linux/kernel/futex.c 2005-12-22 15:30:21.617973325 -0600 @@ -287,7 +287,13 @@ 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. */ + smp_wmb(); q->lock_ptr = NULL; } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |