Re: [PATCH] - Fix memory ordering problem in wake_futex()

[Olof Johansson <olof@lixom.net>]

On Fri, Dec 23, 2005 at 10:38:16AM -0600, Jack Steiner wrote:
> 
> 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.

So what happened to what the comment 10 lines above your patch says?

        /*
         * The lock in wake_up_all() is a crucial memory barrier after
         * the list_del_init() and also before assigning to q->lock_ptr.
         */

On PPC64, the spinlock unlock path has a sync in there for the very
purpose of adding the write barrier. Maybe the ia64 unlock path is
missing something similar?


-Olof
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