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

[Manfred Spraul <manfred@colorfullife.com>]

Jack wrote:

>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.
>
Bad race.
Unfortuantely the scenario that you describe is quite frequent:
- autoremove_wake_function()
- ipc/sem.c (search for IN_WAKEUP)
- ipc/msg.c appears to be correct, there are smp_wmb() calls.

--
    Manfred