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

[Manfred Spraul <manfred@colorfullife.com>]

Manfred Spraul wrote:

> 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.
>>
I'm stupid - first I was certains that your description is wrong, I 
started a long mail, but the I confused myself and accepted your 
description.

Your description can't be correct: The store to q->lock_ptr can't 
overwrite ar.bspstore. As long as q->lock_ptr is not NULL, unqueue_me 
will spin and prevent a race. But the other race seems to be possible_ 
q->lock_ptr is set to NULL early, and then the spin_unlock_irqrestore() 
in __wake_up could overwrite ar.bspstore.