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

[Manfred Spraul]

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.

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