On Mon, 6 Nov 2006, Benjamin Herrenschmidt wrote:
>
> > Yes. On x86 (and x86-64) you'll never see this, because writes are always
> > seen in order regardless, and in addition, the spin_lock is actually
> > totally serializing anyway. On most other architectures, the spin_lock
> > will serialize all the writes too, but it's not guaranteed, so in theory
> > you're right. I suspect no actual architecture will do this, but hey,
> > when talking memory ordering, safe is a lot better than sorry.
>
> PowerPC doesn't serialize the writes on spin_lock, only on spin_unlock.
>
> (That is, previous writes can "leak" into the lock, but writes done
> before the unlock can't leak out of the spinlock).
>
> Now, I've just glanced at the thread, so I don't know if that's relevant
> to the problems you guys are talking about :-)
>
It is relevant. In powerpc, can one write happen before another write?
x = 1;
barrier(); (only compiler barrier)
b = 2;
And have CPU 2 see b=2 before seeing x=1?
If so, then I guess this is indeed a bug on powerpc.
-- Steve
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