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SubjectRe: [RFC PATCH v2] memory-barriers: remove smp_mb__after_unlock_lock()
On Tue, Jul 14, 2015 at 08:43:44AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2015-07-14 at 00:15 +0200, Peter Zijlstra wrote:
>
> > > This is instead the sequence that is of concern:
> > >
> > > store a
> > > unlock M
> > > lock N
> > > load b
> >
> > So its late and that table didn't parse, but that should be ordered too.
> > The load of b should not be able to escape the lock N.
> >
> > If only because LWSYNC is a valid RMB and any LOCK implementation must
> > load the lock state to observe it unlocked.
>
> What happens is that the load passes the store conditional, though it
> doesn't pass the load with reserve. However, both store A and unlock M
> being just stores with an lwsync, can pass a load, so they can pass the
> load with reserve. And thus inside the LL/SC loop, our store A has
> passed our load B.

Ah cute.. Thanks, clearly I wasn't awake enough anymore :-)


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