* H. Peter Anvin <[email protected]> wrote:
> On 12/19/2013 10:09 AM, H. Peter Anvin wrote:
> > On 12/19/2013 09:07 AM, Ingo Molnar wrote:
> >>
> >> Likewise, having a barrier before the MONITOR looks sensible as well.
> >> Having it _after_ monitor looks weird and is probably wrong. [It might
> >> have been the effects of someone seeing the spurious wakeup problems
> >> with realizing the true source, or so.]
> >>
> >
> > Does anyone know the history of this barrier after the monitor? I know
> > Len is looking for a minimal patchset that can go into -stable, and it
> > seems prudent to not preturb the code more than necessary, but going
> > forward it would be nice to know...
> >
>
> Hmm... it *looks* like it is intended to be part of the construct:
>
> smp_mb();
> if (!need_resched())
> ...
>
> I found a note in the HLT variant of the function saying:
>
> /*
> * TS_POLLING-cleared state must be visible before we
> * test NEED_RESCHED:
> */
Yes, that makes sense: the need_resched test is a load, and MONITOR is
a load as well. Can the two ever cross, or does the CPU guarantee that
because it's the same address, the loads don't cross?
Thanks,
Ingo
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