Re: [PATCH -tip/core/rcu 1/6] Cleanups and fixes for RCU in face of heavy CPU-hotplug stress

Mathieu Desnoyers
2009-08-21T15:00:51Z

* Ingo Molnar (mingo@elte.hu) wrote:
>
> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > On Fri, 21 Aug 2009, Ingo Molnar wrote:
> >
> > > * Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> > >
> > > > I would not trust this architecture for synchronization tests.
> > > > There has been reports of a hardware bug affecting the cmpxchg
> > > > instruction in the field. The load fence normally implied by
> > > > the semantic seems to be missing. AFAIK, AMD never
> > > > acknowledged the problem.
> > >
> > > If cmpxchg was broken i'd be having far worse problems and very
> > > widely so.
> >
> > I believe Mathieu is suggesting that the hardware bug is not that
> > the compare and exchange does not work in cmpxchg, but that it
> > does not provide an explicit memory barrier. Such a bug is very
> > hard to trigger, since it requires a race that allows a memory
> > write/read to cross the cmpxchg, and then have this be in such a
> > place that it will cause harm.
>
> We can argue all sorts of exotic hardware bugs really, proof is
> still needed.
>
> [...]
> > > That's not a proof of course (it's near impossible to prove the
> > > lack of a bug), but it's sure a strong indicator and you'll need
> > > to provide far more proof of misbehavior before i discount a
> > > bona fide regression on this box.
> >
> > But with the above said, I totally agree with your point. More
> > proof must be given before we can discount that another bug
> > exists.
>
> Yeah. Especially given that this code was changed recently ;-)
>

Yep, I think we should continue looking for the problem cause, but
stress-testing the hardware with the program I just sent cannot hurt. :)

Mathieu

> Ingo

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Mathieu Desnoyers
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