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

Ingo Molnar
2009-08-21T14:45:04Z

* 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 ;-)

Ingo