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

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Date:	Fri, 21 Aug 2009 16:44:16 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josht@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org, dvhltc@...ibm.com,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	hugh.dickins@...cali.co.uk, benh@...nel.crashing.org
Subject: Re: [PATCH -tip/core/rcu 1/6] Cleanups and fixes for RCU in face
	of heavy CPU-hotplug stress


* Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 21 Aug 2009, Ingo Molnar wrote:
> 
> > * Mathieu Desnoyers <mathieu.desnoyers@...ymtl.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
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