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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Message-ID: <alpine.DEB.2.00.0908211022090.16405@gandalf.stny.rr.com>
Date:	Fri, 21 Aug 2009 10:29:47 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
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


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.

> 
> FYI, this was the same box i prototyped/developed -rt on, which uses 
> cmpxchg for _everything_.

If such a bug exists, then it may not trigger easily, even on -rt.

> 
> 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.

-- Steve

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