From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932337AbZHUOpE (ORCPT ); Fri, 21 Aug 2009 10:45:04 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932214AbZHUOpD (ORCPT ); Fri, 21 Aug 2009 10:45:03 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:38129 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932187AbZHUOpB (ORCPT ); Fri, 21 Aug 2009 10:45:01 -0400 Date: Fri, 21 Aug 2009 16:44:16 +0200 From: Ingo Molnar To: Steven Rostedt Cc: Mathieu Desnoyers , "Paul E. McKenney" , Josh Triplett , linux-kernel@vger.kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, hugh.dickins@tiscali.co.uk, benh@kernel.crashing.org Subject: Re: [PATCH -tip/core/rcu 1/6] Cleanups and fixes for RCU in face of heavy CPU-hotplug stress Message-ID: <20090821144416.GA16810@elte.hu> References: <20090815165153.GA8886@linux.vnet.ibm.com> <1250533487.2709.14.camel@josh-work.beaverton.ibm.com> <20090817192036.GJ6760@linux.vnet.ibm.com> <20090818152643.GA5549@elte.hu> <20090820140335.GA31773@Krystal> <20090821141721.GA11098@elte.hu> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.18 (2008-05-17) X-ELTE-SpamScore: -1.5 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.5 -1.5 BAYES_00 BODY: Bayesian spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Steven Rostedt wrote: > On Fri, 21 Aug 2009, Ingo Molnar wrote: > > > * Mathieu Desnoyers 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