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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <20090821153753.GA6735@linux.vnet.ibm.com>
Date:	Fri, 21 Aug 2009 08:37:53 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	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, Aug 21, 2009 at 04:44:16PM +0200, Ingo Molnar wrote:
> 
> * 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 ;-)

Please rest assured that I consider my code in this area to be suspect.  

							Thanx, Paul
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/