linux-kernel - Re: [patch 2/2] x86 amd fix cmpxchg read acquire barrier

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Date:	Thu, 23 Apr 2009 15:41:17 +0200
From:	Arkadiusz Miskiewicz <a.miskiewicz@...il.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...e.hu>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mark.langsdorf@....com, "H. Peter Anvin" <hpa@...or.com>,
	Andi Kleen <andi@...stfloor.org>, Avi Kivity <avi@...ranet.com>
Subject: Re: [patch 2/2] x86 amd fix cmpxchg read acquire barrier

On Thursday 23 of April 2009, Mathieu Desnoyers wrote:
> * Ingo Molnar (mingo@...e.hu) wrote:
> > * Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> > > " // Opteron Rev E has a bug in which on very rare occasions a locked
> > >   // instruction doesn't act as a read-acquire barrier if followed by a
> > >   // non-locked read-modify-write instruction.  Rev F has this bug in
> > >   // pre-release versions, but not in versions released to customers,
> > >   // so we test only for Rev E, which is family 15, model 32..63
> > > inclusive.
> >
> > Dunno. The fix looks a bit intrusive (emits a NOP even on good
> > CPUs). Also, the text above says "not in versions released to
> > customers".
> >
> > So unless there's an official erratum or reports in the field (not
> > from early prototype systems shipped to developers) i'd not rush to
> > apply it, just yet.
>
> Actually, Operon Rev E has this bug in the field (family 15, model
> 32..64). Rev F only had the bug in pre-releases.
>
> But yes, it's bad that it drags so many code additions to something as
> critical as cmpxchg. I start to think it might be better to just
> disallow bringing up more than one CPU on these machines.

That probably would be even worse than what we have now. This bug doesn't 
manifest too often in a noticeable way here (I have few such machines here, 
mostly 2 x dual core; once per few months mysql dies) and loosing 3 of 4 cores 
(or 1 cpu of 2; depends on what you mean) doesn't sound like fun.

> Mathieu


-- 
Arkadiusz Miśkiewicz        PLD/Linux Team
arekm / maven.pl            http://ftp.pld-linux.org/

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