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

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Date:	Thu, 23 Apr 2009 09:19:42 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, mark.langsdorf@....com,
	arekm@...en.pl, "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

* 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.

Mathieu

> 	Ingo

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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