linux-kernel - Re: [patch 01/23] Fall back on interrupt disable in cmpxchg8b on 80386 and 80486

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Message-ID: <20070812162341.GA30075@Krystal>
Date:	Sun, 12 Aug 2007 12:23:41 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	Christoph Lameter <clameter@....com>, mingo@...hat.com
Subject: Re: [patch 01/23] Fall back on interrupt disable in cmpxchg8b on 80386 and 80486

* Jan Engelhardt (jengelh@...putergmbh.de) wrote:
> 
> On Aug 12 2007 10:54, Mathieu Desnoyers wrote:
> 
> >Date: Sun, 12 Aug 2007 10:54:35 -0400
> >From: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
> >To:  <akpm@...ux-foundation.org>,  <linux-kernel@...r.kernel.org>
> >Cc: Christoph Lameter <clameter@....com>,
> >    Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,  <mingo@...hat.com>
> >Subject: [patch 01/23] Fall back on interrupt disable in cmpxchg8b on 80386 and
> >     80486
> >
> >Actually, on 386, cmpxchg and cmpxchg_local fall back on
> >cmpxchg_386_u8/16/32: it disables interruptions around non atomic
> >updates to mimic the cmpxchg behavior.
> >
> >The comment:
> >/* Poor man's cmpxchg for 386. Unsuitable for SMP */
> >
> >already present in cmpxchg_386_u32 tells much about how this cmpxchg
> >implementation should not be used in a SMP context. However, the cmpxchg_local
> >can perfectly use this fallback, since it only needs to be atomic wrt the local
> >cpu.
> >
> >This patch adds a cmpxchg_486_u64 and uses it as a fallback for cmpxchg64
> >and cmpxchg64_local on 80386 and 80486.
> 
> hm, but why is it called cmpxchg_486 when the other functions are called
> cmpxchg_386?
> 

Because the standard cmpxchg is missing only on  386, but cmpxchg8b is
missing both on 386 and 486.

Citing Intel's Instruction set reference:

cmpxchg:
This instruction is not supported on Intel processors earlier than the
Intel486 processors.

cmpxchg8b:
This instruction encoding is not supported on Intel processors earlier
than the Pentium processors.

Mathieu

> 
> 	Jan
> -- 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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