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

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Message-ID: <20070820204126.GA22507@Krystal>
Date:	Mon, 20 Aug 2007 16:41:26 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Christoph Lameter <clameter@....com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...hat.com
Subject: Re: [patch 01/23] Fall back on interrupt disable in cmpxchg8b on 80386 and 80486

* Christoph Lameter (clameter@....com) wrote:
> On Mon, 20 Aug 2007, Mathieu Desnoyers wrote:
> 
> > 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.
> 
> Do you have an example where the use of cmpxchg_local is a performance 
> benefit when actually used in code? cmpxchg is already problematic
> functionality since it is not available on all platforms.

Yes, I use it as synchronization mechanism for my buffer management algorithm
in LTTng. Since I write in per-cpu buffers and want to be as reentrant
as possible wrt other contexts (dealing with NMI as worse case, but also
applies to MCE..), I use local_cmpxchg to reserve space in my buffers.
It is faster than the standard cmpxchg.

Mathieu

-- 
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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