linux-kernel - Re: [PATCH 13/15] x86: add cmpxchg

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Date:	Wed, 24 Aug 2011 08:53:44 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 13/15] x86: add cmpxchg_flag() variant

On Tue, 23 Aug 2011, Jeremy Fitzhardinge wrote:

> However, having prototyped it, I dunno, it doesn't really seem like much
> of a win for all the extra code it adds.  I just can't get too excited
> about an extra test instruction adjacent to a monster like a locked
> cmpxchg.  The jump variant avoids the test, but gcc still generates some
> pretty bogus stuff:

There are also unlocked cmpxchges in use. And if the cmpxchg is a 16 byte
cmpxchg (cmpxchg_double) then the comparison is getting more expensive.
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