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

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Message-ID: <4E555BFA.3010801@goop.org>
Date:	Wed, 24 Aug 2011 13:15:54 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Christoph Lameter <cl@...ux.com>
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 08/24/2011 12:27 PM, Christoph Lameter wrote:
> On Wed, 24 Aug 2011, Jeremy Fitzhardinge wrote:
>
>> I only looked in arch/x86, but I didn't find any that were
>> straightforward candidates for cmpxchg_flag.
> Look at core code: mm/slub.c

These changes are currently only in arch/x86.  I haven't looked at
extending the cmpxchg API kernel-wide.

> But still the solution with the flags would save another instruction and
> the generated code would not be as ugly. For not only do you have an
> additional sete you will then also have to check the result again. This
> means at least two additional instruction.

Sure.  And the asm goto() variant avoids the sete and subsequent
gcc-generated test, but at the cost of generating a number of
unnecessary jumps - so it doesn't look like much of a win.  Perhaps a
hypothetical gcc extension to add "cc" as an output for asms would help,
but that's conjecture at this point.


    J
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