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

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Date:	Wed, 24 Aug 2011 09:33:06 -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 06:53 AM, Christoph Lameter wrote:
> 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.

I only looked in arch/x86, but I didn't find any that were
straightforward candidates for cmpxchg_flag.

>  And if the cmpxchg is a 16 byte
> cmpxchg (cmpxchg_double) then the comparison is getting more expensive.

We're talking about the difference between cmpxchg_flag()  - which does
a sete based on the flags set up cmpxchg - and a variant based on "asm
goto" which could, in principle, avoid the need for sete by allowing a
control flow statment to directly use the asm goto's conditional jump. 
The performance of both is invariant wrt the cmpxchg argument size.

    J
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