linux-kernel - Re: [PATCH 07/12] x86: use cmpxchg

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Message-ID: <4E5574C3.6080104@zytor.com>
Date:	Wed, 24 Aug 2011 15:01:39 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.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 07/12] x86: use cmpxchg_flag() where applicable

On 08/24/2011 02:56 PM, Linus Torvalds wrote:
> Ok, I see nothing horrible in this series.
> 
> The one reaction I have is that the cmpxchg_flag() thing returns an
> 8-bit value, but then a lot of the users end up having to extend it to
> a full "int" purely for calling convention reasons (eg I think
> 'down_write_trylock()' will have 'sete + movzl' - not a new problem,
> but since the whole point was to remove extraneous instructions and we
> no longer have the silly 'testl', it now annoys me more).
> 
> So it seems a bit sad. But I guess it doesn't really matter.
> 

I think it is a net lose.  The most common case is probably going to be
to use it immediately, in which case we have:

cmpxchg -> sete -> compare -> conditional

versus

cmpxchg -> compare -> conditional

For doubleword cmpxchg it's another matter entirely, because doubleword
comparisons are significantly more expensive that sete + test.

So unless there is actual data showing this is better, I would like to
see this dropped for now.

	-hpa
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