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

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Message-ID: <alpine.DEB.2.00.1108231355380.26421@router.home>
Date:	Tue, 23 Aug 2011 14:01:38 -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 Mon, 22 Aug 2011, Jeremy Fitzhardinge wrote:

> Most callers of cmpxchg() direcly compare RETURN with OLD to see if it was
> successful.  This results in unnecessary conditional comparisons
> and conditionals since the cmpxchg instruction directly sets the flags
> to indicate success/failure.

> Add cmpxchg_flag() variants which return a boolean flag directly indicating
> success.  Unfortunately an asm() statement can't directly export status
> status flags, but sete isn't too bad.

And so what happens through this patch is that a cmp with a value that is
likely in a register is replaced by a sete. Is there really a benefit?

What I wish we would have is the actual use of the processor flag.

if (cmpxchg_flags(....)) {
}

where the cmpxchg is followed immediately by a jump. I tried in the past
to pass a goto label to cmpxchg but that did not work.



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