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Date: Tue, 23 Aug 2011 14:01:38 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
cc: "H. Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@elte.hu>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nick Piggin <npiggin@kernel.dk>,
        Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Subject: Re: [PATCH 13/15] x86: add cmpxchg_flag() variant
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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.