linux-kernel - Re: locking/atomic: Introduce atomic_try

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Message-ID: <20170325211330.duvzsioxs5ll4qbe@hirez.programming.kicks-ass.net>
Date:   Sat, 25 Mar 2017 22:13:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()

On Sat, Mar 25, 2017 at 11:34:32AM -0700, Linus Torvalds wrote:
> On Sat, Mar 25, 2017 at 11:28 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Hmm. Sad. The label approach looked like it would match the semantics
> > of cmpxchg perfectly, but it's not as optimal as it superficially
> > would have seemed.
> 
> Oh, I just noticed that at least your other one didn't mark "success"
> as being likely.

10730509        4540256  843776 16114541         f5e36d defconfig-build/vmlinux


---
 arch/x86/include/asm/cmpxchg.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
index fb961db..d347abc 100644
--- a/arch/x86/include/asm/cmpxchg.h
+++ b/arch/x86/include/asm/cmpxchg.h
@@ -212,8 +212,9 @@ extern void __add_wrong_size(void)
 	default:							\
 		__cmpxchg_wrong_size();					\
 	}								\
+	if (unlikely(!success)) \
 	*_old = __old;							\
-	success;							\
+	likely(success);						\
 })
 
 #define __try_cmpxchg(ptr, pold, new, size)				\