Re: locking/atomic: Introduce atomic_try_cmpxchg()

[Peter Zijlstra]

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@xxxxxxxxxxxxxxxxxxxx> 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)				\