On Wed, Sep 02, 2015 at 04:36:09PM +0100, Pranith Kumar wrote:

On Wed, Sep 2, 2015 at 11:23 AM, Pranith Kumar [off-list ref] wrote:

quoted

On 09/02/2015 05:59 AM, Will Deacon wrote:

quoted

I just thought it was worth making this point, because it is prohibited
in SC and I don't want people to think that our RELEASE/ACQUIRE operations
are SC (even though they happen to be on arm64).

This is interesting information. Does that mean that the following patch
should work? (I am not proposing to use it, just trying to understand if
REL+ACQ will act as a full barrier on ARM64, which you say it does).

Thanks,
Pranith.

diff --git a/arch/arm64/include/asm/cmpxchg.h b/arch/arm64/include/asm/cmpxchg.h
index d8c25b7..14a1b35 100644
--- a/arch/arm64/include/asm/cmpxchg.h
+++ b/arch/arm64/include/asm/cmpxchg.h

@@ -68,8 +68,7 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
                BUILD_BUG();
        }

-       smp_mb();
-       return ret;
+       return smp_load_acquire(ret);

I meant 'smp_load_acquire(&ret);'

Yes, I think that would work on arm64, but it's not portable between
architectures.

Will