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SubjectRe: [RFC v2 5/7] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg_* variants
On Thu, Oct 01, 2015 at 02:27:15PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 16, 2015 at 11:49:33PM +0800, Boqun Feng wrote:
> > Unlike other atomic operation variants, cmpxchg{,64}_acquire and
> > atomic{,64}_cmpxchg_acquire don't have acquire semantics if the cmp part
> > fails, so we need to implement these using assembly.
>
> I think that is actually expected and documented. That is, a cmpxchg
> only implies barriers on success. See:
>
> ed2de9f74ecb ("locking/Documentation: Clarify failed cmpxchg() memory ordering semantics")

Also:

654672d4ba1a6 (Will Deacon 2015-08-06 17:54:37 +0100 28) * store portion of the operation. Note that a failed cmpxchg_acquire
654672d4ba1a6 (Will Deacon 2015-08-06 17:54:37 +0100 29) * does -not- imply any memory ordering constraints.


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