[RFC v2 5/7] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg_* variants

[Peter Zijlstra]

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")