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

[Boqun Feng]

Hi Peter,

Sorry for replying late.

On Thu, Oct 01, 2015 at 02:27:16PM +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")

I probably didn't make myself clear here, my point is that if we use
__atomic_op_acquire() to built *_cmpchg_acquire(For ARM and PowerPC),
the barrier will be implied _unconditionally_, meaning no matter cmp
fails or not, there will be a barrier after the cmpxchg operation.
Therefore we have to use assembly to implement the operations right now.

Regards,
Boqun

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