Messages in this thread | | | Date | Thu, 1 Oct 2015 08:13:10 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC v2 5/7] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg_* variants |
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On Thu, Oct 01, 2015 at 02:36:26PM +0200, Peter Zijlstra wrote: > 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.
Agreed, no need for ordering on failed cmpxchg.
Thanx, Paul
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