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.
What C11 does is to allow the developer to specify different orderings
on success and failure. But it is no harder to supply a barrier (if
needed) on the failure path, right?
Thanx, Paul
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