Re: [RESEND PATCH v5] locking/pvqspinlock: Relax cmpxchg's to improve performance on some archs

[Peter Zijlstra]

On Thu, Aug 10, 2017 at 02:18:30PM -0400, Waiman Long wrote:
> On 08/10/2017 12:22 PM, Waiman Long wrote:
> > On 08/10/2017 12:15 PM, Peter Zijlstra wrote:

> >> Might as well do an explicit:
> >>
> >> 	smp_mb__before_atomic()
> >> 	cmpxchg_relaxed()
> >> 	smp_mb__after_atomic()
> >>
> >> I suppose and not introduce new primitives.
> 
> I think we don't need smp_mb__after_atomic(). The read has to be fully
> ordered, but the write part may not need it as the control dependency of
> the old value should guard against incorrect action. Right?

You'd think that, but IIRC there was something funny about using the SC
return flag for control dependencies. Will?