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

[Will Deacon]

On Fri, Aug 11, 2017 at 11:06:01AM +0200, Peter Zijlstra wrote:
> 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?

Yeah, that's right, you can't use the STXR status flag to create control
dependencies.

Will