On Thu, Oct 13, 2016 at 04:28:01PM +0100, Will Deacon wrote:
> On Fri, Oct 07, 2016 at 04:52:51PM +0200, Peter Zijlstra wrote:
> > @@ -457,15 +472,20 @@ static bool mutex_optimistic_spin(struct
> > * release the lock or go to sleep.
> > */
> > owner = __mutex_owner(lock);
> > - if (owner && !mutex_spin_on_owner(lock, owner))
> > - break;
> > + if (owner) {
> > + if (waiter && owner == task) {
> > + smp_mb(); /* ACQUIRE */
>
> Hmm, is this barrier actually needed? This only happens on the handoff path,
> and we take the wait_lock immediately after this succeeds anyway. That
> control dependency, coupled with the acquire semantics of the spin_lock,
> should be sufficient, no?
Yes, I think you're right. But like said in that earlier email, I'd like
to keep this for now.
Once this code has settled we can reconsider this.
