Re: [PATCH v2 05/11] locking/ww_mutex: Add waiters in stamp order

[Nicolai HÃhnle]

On 16.12.2016 18:20, Peter Zijlstra wrote:
> On Fri, Dec 16, 2016 at 03:19:43PM +0100, Nicolai Hähnle wrote:
> > > > @@ -716,7 +775,20 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
> > > > 		spin_unlock_mutex(&lock->wait_lock, flags);
> > > > 		schedule_preempt_disabled();
> > > >
> > > > -		if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> > > > +		if (use_ww_ctx && ww_ctx) {
> > > > +			/*
> > > > +			 * Always re-check whether we're in first position. We
> > > > +			 * don't want to spin if another task with a lower
> > > > +			 * stamp has taken our position.
> > > > +			 *
> > > > +			 * We also may have to set the handoff flag again, if
> > > > +			 * our position at the head was temporarily taken away.
> > > > +			 */
> > > > +			first = __mutex_waiter_is_first(lock, &waiter);
> > > > +
> > > > +			if (first)
> > > > +				__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> > > > +		} else if (!first && __mutex_waiter_is_first(lock, &waiter)) {
> > > > 			first = true;
> > > > 			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
> > > > 		}
> > >
> > > So the point is that !ww_ctx entries are 'skipped' during the insertion
> > > and therefore, if one becomes first, it must stay first?
> >
> > Yes. Actually, it should be possible to replace all the cases of use_ww_ctx
> > || first with ww_ctx. Similarly, all cases of use_ww_ctx && ww_ctx could be
> > replaced by just ww_ctx.
>
>
> I'm not seeing how "use_ww_ctx || first" -> "ww_ctx" works.

My bad, missing the '|| first'.


> And while
> "use_ww_ctx && ww_ctx" -> "ww_ctx" is correct, it didn't work right on
> some older GCCs, they choked on value propagation for ww_ctx and kept
> emitting code even if we passed in NULL. Hence use_ww_ctx.

Okay, I'll stick with use_ww_ctx. Thanks for the explanation.

Nicolai


> Arnd is now looking to raise the minimum supported GCC version, so maybe
> we should look at that again if he gets anywhere.