On Mon, Nov 02, 2015 at 05:36:26PM +0100, Peter Zijlstra wrote:
> On Fri, Oct 30, 2015 at 07:26:33PM -0400, Waiman Long wrote:
> > @@ -426,6 +437,15 @@ queue:
> > cpu_relax();
> >
> > /*
> > + * If the next pointer is defined, we are not tail anymore.
> > + * In this case, claim the spinlock & release the MCS lock.
> > + */
> > + if (next) {
> > + set_locked(lock);
> > + goto mcs_unlock;
> > + }
> > +
> > + /*
> > * claim the lock:
> > *
> > * n,0,0 -> 0,0,1 : lock, uncontended
> > @@ -458,6 +478,7 @@ queue:
> > while (!(next = READ_ONCE(node->next)))
> > cpu_relax();
> >
> > +mcs_unlock:
> > arch_mcs_spin_unlock_contended(&next->locked);
> > pv_kick_node(lock, next);
> >
>
> This however appears an independent optimization. Is it worth it? Would
> we not already have observed a val != tail in this case? At which point
> we're just adding extra code for no gain.
>
> That is, if we observe @next, must we then not also observe val != tail?
Not quite; the ordering is the other way around. If we observe next we
must also observe val != tail. But its a narrow thing. Is it really
worth it?
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