Re: [PATCH tip/locking/core v9 2/6] locking/qspinlock: prefetch next node cacheline

[Peter Zijlstra <peterz@infradead.org>]

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?