Re: [PATCH -tip/master 4/7] locking/mutex: Refactor optimistic spinning code

[Jason Low <jason.low2@hp.com>]

On Mon, 2014-07-28 at 11:08 +0200, Peter Zijlstra wrote:
> On Sun, Jul 27, 2014 at 10:18:41PM -0700, Davidlohr Bueso wrote:
> > +static bool mutex_optimistic_spin(struct mutex *lock,
> > +				  struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
> > +{
> 
> 
> > +	/*
> > +	 * If we fell out of the spin path because of need_resched(),
> > +	 * reschedule now, before we try-lock the mutex. This avoids getting
> > +	 * scheduled out right after we obtained the mutex.
> > +	 */
> > +	if (need_resched())
> > +		schedule_preempt_disabled();
> > +
> > +	return false;
> > +}
> 
> 
> > +	if (mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {
> > +		/* got it, yay! */
> > +		preempt_enable();
> > +		return 0;
> >  	}
> > +
> >  	/*
> >  	 * If we fell out of the spin path because of need_resched(),
> >  	 * reschedule now, before we try-lock the mutex. This avoids getting
> > @@ -475,7 +512,7 @@ slowpath:
> >  	 */
> >  	if (need_resched())
> >  		schedule_preempt_disabled();
> > +
> >  	spin_lock_mutex(&lock->wait_lock, flags);
> 
> We now have two if (need_resched) schedule_preempt_disable() instances,
> was that on purpose?

I think we can delete the extra check in mutex_optimistic_spin(). It is
sufficient to have it here and it also covers the case where the task
need_resched() without attempting to spin.