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

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Message-ID: <1406565669.25428.2.camel@buesod1.americas.hpqcorp.net>
Date:	Mon, 28 Jul 2014 09:41:09 -0700
From:	Davidlohr Bueso <davidlohr@...com>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	aswin@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH -tip/master 4/7] locking/mutex: Refactor optimistic
 spinning code

On Mon, 2014-07-28 at 09:39 -0700, Jason Low wrote:
> 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.

Yes, I need to delete the second check, one is enough.

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