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Date: Mon, 17 Oct 2016 11:32:42 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Will Deacon <will.deacon@arm.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Waiman Long <waiman.long@hpe.com>, Jason Low <jason.low2@hpe.com>,
        Ding Tianhong <dingtianhong@huawei.com>,
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        "Paul E. McKenney" <paulmck@us.ibm.com>, Jason Low <jason.low2@hp.com>,
        Chris Wilson <chris@chris-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@ffwll.ch>
Subject: Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of
 woken waiter
Message-ID: <20161017093242.GN3117@twins.programming.kicks-ass.net>
References: <20161007145243.361481786@infradead.org>
 <20161007150211.416377482@infradead.org>
 <20161013152801.GD13138@arm.com>
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On Thu, Oct 13, 2016 at 04:28:01PM +0100, Will Deacon wrote:
> On Fri, Oct 07, 2016 at 04:52:51PM +0200, Peter Zijlstra wrote:

> > @@ -457,15 +472,20 @@ static bool mutex_optimistic_spin(struct
> >  		 * release the lock or go to sleep.
> >  		 */
> >  		owner = __mutex_owner(lock);
> > -		if (owner && !mutex_spin_on_owner(lock, owner))
> > -			break;
> > +		if (owner) {
> > +			if (waiter && owner == task) {
> > +				smp_mb(); /* ACQUIRE */
> 
> Hmm, is this barrier actually needed? This only happens on the handoff path,
> and we take the wait_lock immediately after this succeeds anyway. That
> control dependency, coupled with the acquire semantics of the spin_lock,
> should be sufficient, no?

Yes, I think you're right. But like said in that earlier email, I'd like
to keep this for now.

Once this code has settled we can reconsider this.