linux-kernel - Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of woken waiter

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Date:   Tue, 18 Oct 2016 14:19:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <waiman.long@....com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [PATCH -v4 8/8] locking/mutex: Enable optimistic spinning of
 woken waiter

On Mon, Oct 17, 2016 at 07:21:28PM -0400, Waiman Long wrote:
> On 10/07/2016 10:52 AM, Peter Zijlstra wrote:
> >@@ -600,7 +630,7 @@ __mutex_lock_common(struct mutex *lock,
> >  	mutex_acquire_nest(&lock->dep_map, subclass, 0, nest_lock, ip);
> >
> >  	if (__mutex_trylock(lock, false) ||
> >-	    mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx)) {
> >+	    mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, false)) {
> >  		/* got the lock, yay! */
> >  		lock_acquired(&lock->dep_map, ip);
> >  		if (use_ww_ctx)
> >@@ -669,7 +699,8 @@ __mutex_lock_common(struct mutex *lock,
> >  		 * state back to RUNNING and fall through the next schedule(),
> >  		 * or we must see its unlock and acquire.
> >  		 */
> >-		if (__mutex_trylock(lock, first))
> >+		if ((first&&  mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, true)) ||
> >+		     __mutex_trylock(lock, first))
> 
> Do we need a __mutex_trylock() here? mutex_optimistic_spin() will do the
> trylock and we have one at the top of the loop.

Yes, mutex_optimistic_spin() can be a no-op.