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Date: Tue, 18 Oct 2016 14:19:41 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Jason Low <jason.low2@hpe.com>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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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: <20161018121941.GU3117@twins.programming.kicks-ass.net>
References: <20161007145243.361481786@infradead.org>
 <20161007150211.416377482@infradead.org>
 <58055CF8.2060806@hpe.com>
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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.