From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752562AbcBPCXQ (ORCPT <rfc822;w@1wt.eu>);
	Mon, 15 Feb 2016 21:23:16 -0500
Received: from g1t6214.austin.hp.com ([15.73.96.122]:58620 "EHLO
	g1t6214.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752229AbcBPCXP (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 15 Feb 2016 21:23:15 -0500
X-Greylist: delayed 414 seconds by postgrey-1.27 at vger.kernel.org; Mon, 15 Feb 2016 21:23:15 EST
Message-ID: <1455589334.2276.39.camel@j-VirtualBox>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()
From: Jason Low <jason.low2@hp.com>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>, Waiman Long <Waiman.Long@hpe.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Jason Low <jason.low2@hpe.com>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Waiman Long <Waiman.Long@hp.com>, jason.low2@hp.com
Date: Mon, 15 Feb 2016 18:22:14 -0800
In-Reply-To: <1455588930.2276.36.camel@j-VirtualBox>
References: <1455298335-53229-1-git-send-email-Waiman.Long@hpe.com>
	 <1455298335-53229-2-git-send-email-Waiman.Long@hpe.com>
	 <20160212202355.GY6357@twins.programming.kicks-ass.net>
	 <20160212221444.GC16417@linux-uzut.site>
	 <1455588930.2276.36.camel@j-VirtualBox>
Content-Type: text/plain; charset="UTF-8"
X-Mailer: Evolution 3.10.4-0ubuntu2 
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, 2016-02-15 at 18:15 -0800, Jason Low wrote:
> On Fri, 2016-02-12 at 14:14 -0800, Davidlohr Bueso wrote:
> > On Fri, 12 Feb 2016, Peter Zijlstra wrote:
> > 
> > >On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> > >>  static bool mutex_optimistic_spin(struct mutex *lock,
> > >> +				  struct ww_acquire_ctx *ww_ctx,
> > >> +				  const bool use_ww_ctx, int waiter)
> > >>  {
> > >>  	struct task_struct *task = current;
> > >> +	bool acquired = false;
> > >>
> > >> +	if (!waiter) {
> > >> +		if (!mutex_can_spin_on_owner(lock))
> > >> +			goto done;
> > >
> > >Why doesn't the waiter have to check mutex_can_spin_on_owner() ?
> > 
> > afaict because mutex_can_spin_on_owner() fails immediately when the counter
> > is -1, which is a nono for the waiters case.
> 
> mutex_can_spin_on_owner() returns false if the task needs to reschedule
> or if the lock owner is not on_cpu. In either case, the task will end up
> not spinning when it enters the spin loop. So it makes sense if the
> waiter also checks mutex_can_spin_on_owner() so that the optimistic spin
> queue overhead can be avoided in those cases.

Actually, since waiters bypass the optimistic spin queue, that means the
the mutex_can_spin_on_owner() isn't really beneficial. So Waiman is
right in that it's fine to skip this in the waiter case.