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 S1751067AbcBLUYK (ORCPT <rfc822;w@1wt.eu>);
	Fri, 12 Feb 2016 15:24:10 -0500
Received: from casper.infradead.org ([85.118.1.10]:47402 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750898AbcBLUYD (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 12 Feb 2016 15:24:03 -0500
Date: Fri, 12 Feb 2016 21:23:55 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <Waiman.Long@hpe.com>
Cc: 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>, Davidlohr Bueso <dave@stgolabs.net>,
        "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>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()
Message-ID: <20160212202355.GY6357@twins.programming.kicks-ass.net>
References: <1455298335-53229-1-git-send-email-Waiman.Long@hpe.com>
 <1455298335-53229-2-git-send-email-Waiman.Long@hpe.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1455298335-53229-2-git-send-email-Waiman.Long@hpe.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> This patch adds a new waiter parameter to the mutex_optimistic_spin()
> function to prepare it to be used by a waiter-spinner that doesn't
> need to go into the OSQ as there can only be one waiter-spinner which
> is the head of the waiting queue.

Does not explain why..

>  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() ?

>  
> +		/*
> +		 * In order to avoid a stampede of mutex spinners trying to
> +		 * acquire the mutex all at once, the spinners need to take a
> +		 * MCS (queued) lock first before spinning on the owner field.
> +		 */
> +		if (!osq_lock(&lock->osq))
> +			goto done;
> +	}
>  
>  	while (true) {
>  		struct task_struct *owner;