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 S932289AbcGOHJa (ORCPT <rfc822;w@1wt.eu>);
	Fri, 15 Jul 2016 03:09:30 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:40394 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S932094AbcGOHJ3 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 15 Jul 2016 03:09:29 -0400
Date: Fri, 15 Jul 2016 09:09:25 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Wanpeng Li <kernellwp@gmail.com>
Cc: Waiman Long <waiman.long@hpe.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Wanpeng Li <wanpeng.li@hotmail.com>, Ingo Molnar <mingo@kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>
Subject: Re: [PATCH v3] locking/pvqspinlock: restore/set vcpu_hashed state
 after failing adaptive locking spinning
Message-ID: <20160715070925.GO30154@twins.programming.kicks-ass.net>
References: <1468496357-6400-1-git-send-email-wanpeng.li@hotmail.com>
 <5787A718.5020901@hpe.com>
 <CANRm+Cy-cOYPmEbaRFywysxc2vg5BpKR-UXsqzXA-sX1XBVeRw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CANRm+Cy-cOYPmEbaRFywysxc2vg5BpKR-UXsqzXA-sX1XBVeRw@mail.gmail.com>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Jul 15, 2016 at 05:26:40AM +0800, Wanpeng Li wrote:
> 2016-07-14 22:52 GMT+08:00 Waiman Long <waiman.long@hpe.com>:
> [...]
> > As pv_kick_node() is called immediately after designating the next node as
> > the queue head, the chance of this racing is possible, but is not likely
> > unless the lock holder vCPU gets preempted for a long time at that right
> > moment. This change does not do any harm though, so I am OK with that.
> > However, I do want you to add a comment about the possible race in the code
> > as it isn't that obvious or likely.
> 
> How about something like:
> 
> /*
>  * If the lock holder vCPU gets preempted for a long time, pv_kick_node will
>  * advance its state and hash the lock, restore/set the vcpu_hashed state to
>  * avoid the race.
>  */

So I'm not sure. Yes it was a bug, but its fairly 'obvious' it should be
vcpu_hashed, we did after all hash the thing.

> Btw, do you think patch title should be improved, what do you like?

I changed it to: "locking/pvqspinlock: Fix double hash race"