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 S932216Ab1ACPOd (ORCPT <rfc822;w@1wt.eu>);
	Mon, 3 Jan 2011 10:14:33 -0500
Received: from casper.infradead.org ([85.118.1.10]:54078 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1755003Ab1ACPOc convert rfc822-to-8bit (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 3 Jan 2011 10:14:32 -0500
Subject: Re: [PATCH UPDATED] workqueue: relax lockdep annotation on
 flush_work()
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>, "Rafael J. Wysocki" <rjw@sisk.pl>,
        linux-kernel@vger.kernel.org
In-Reply-To: <20110103150052.GU18831@htj.dyndns.org>
References: <20101229125711.GL488@htj.dyndns.org>
	 <1294047091.2016.38.camel@laptop> <20110103141735.GQ18831@htj.dyndns.org>
	 <1294066490.2016.81.camel@laptop>  <20110103150052.GU18831@htj.dyndns.org>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8BIT
Date: Mon, 03 Jan 2011 16:14:45 +0100
Message-ID: <1294067685.2016.83.camel@laptop>
Mime-Version: 1.0
X-Mailer: Evolution 2.30.3 
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Mon, 2011-01-03 at 16:00 +0100, Tejun Heo wrote:
> Hello,
> 
> On Mon, Jan 03, 2011 at 03:54:50PM +0100, Peter Zijlstra wrote:
> > On Mon, 2011-01-03 at 15:17 +0100, Tejun Heo wrote:
> > 
> > > @@ -2384,8 +2384,18 @@ static bool start_flush_work(struct work_struct *work, struct wq_barrier *barr,
> > >  	insert_wq_barrier(cwq, barr, work, worker);
> > >  	spin_unlock_irq(&gcwq->lock);
> > >  
> > > -	lock_map_acquire(&cwq->wq->lockdep_map);
> > > +	/*
> > > +	 * If @max_active is 1 or rescuer is in use, flushing another work
> > > +	 * item on the same workqueue may lead to deadlock.  Make sure the
> > > +	 * flusher is not running on the same workqueue by verifying write
> > > +	 * access.
> > > +	 */
> > > +	if (cwq->wq->saved_max_active == 1 || cwq->wq->flags & WQ_RESCUER)
> > > +		lock_map_acquire(&cwq->wq->lockdep_map);
> > > +	else
> > > +		lock_map_acquire_read(&cwq->wq->lockdep_map);
> > >  	lock_map_release(&cwq->wq->lockdep_map);
> > > +
> > >  	return true;
> > >  already_gone:
> > >  	spin_unlock_irq(&gcwq->lock);
> > 
> > Ah, but this violates the rule that you must always use the most strict
> > constraints. Code doesn't know if it will run in a rescue thread or not,
> > hence it must assume it does.
> 
> Hmmm?  The code applies the most strict contraints.  If the workqueue
> has a rescuer, flushing another work from the workqueue will always
> trigger lockdep warning.  The rule is relaxed only for workqueues
> which aren't used for memory reclaiming && support parallel execution.

Ah, ok. I read it like: if the current thread is a rescue thread.