linux-kernel - Re: [PATCH UPDATED] workqueue: relax lockdep annotation on flush

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Message-ID: <20110103150052.GU18831@htj.dyndns.org>
Date:	Mon, 3 Jan 2011 16:00:52 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...hat.com>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH UPDATED] workqueue: relax lockdep annotation on
 flush_work()

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.

Thanks.

-- 
tejun
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