linux-kernel - Re: [RFC/PATCH] debug workqueue deadlocks with lockdep

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Date:	Wed, 4 Jul 2007 14:21:59 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Oleg Nesterov <oleg@...sign.ru>, Ingo Molnar <mingo@...hat.com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Sattler <tsattler@....de>
Subject: Re: [RFC/PATCH] debug workqueue deadlocks with lockdep


* Johannes Berg <johannes@...solutions.net> wrote:

> > > @@ -257,7 +260,9 @@ static void run_workqueue(struct cpu_wor
> > >
> > >  		BUG_ON(get_wq_data(work) != cwq);
> > >  		work_clear_pending(work);
> > > +		lock_acquire(&cwq->wq->lockdep_map, 0, 0, 0, 2, _THIS_IP_);
> > >  		f(work);
> > > +		lock_release(&cwq->wq->lockdep_map, 0, _THIS_IP_);
> >                                                    ^^^
> > Isn't it better to call lock_release() with nested == 1 ?
> 
> Not sure, Ingo?

well, in this case the lock/unlock should nest perfectly (i.e. it should 
always be balanced perfectly), so indeed calling with nested==1 leads to 
stricter checking.

non-nested unlocks occur when people do stuff like:

	spin_lock(&lock1);
	spin_lock(&lock2);
	spin_unlock(&lock1);
	spin_unlock(&lock2);

the first unlock is not 'nested perfectly'. Now for the workqueue 
dep_map this shouldnt be a legal combination, right?

	Ingo
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