linux-kernel - Re: 2.6.19-rc1-mm1

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Message-Id: <1160565786.3000.369.camel@laptopd505.fenrus.org>
Date:	Wed, 11 Oct 2006 13:23:06 +0200
From:	Arjan van de Ven <arjan@...radead.org>
To:	Michal Piotrowski <michal.k.k.piotrowski@...il.com>
Cc:	Neil Brown <neilb@...e.de>, Andrew Morton <akpm@...l.org>,
	Pavel Machek <pavel@....cz>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.19-rc1-mm1


> > blocking_notifier_call_chain is
> >         down_read(&nh->rwsem);
> >         ret = notifier_call_chain(&nh->head, val, v);
> >         up_read(&nh->rwsem);
> >
> > and so holds ->rwsem while calling the callback.
> > So the locking sequence ends up as:
> >
> >  down_read(&cpu_chain.rwsem);
> >  mutex_lock(&workqueue_mutex);
> >  up_read(&cpu_chain.rwsem);
> >
> >  down_read(&cpu_chain.rwsem);
> >  mutex_unlock(&workqueue_mutex);
> >  up_read(&workqueue_mutex);
> >
> > and lockdep doesn't seem to like this.  It sees workqueue_mutex
> > claimed while cpu_chain.rwsem is held. and then it sees
> > cpu_chain.rwsem claimed while workqueue_mutex is held, which looks a
> > bit like a class ABBA deadlock.
> > Of course because it is a 'down_read' rather than a 'down', it isn't
> > really a dead lock.

ok can you explain to me why "down_read" doesn't make this a deadlock
while "down" would make it a deadlock? I have trouble following your
reasoning.....

(remember that rwsems are strictly fair)


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