linux-kernel - Re: 2.6.19-rc1-mm1

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Message-ID: <17708.33450.608010.113968@cse.unsw.edu.au>
Date:	Wed, 11 Oct 2006 15:35:38 +1000
From:	Neil Brown <neilb@...e.de>
To:	"Michal Piotrowski" <michal.k.k.piotrowski@...il.com>
Cc:	"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

On Tuesday October 10, michal.k.k.piotrowski@...il.com wrote:
> On 10/10/06, Michal Piotrowski <michal.k.k.piotrowski@...il.com> wrote:
> > Hi,
> >
> > On 10/10/06, Andrew Morton <akpm@...l.org> wrote:
> > >
> > > ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.19-rc1/2.6.19-rc1-mm1/
> > >
> >
> > Kernel 2.6.19-rc1-mm1 + Neil's avoid_lockdep_warning_in_md.patch
> > (http://www.ussg.iu.edu/hypermail/linux/kernel/0610.1/0642.html)
> >
> > (I'll try to reproduce this without Neil's patch).
> 
> I can't reproduce this without Neil's patch.
> 

Despite this circumstantial evidence, I don't see how my patch could
possible have an effect here....

Looking at the code, starting at _cpu_down in the CONFIG_HOTPLUG_CPU
case, the call notifier chain 'cpu_chain' contains
workqueue_cpu_callback which does 'mutex_lock(&workqueue_mutex)' in
the "DOWN_PREPARE" case and mutex_unlock(&workqueue_mutex) in the
DOWN_FAILED and DEAD cases.

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.

I don't know how to tell lockdep to do the right thing, but I'll leave
that up to Ingo et al.

Why it didn't trigger without my patch I cannot imagine.  Are you sure
the config was identical (you didn't remove CONFIG_HOTPLUG_CPU or
anything did you?).

NeilBrown

> >
> > echo shutdown > /sys/power/disk; echo disk > /sys/power/state
> >
> > =======================================================
> > [ INFO: possible circular locking dependency detected ]
> > 2.6.19-rc1-mm1 #4
> > -------------------------------------------------------
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