On 11/10/06, Neil Brown <[email protected]> wrote:
On Tuesday October 10, [email protected] wrote:
> On 10/10/06, Michal Piotrowski <[email protected]> wrote:
> > Hi,
> >
> > On 10/10/06, Andrew Morton <[email protected]> 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?).
No, I didn't remove CONFIG_HOTPLUG_CPU or anything else.
I didn't do enough testing - only a few hibernatins.
NeilBrown
Regards,
Michal
--
Michal K. K. Piotrowski
LTG - Linux Testers Group
(http://www.stardust.webpages.pl/ltg/)
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