Re: _cpu_down deadlock [was Re: 2.6.19-rc1-mm1]

[Andrew Morton]

On Wed, 11 Oct 2006 23:08:21 +1000
Neil Brown <neilb@xxxxxxx> wrote:

> On Wednesday October 11, arjan@xxxxxxxxxxxxx wrote:
> > 
> > > > 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)
> 
> I see your point.
> 
> While thread A holds just workqueue_mutex,
> thread B takes cpu_chain.rwsem for read then tries to take
> workqueue_mutex and blocks.
> Now thread C tries to get a write lock on cpu_chain.rwsem and blocks
> as well.
> Finally thread A moves on to try to get a read lock on cpu_chain.rwsem
> and this blocks because thread C is waiting for a write lock.
> 
> So A waits on B and C, C waits on B, B waits on A.
> Deadlock.

Except the entire operation is serialised by the the two top-level callers
(cpu_up() and cpu_down()) taking mutex_lock(&cpu_add_remove_lock).  Can
lockdep be taught about that?

> Who do we blame this on?  Are you still the cpu-hot-plug guy Rusty?

It's fun blaming Rusty for stuff, but he can dodge this one with
more-than-usual ease, I'm afraid.
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