Re: [RFC][PATCH 09/13] hotplug: Replace hotplug lock with percpu-rwsem

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 23, 2015 at 07:01:22PM +0200, Oleg Nesterov wrote:
> On 06/23, Peter Zijlstra wrote:
> >
> > On Tue, Jun 23, 2015 at 12:57:39AM +0200, Oleg Nesterov wrote:
> > > > +
> > > > +	lock_map_acquire_read(&cpu_hotplug.rwsem.rw_sem.dep_map);
> > > > +	_percpu_down_read(&cpu_hotplug.rwsem);
> > > >  }
> > >
> > > Confused... Why do we need _percpu_down_read()? Can't get_online_cpus()
> > > just use percpu_down_read() ?
> > >
> > > Yes, percpu_down_read() is not recursive, like the normal down_read().
> > > But this does not matter because we rely on ->cpuhp_ref anyway?
> >
> > While we will not call the actual lock, lockdep will still get confused
> > by the inconsistent locking order observed.
> >
> > Change it and boot, you'll find lockdep output pretty quickly.
> 
> Hmm. and I simply can't understand why...

If in one callchain we do:

	get_online_cpus();
	lock(A);

in another we do:

	lock(A);
	get_online_cpus();

lockdep will complain about the inverted lock order, however this is not
a problem at all for recursive locks.

I think the example you get on boot is slightly more complicated, but
ends up like the above iirc.