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Date: Tue, 23 Jun 2015 19:53:18 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: paulmck@linux.vnet.ibm.com, tj@kernel.org, mingo@redhat.com,
        linux-kernel@vger.kernel.org, der.herr@hofr.at, dave@stgolabs.net,
        riel@redhat.com, viro@ZenIV.linux.org.uk,
        torvalds@linux-foundation.org
Subject: Re: [RFC][PATCH 09/13] hotplug: Replace hotplug lock with
 percpu-rwsem
Message-ID: <20150623175318.GE3644@twins.programming.kicks-ass.net>
References: <20150622121623.291363374@infradead.org>
 <20150622122256.480062572@infradead.org>
 <20150622225739.GA5582@redhat.com>
 <20150623071637.GA3644@twins.programming.kicks-ass.net>
 <20150623170122.GA26854@redhat.com>
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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.