From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: Deadlock scenario in regulator core
Date: Fri, 25 Mar 2011 11:55:58 +0100
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To: Steven Rostedt <rostedt@goodmis.org>
Cc: David Collins <collinsd@codeaurora.org>, Liam Girdwood <lrg@slimlogic.co.uk>, Mark Brown <broonie@opensource.wolfsonmicro.com>, linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>

On Tue, 2011-03-22 at 20:07 -0400, Steven Rostedt wrote:
> On Tue, 2011-03-22 at 16:41 -0700, David Collins wrote:
> 
> > There seem to be very few uses of mutex_lock_nested() in the kernel.  Most
> > of them use subclass = SINGLE_DEPTH_NESTING.  Would this be sufficient for
> > usage in the regulator core in _notifier_call_chain (and perhaps other
> > places) or should some other subclass be used?
> 
> Note, I do not know this code well enough to say. I'm assuming that an
> rdevA on a rdevB->supply_list never has rdevB on its own
> rdevA->supply_list.
> 
> If this is the case, and that you only ever have a lock nesting of one,
> then sure, use the SINGLE_DEPTH_NESTING.
> 
> Peter or Ingo could correct me if I'm wrong.

Right, so be aware that you can annotate an actual deadlock away with
mutex_lock_nested(), so use with care. The thing to avoid is something
like:

  mutex_lock(instance1);
  mutex_lock_nested(instance2, SINGLE_DEPTH_NESTING);

vs

  mutex_lock(instance2);
  mutex_lock_nested(instance1, SINGLE_DEPTH_NESTING);

Lockdep will not complain anymore but it will cause deadlocks.