linux-kernel - Re: Deadlock scenario in regulator core

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Message-ID: <4D892C0A.1090606@codeaurora.org>
Date:	Tue, 22 Mar 2011 16:08:58 -0700
From:	David Collins <collinsd@...eaurora.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Liam Girdwood <lrg@...mlogic.co.uk>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>,
	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Deadlock scenario in regulator core

On 03/22/2011 03:37 PM, Steven Rostedt wrote:
> On Tue, Mar 22, 2011 at 03:02:01PM -0700, David Collins wrote:
>> Assume that A has already called regulator_enable for S1 some time in the
>> past.
>>
>> Consumer A thread execution:
>> 	regulator_disable(S1)
>> 	mutex_lock(S1)
>> 	_regulator_disable(S1)
>> 	_notifier_call_chain(S1)
>> 	mutex_lock(L2)
>>
>> Consumer B thread execution:
>> 	regulator_enable(L2)
>> 	mutex_lock(L2)
>> 	_regulator_enable(L2)
>> 	mutex_lock(S1)
>>
>> The locks for S1 and L2 are taken in opposite orders in the two threads;
>> therefore, it is possible to achieve deadlock.  I am not sure about the
>> best way to resolve this situation.  Is there a correctness requirement
>> that regulator_enable holds the child regulator's lock when it attempts to
>> enable the parent regulator?  Likewise, is the lock around
>> _notifier_call_chain required?
> 
> I'm curious, if you had enabled lockdep, do you get a warning? If not,
> why not?
> 
> Thanks,
> 
> -- Steve
> 
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I have tried running with lockdep enabled.  It does not produce a warning
about possible deadlock from locks being taken in opposite orders in two
threads.  I assume that this is because it can only keep track of locks
taken in the current stack backtrace.

It does produce a warning for regulator_disable by itself though on a
regulator with a non-empty supply_list:

 =============================================
 [ INFO: possible recursive locking detected ]
 2.6.38-rc7+ #231
 ---------------------------------------------
 sh/25 is trying to acquire lock:
  (&rdev->mutex){+.+...}, at: [<c0137ae4>] _notifier_call_chain+0x28/0x6c

 but task is already holding lock:
  (&rdev->mutex){+.+...}, at: [<c0138410>] regulator_disable+0x24/0x74

The locks that it is noting are different; one is for the parent regulator
and the other is for the child regulator. Any thoughts?

Thanks,
David

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.
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