linux-kernel - Deadlock scenario in regulator core

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Message-ID: <4D891C59.1030009@codeaurora.org>
Date:	Tue, 22 Mar 2011 15:02:01 -0700
From:	David Collins <collinsd@...eaurora.org>
To:	Liam Girdwood <lrg@...mlogic.co.uk>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
CC:	linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Deadlock scenario in regulator core

Hi Liam and Mark,

I was analyzing the mutex lock usage in drivers/regulator/core.c and found
at least one way to reach deadlock: regulator_enable is called for a
regulator at the same time that regulator_disable is called for that
regulator's supply.  Consider this simple example.  There are two
regulators: S1 and L2, as well as two consumers: A and B.  They are
connected as follows:

S1 --> L2 --> B
|
|--> A

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?

Thanks,
David Collins

-- 
Sent by an employee of the Qualcomm Innovation Center, Inc.
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