Deadlock scenario in regulator core

[David Collins <collinsd@codeaurora.org>]

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

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