Re: Deadlock scenario in regulator core

[Steven Rostedt]

[ Added Peter and Ingo on Cc ]

On Tue, 2011-03-22 at 16:08 -0700, David Collins wrote:
> 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?

Looks to me that the mutex_lock() in _notifier_call_chain needs to be a
mutex_lock_nested().

The "_nested()" versions are when you have the same type of mutex taken
but belonging to two different instances. Like you have here:

	blocking_notifier_call_chain(&rdev->notifier, event, NULL);

	/* now notify regulator we supply */
	list_for_each_entry(_rdev, &rdev->supply_list, slist) {
		mutex_lock(&_rdev->mutex);
		_notifier_call_chain(_rdev, event, data);
		mutex_unlock(&_rdev->mutex);
	}

The rdev->mutex is already held, so we don't need to take it to call the
blocking_notifier_call_chain() with the rdev->notifier. But then the
list of rdev's in the rdev->supply_list are different instances but we
are still taking the same type of lock. lockdep treats all instances of
the same lock the same, so to lockdep this looks like a deadlock. To
teach lockdep that this is a different instance, simply use
mutex_lock_nested() instead.

-- Steve


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