linux-kernel - Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop

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Message-ID: <506BB950.3000102@linux.vnet.ibm.com>
Date:	Wed, 03 Oct 2012 09:34:32 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Jiri Kosina <jkosina@...e.cz>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier()
 dependency on __stop_machine()")

On 10/03/2012 09:14 AM, Paul E. McKenney wrote:
> On Wed, Oct 03, 2012 at 09:05:31AM +0530, Srivatsa S. Bhat wrote:
>> On 10/03/2012 03:47 AM, Jiri Kosina wrote:
>>> On Wed, 3 Oct 2012, Srivatsa S. Bhat wrote:
>>>
>>>> I don't see how this circular locking dependency can occur.. If you are using SLUB,
>>>> kmem_cache_destroy() releases slab_mutex before it calls rcu_barrier(). If you are
>>>> using SLAB, kmem_cache_destroy() wraps its whole operation inside get/put_online_cpus(),
>>>> which means, it cannot run concurrently with a hotplug operation such as cpu_up(). So, I'm
>>>> rather puzzled at this lockdep splat..
>>>
>>> I am using SLAB here.
>>>
>>> The scenario I think is very well possible:
>>>
>>>
>>> 	CPU 0				CPU 1
>>> 	kmem_cache_destroy()
>>
>> What about the get_online_cpus() right here at CPU0 before
>> calling mutex_lock(slab_mutex)? How can the cpu_up() proceed
>> on CPU1?? I still don't get it... :(
>>
>> (kmem_cache_destroy() uses get/put_online_cpus() around acquiring
>> and releasing slab_mutex).
> 
> The problem is that there is a CPU-hotplug notifier for slab, which
> establishes hotplug->slab.

Agreed.

>  Then having kmem_cache_destroy() call
> rcu_barrier() under the lock

Ah, that's where I disagree. kmem_cache_destroy() *cannot* proceed at
this point in time, because it has invoked get_online_cpus()! It simply
cannot be running past that point in the presence of a running hotplug
notifier! So, kmem_cache_destroy() should have been sleeping on the
hotplug lock, waiting for the notifier to release it, no?

> establishes slab->hotplug, which results
> in deadlock.  Jiri really did explain this in an earlier email
> message, but both of us managed to miss it.  ;-)
> 

Maybe I'm just being blind, sorry! ;-)

Regards,
Srivatsa S. Bhat

> 							Thanx, Paul
> 
>> Regards,
>> Srivatsa S. Bhat
>>
>>> 	mutex_lock(slab_mutex)
>>> 	 				_cpu_up()
>>> 					cpu_hotplug_begin()
>>> 					mutex_lock(cpu_hotplug.lock)
>>> 	rcu_barrier()
>>> 	_rcu_barrier()
>>> 	get_online_cpus()
>>> 	mutex_lock(cpu_hotplug.lock)
>>> 	 (blocks, CPU 1 has the mutex)
>>> 					__cpu_notify()
>>> 					mutex_lock(slab_mutex)
>>>
>>> Deadlock.
>>>
>>> Right?
>>>
>>
>>

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