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

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Message-ID: <506BB283.4010800@linux.vnet.ibm.com>
Date:	Wed, 03 Oct 2012 09:05:31 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Jiri Kosina <jkosina@...e.cz>
CC:	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier()
 dependency on __stop_machine()")

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).

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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