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

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Date:	Wed, 3 Oct 2012 00:17:00 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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 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()
	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?

-- 
Jiri Kosina
SUSE Labs
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