Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove_rcu_barrier() dependency on __stop_machine()")

[Jiri Kosina]

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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