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

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Date:	Wed, 3 Oct 2012 01:48:21 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Josh Triplett <josh@...htriplett.org>,
	linux-kernel@...r.kernel.org,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove
 _rcu_barrier() dependency on __stop_machine()")

On Tue, 2 Oct 2012, Paul E. McKenney wrote:

> Indeed.  Slab seems to be doing an rcu_barrier() in a CPU hotplug 
> notifier, which doesn't sit so well with rcu_barrier() trying to exclude 
> CPU hotplug events.  I could go back to the old approach, but it is 
> significantly more complex.  I cannot say that I am all that happy about 
> anyone calling rcu_barrier() from a CPU hotplug notifier because it 
> doesn't help CPU hotplug latency, but that is a separate issue.
> 
> But the thing is that rcu_barrier()'s assumptions work just fine if either
> (1) it excludes hotplug operations or (2) if it is called from a hotplug
> notifier.  You see, either way, the CPU cannot go away while rcu_barrier()
> is executing.  So the right way to resolve this seems to be to do the
> get_online_cpus() only if rcu_barrier() is -not- executing in the context
> of a hotplug notifier.  Should be fixable without too much hassle...

Sorry, I don't think I understand what you are proposing just yet.

If I understand it correctly, you are proposing to introduce some magic 
into _rcu_barrier() such as (pseudocode of course):

	if (!being_called_from_hotplug_notifier_callback)
		get_online_cpus()

How does that protect from the scenario I've outlined before though?

	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)	

CPU 0 grabs both locks anyway (it's not running from notifier callback). 
CPU 1 grabs both locks as well, as there is no _rcu_barrier() being called 
from notifier callback either.

What did I miss?

Thanks,

-- 
Jiri Kosina
SUSE Labs
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