Date:	Wed, 03 Oct 2012 09:45:00 +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 09:37 AM, Paul E. McKenney wrote:
> On Wed, Oct 03, 2012 at 09:29:01AM +0530, Srivatsa S. Bhat wrote:
>> On 10/03/2012 05:01 AM, Paul E. McKenney wrote:
>>> On Tue, Oct 02, 2012 at 11:58:36PM +0200, Jiri Kosina wrote:
>>>> On Tue, 2 Oct 2012, Jiri Kosina wrote:
>>>>
>>>>>>>> 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is the first bad commit
>>>>>>>> commit 1331e7a1bbe1f11b19c4327ba0853bee2a606543
>>>>>>>> Author: Paul E. McKenney <paul.mckenney@...aro.org>
>>>>>>>> Date:   Thu Aug 2 17:43:50 2012 -0700
>>>>>>>>
>>>>>>>>     rcu: Remove _rcu_barrier() dependency on __stop_machine()
>>>>>>>>     
>>>>>>>>     Currently, _rcu_barrier() relies on preempt_disable() to prevent
>>>>>>>>     any CPU from going offline, which in turn depends on CPU hotplug's
>>>>>>>>     use of __stop_machine().
>>>>>>>>     
>>>>>>>>     This patch therefore makes _rcu_barrier() use get_online_cpus() to
>>>>>>>>     block CPU-hotplug operations.  This has the added benefit of removing
>>>>>>>>     the need for _rcu_barrier() to adopt callbacks:  Because CPU-hotplug
>>>>>>>>     operations are excluded, there can be no callbacks to adopt.  This
>>>>>>>>     commit simplifies the code accordingly.
>>>>>>>>     
>>>>>>>>     Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
>>>>>>>>     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>>>>>>>>     Reviewed-by: Josh Triplett <josh@...htriplett.org>
>>>>>>>> ==
>>>>>>>>
>>>>>>>> is causing lockdep to complain (see the full trace below). I haven't yet 
>>>>>>>> had time to analyze what exactly is happening, and probably will not have 
>>>>>>>> time to do so until tomorrow, so just sending this as a heads-up in case 
>>>>>>>> anyone sees the culprit immediately.
>>>>>>>
>>>>>>> Hmmm...  Does the following patch help?  It swaps the order in which
>>>>>>> rcu_barrier() acquires the hotplug and rcu_barrier locks.
>>>>>>
>>>>>> It changed the report slightly (see for example the change in possible 
>>>>>> unsafe locking scenario, rcu_sched_state.barrier_mutex vanished and it's 
>>>>>> now directly about cpu_hotplug.lock). With the patch applied I get
>>>>>>
>>>>>>
>>>>>>
>>>>>> ======================================================
>>>>>> [ INFO: possible circular locking dependency detected ]
>>>>>> 3.6.0-03888-g3f99f3b #145 Not tainted
>>>>>
>>>>> And it really seems valid. 
>>>
>>> Yep, it sure is.  I wasn't getting the full picture earlier, so please
>>> accept my apologies for the bogus patch.
>>>
>>>>> kmem_cache_destroy() calls rcu_barrier() with slab_mutex locked, which 
>>>>> introduces slab_mutex -> cpu_hotplug.lock dependency (through 
>>>>> rcu_barrier() -> _rcu_barrier() -> get_online_cpus()).
>>>>>
>>>>> On the other hand, _cpu_up() acquires cpu_hotplug.lock through 
>>>>> cpu_hotplug_begin(), and with this lock held cpuup_callback() notifier 
>>>>> gets called, which acquires slab_mutex. This gives the reverse dependency, 
>>>>> i.e. deadlock scenario is valid one.
>>>>>
>>>>> 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is triggering this, because 
>>>>> before that, there was no slab_mutex -> cpu_hotplug.lock dependency.
>>>>>
>>>>> Simply put, the commit causes get_online_cpus() to be called with 
>>>>> slab_mutex held, which is invalid.
>>>>
>>>> Oh, and it seems to be actually triggering in real.
>>>>
>>>> With HEAD being 974a847e00c, machine suspends nicely. With 974a847e00c + 
>>>> your patch, changing the order in which rcu_barrier() acquires hotplug and 
>>>> rcu_barrier locks, the machine hangs 100% reliably during suspend, which 
>>>> very likely actually is the deadlock described above.
>>>
>>> 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.
>>
>> Why not? IMHO it should have been perfectly fine! See below...
>>
>>>  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...
>>>
>>
>> The thing is, get_online_cpus() is smart: it *knows* when you are calling
>> it in a hotplug-writer, IOW, when you are in a hotplug notifier.
>>
>> The relevant code is:
>>
>> void get_online_cpus(void)
>> {
>>         might_sleep();
>>         if (cpu_hotplug.active_writer == current)
>>                 return;
>> 	....
>> }
>>
>> So calling rcu_barrier() (and hence get_online_cpus()) from within a hotplug
>> notifier should pose no problem at all!
> 
> Indeed, that was my confusion.  The deadlock can happen with
> the slab CPU-hotplug notifier (without calling rcu_barrier()!), which
> establishes hotplug->slab.  The some other unrelated thread calls
> kmem_cache_destroy(), which acquires slab and then calls rcu_barrier(),
> which acquires hotplug.  So the deadlock can happen independently of
> rcu_barrier() being called from a CPU-hotplug notifier.
> 

Right, this is exactly what I thought yesterday. I had drafted a mail explaining
why the length of the circular locking dependency is really 2 but not 3 and
why the rcu_barrier() (barrier_mutex) is only aggravating a problem that is
there even without using rcu_barrier() at all. But then I stopped short of posting
it when I noticed the get/put_online_cpus() in kmem_cache_destroy() which really
looked puzzling to me. I (still) can't get myself to believe that kmem_cache_destroy()
could go beyond its get_online_cpus() and call rcu_barrier() at all, in the
presence of a concurrent CPU hotplug notifier!

Regards,
Srivatsa S. Bhat

> Making kmem_cache_destroy() release slab before calling rcu_barrier()
> seems to clear things up for Jiri, but we need Pekka's or Christoph
> Lameter's view on whether this is really safe.  (It looks safe to
> both Jiri and I, but...)
> 
> 							Thanx, Paul
> 

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