[PATCH] mm, slab: release slab_mutex earlier in kmem_cache_destroy()(was Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier()dependency on __stop_machine()"))

[Jiri Kosina]

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

> On Wed, Oct 03, 2012 at 01:48:21AM +0200, Jiri Kosina wrote:
> > 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?
> 
> You didn't miss anything, I was suffering a failure to read carefully.
> 
> So my next stupid question is "Why can't kmem_cache_destroy drop
> slab_mutex early?" like the following:
> 
> 	void kmem_cache_destroy(struct kmem_cache *cachep)
> 	{
> 		BUG_ON(!cachep || in_interrupt());
> 
> 		/* Find the cache in the chain of caches. */
> 		get_online_cpus();
> 		mutex_lock(&slab_mutex);
> 		/*
> 		 * the chain is never empty, cache_cache is never destroyed
> 		 */
> 		list_del(&cachep->list);
> 		if (__cache_shrink(cachep)) {
> 			slab_error(cachep, "Can't free all objects");
> 			list_add(&cachep->list, &slab_caches);
> 			mutex_unlock(&slab_mutex);
> 			put_online_cpus();
> 			return;
> 		}
> 		mutex_unlock(&slab_mutex);
> 
> 		if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU))
> 			rcu_barrier();
> 
> 		__kmem_cache_destroy(cachep);
> 		put_online_cpus();
> 	}
> 
> Or did I miss some reason why __kmem_cache_destroy() needs that lock?
> Looks to me like it is just freeing now-disconnected memory.

Good question. I believe it should be safe to drop slab_mutex earlier, as 
cachep has already been unlinked. I am adding slab people and linux-mm to 
CC (the whole thread on LKML can be found at 
https://lkml.org/lkml/2012/10/2/296 for reference).

How about the patch below? Pekka, Christoph, please?

It makes the lockdep happy again, and obviously removes the deadlock (I 
tested it).



From: Jiri Kosina <jkosina@xxxxxxx>
Subject: mm, slab: release slab_mutex earlier in kmem_cache_destroy()

Commit 1331e7a1bbe1 ("rcu: Remove _rcu_barrier() dependency on
__stop_machine()") introduced slab_mutex -> cpu_hotplug.lock
dependency through kmem_cache_destroy() -> rcu_barrier() ->
_rcu_barrier() -> get_online_cpus().

This opens a possibilty for deadlock:

        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)

It turns out that slab's kmem_cache_destroy() might release slab_mutex
earlier before calling out to rcu_barrier(), as cachep has already been
unlinked.

This patch removes the AB-BA dependency by calling rcu_barrier() with 
slab_mutex already unlocked.

Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
---
 mm/slab.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index 1133911..693c7cb 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -2801,12 +2801,12 @@ void kmem_cache_destroy(struct kmem_cache *cachep)
 		put_online_cpus();
 		return;
 	}
+	mutex_unlock(&slab_mutex);
 
 	if (unlikely(cachep->flags & SLAB_DESTROY_BY_RCU))
 		rcu_barrier();
 
 	__kmem_cache_destroy(cachep);
-	mutex_unlock(&slab_mutex);
 	put_online_cpus();
 }
 EXPORT_SYMBOL(kmem_cache_destroy);

-- 
Jiri Kosina
SUSE Labs
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