Re: [PATCH v3] mm, slab: release slab_mutex earlier in kmem_cache_destroy()

[Srivatsa S. Bhat]

On 10/03/2012 08:04 PM, Jiri Kosina wrote:
> On Wed, 3 Oct 2012, Christoph Lameter wrote:
> 
>>> How about the patch below? Pekka, Christoph, please?
>>
>> Looks fine for -stable. For upstream there is going to be a move to
>> slab_common coming in this merge period. We would need a fix against -next
>> or Pekka's tree too.
> 
> Thanks Christoph. Patch against Pekka's slab/for-linus branch below.
> 
> I have kept the Acked-by/Reviewed-by from the version of the patch against 
> current Linus' tree, if anyone object, please shout loudly. Ideally should 
> go in during this merge window to keep lockdep happy.
> 
> 
> 
> 
> 
> From: Jiri Kosina <jkosina@xxxxxxx>
> Subject: [PATCH] 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().
> 
> Lockdep thinks that this might actually result in ABBA deadlock,
> and reports it as below:
> 
[...] 
> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Acked-by: Christoph Lameter <cl@xxxxxxxxx>
> Signed-off-by: Jiri Kosina <jkosina@xxxxxxx>
> ---
>  mm/slab_common.c |    4 +++-
>  1 files changed, 3 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 9c21725..90c3053 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -166,6 +166,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  	s->refcount--;
>  	if (!s->refcount) {
>  		list_del(&s->list);
> +		mutex_unlock(&slab_mutex);
> 
>  		if (!__kmem_cache_shutdown(s)) {

__kmem_cache_shutdown() calls __cache_shrink(). And __cache_shrink() has this
comment over it:
/* Called with slab_mutex held to protect against cpu hotplug */

So, I guess the question is whether to modify your patch to hold the slab_mutex
while calling this function, or to update the comment on top of this function
saying that we are OK to call this function (even without slab_mutex) when we
are inside a get/put_online_cpus() section.

>  			if (s->flags & SLAB_DESTROY_BY_RCU)
> @@ -179,8 +180,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
>  				s->name);
>  			dump_stack();

There is a list_add() before this dump_stack(). I assume we need to hold the
slab_mutex while calling it.

>  		}
> +	} else {
> +		mutex_unlock(&slab_mutex);
>  	}
> -	mutex_unlock(&slab_mutex);
>  	put_online_cpus();
>  }
>  EXPORT_SYMBOL(kmem_cache_destroy);
> 

Regards,
Srivatsa S. Bhat

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