Re: netfilter: active obj WARN when cleaning up

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Wed, Nov 27, 2013 at 01:32:31PM +0000, Russell King - ARM Linux wrote:
> On Wed, Nov 27, 2013 at 02:29:41PM +0100, Thomas Gleixner wrote:
> > Though the kobject is the only thing which has a delayed work embedded
> > inside struct kmem_cache. And the debug object splat points at the
> > kmem_cache_free() of the struct kmem_cache itself. That's why I
> > assumed the wreckage around that place. And indeed:
> > 
> > kmem_cache_destroy(s)
> >     __kmem_cache_shutdown(s)
> >       sysfs_slab_remove(s)
> >         ....
> > 	kobject_put(&s->kobj)
> >            kref_put(&kobj->kref, kobject_release);
> > 	     kobject_release(kref)
> >     	       #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
> > 	         schedule_delayed_work(&kobj->release)
> > 	       #else
> > 	        kobject_cleanup(kobj)
> > 	       #endif
> > 
> > So in the CONFIG_DEBUG_KOBJECT_RELEASE=y case, schedule_delayed_work()
> > _IS_ called which arms the timer. debugobjects catches the attempt to
> > free struct kmem_cache which contains the armed timer.
> 
> You fail to show where the free is in the above path.

Right, there's a kmem_cache_free(kmem_cache, s); by the slob code
after the above sequence, which is The Bug(tm).

As I said, a kobject has its own lifetime.  If you embed that into
another structure, that structure inherits the lifetime of the kobject,
which is from the point at which it's created to the point at which the
kobject's release function is called.

So no, the code here is buggy.  The kobject debugging has yet again
found a violation of the kobject lifetime rules.  slub needs fixing.