Re: netfilter: active obj WARN when cleaning up

[Greg KH <greg@kroah.com>]

On Wed, Nov 27, 2013 at 02:44:58PM +0100, Thomas Gleixner wrote:
> On Wed, 27 Nov 2013, Russell King - ARM Linux wrote:
> 
> > 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.
> 
> I leave that discussion to you, greg and the slub folks.

/me grabs some popcorn from tglx

It's really not that much of a discussion, Documentation/kobject.txt has
said this for years, it's as if no one even reads documentation
anymore...

If you embed a kobject into a structure, you have to use the kobject for
the reference counting of the structure, otherwise it's a bug.  If you
don't want to use a kobject to reference count the structure, don't
embed it into it, use a pointer.

Are "slabs" never freed in the slub allocator?  Surely someone should
have seen the huge "this kobject doesn't have a release function" error
message that the kernel should have spit out for it?

Just make the kobject "dynamic" instead of embedded in struct kmem_cache
and all will be fine.  I can't believe this code has been broken for
this long.

thanks,

greg k-h

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>