Re: netfilter: active obj WARN when cleaning up

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

On Wed, Nov 27, 2013 at 11:45:17AM +0100, Thomas Gleixner wrote:
> On Wed, 27 Nov 2013, Pablo Neira Ayuso wrote:
> 
> > On Tue, Nov 26, 2013 at 02:11:57PM -0500, Sasha Levin wrote:
> > > Ping? I still see this warning.
> > 
> > Did your test include patch 0c3c6c00c6?
> 
> And how is that patch supposed to help?
>  
> > > >[  418.312449] WARNING: CPU: 6 PID: 4178 at lib/debugobjects.c:260 debug_print_object+0x8d/0xb0()
> > > >[  418.313243] ODEBUG: free active (active state 0) object type: timer_list hint:
> > > >delayed_work_timer_fn+0x0/0x20
> 
> > > >[  418.321101]  [<ffffffff812874d7>] kmem_cache_free+0x197/0x340
> > > >[  418.321101]  [<ffffffff81249e76>] kmem_cache_destroy+0x86/0xe0
> > > >[  418.321101]  [<ffffffff83d5d681>] nf_conntrack_cleanup_net_list+0x131/0x170
> 
> The debug code detects an active timer, which itself is part of a
> delayed work struct. The call comes from kmem_cache_destroy().
> 
>          kmem_cache_free(kmem_cache, s);
> 
> So debug object says: s contains an active timer. s is the kmem_cache
> which is destroyed from nf_conntrack_cleanup_net_list.
> 
> Now struct kmem_cache has in case of SLUB:
> 
>     struct kobject kobj;    /* For sysfs */
> 
> and struct kobject has:
> 
> #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
>     struct delayed_work     release;
> #endif
> 
> So this is the thing you want to look at:
> 
> commit c817a67ec (kobject: delayed kobject release: help find buggy
> drivers) added that delayed work thing.
> 
> I fear that does not work for kobjects which are embedded into
> something else.

No, kobjects embedded into something else have their lifetime determined
by the embedded kobject.  That's rule #1 of kobjects - or rather reference
counted objects.

The point at which the kobject gets destructed is when the release function
is called.  If it is destructed before that time, that's a violation of
the reference counted nature of kobjects, and that's what the delay on
releasing is designed to catch.

It's designed to catch code which does this exact path:

	put(obj)
	free(obj)

rather than code which does it the right way:

	put(obj)
		-> refcount becomes 0
			-> release function gets called
				->free(obj)

The former is unsafe because obj may have other references.