On Wed, 27 Nov 2013, Russell King - ARM Linux wrote: > 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. 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. So much for rule #1 Thanks, tglx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>