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Date:	Thu, 19 Jun 2014 21:29:08 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Christoph Lameter <cl@...two.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: slub/debugobjects: lockup when freeing memory

On Thu, 19 Jun 2014, Paul E. McKenney wrote:

> On Thu, Jun 19, 2014 at 10:03:04AM -0500, Christoph Lameter wrote:
> > On Thu, 19 Jun 2014, Sasha Levin wrote:
> > 
> > > [  690.770137] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
> > > [  690.770137] __slab_alloc (mm/slub.c:1732 mm/slub.c:2205 mm/slub.c:2369)
> > > [  690.770137] ? __lock_acquire (kernel/locking/lockdep.c:3189)
> > > [  690.770137] ? __debug_object_init (lib/debugobjects.c:100 lib/debugobjects.c:312)
> > > [  690.770137] kmem_cache_alloc (mm/slub.c:2442 mm/slub.c:2484 mm/slub.c:2489)
> > > [  690.770137] ? __debug_object_init (lib/debugobjects.c:100 lib/debugobjects.c:312)
> > > [  690.770137] ? debug_object_activate (lib/debugobjects.c:439)
> > > [  690.770137] __debug_object_init (lib/debugobjects.c:100 lib/debugobjects.c:312)
> > > [  690.770137] debug_object_init (lib/debugobjects.c:365)
> > > [  690.770137] rcuhead_fixup_activate (kernel/rcu/update.c:231)
> > > [  690.770137] debug_object_activate (lib/debugobjects.c:280 lib/debugobjects.c:439)
> > > [  690.770137] ? discard_slab (mm/slub.c:1486)
> > > [  690.770137] __call_rcu (kernel/rcu/rcu.h:76 (discriminator 2) kernel/rcu/tree.c:2585 (discriminator 2))
> > 
> > __call_rcu does a slab allocation? This means __call_rcu can no longer be
> > used in slab allocators? What happened?
> 
> My guess is that the root cause is a double call_rcu(), call_rcu_sched(),
> call_rcu_bh(), or call_srcu().
> 
> Perhaps the DEBUG_OBJECTS code now allocates memory to report errors?
> That would be unfortunate...

Well, no. Look at the callchain:

__call_rcu
    debug_object_activate
       rcuhead_fixup_activate
          debug_object_init
              kmem_cache_alloc

So call rcu activates the object, but the object has no reference in
the debug objects code so the fixup code is called which inits the
object and allocates a reference ....

Thanks,

	tglx
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