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Date:	Thu, 19 Jun 2014 13:36:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <cl@...two.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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, Jun 19, 2014 at 03:19:39PM -0500, Christoph Lameter wrote:
> On Thu, 19 Jun 2014, Thomas Gleixner wrote:
> 
> > 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 ....
> 
> So we need to init the object in the page struct before the __call_rcu?

Good point.  The patch I just sent will complain at callback-invocation
time because the debug-object information won't be present.

One way to handle this would be for rcu_do_batch() to avoid complaining
if it gets a callback that has not been through call_rcu()'s
debug_rcu_head_queue().  One way to do that would be to have an
alternative to debug_object_deactivate() that does not complain
if it is handed an unactivated object.

Another way to handle this would be for me to put the definition of
debug_rcu_head_queue() somewhere where the sl*b allocator could get
at it, and have the sl*b allocators invoke it some at initialization
and within the RCU callback.

Other thoughts?

							Thanx, Paul

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