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Date:	Thu, 19 Jun 2014 23:32:41 +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 10:37:17PM +0200, Thomas Gleixner wrote:
> > On Thu, 19 Jun 2014, Paul E. McKenney wrote:
> > > On Thu, Jun 19, 2014 at 09:29:08PM +0200, Thomas Gleixner wrote:
> > > > On Thu, 19 Jun 2014, Paul E. McKenney 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 ....
> > > 
> > > OK, got it.  And you are right, call_rcu() has done this for a very
> > > long time, so not sure what changed.  But it seems like the right
> > > approach is to provide a debug-object-free call_rcu_alloc() for use
> > > by the memory allocators.
> > > 
> > > Seem reasonable?  If so, please see the following patch.
> > 
> > Not really, you're torpedoing the whole purpose of debugobjects :)
> > 
> > So, why can't we just init the rcu head when the stuff is created?
> 
> That would allow me to keep my code unchanged, so I am in favor.  ;-)

Almost unchanged. You need to provide a function to do so, i.e. make
use of

    debug_init_rcu_head()

Thanks,

	tglx
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