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Date:	Thu, 19 Jun 2014 22:37:17 +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 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?

If that's impossible due to other memory allocator constraints, then
instead of inventing a whole new API we can simply flag the relevent
data in the memory allocator as we do with the debug objects mem cache
itself (SLAB_DEBUG_OBJECTS).

Thanks,

	tglx

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