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Date:	Thu, 19 Jun 2014 13:53:07 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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, 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.  ;-)

							Thanx, Paul

> 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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