linux-kernel - Re: kmemcheck caught read from freed memory (cfq_free_io

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Date:	Wed, 2 Apr 2008 13:43:38 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	Pekka J Enberg <penberg@...helsinki.fi>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)

On Wed, Apr 02 2008, Fabio Checconi wrote:
> > From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date: Wed, Apr 02, 2008 12:59:21PM +0200
> >
> > On Wed, 2008-04-02 at 03:55 -0700, Paul E. McKenney wrote:
> > > On Wed, Apr 02, 2008 at 09:28:46AM +0200, Ingo Molnar wrote:
> > > > 
> > > > * Jens Axboe <jens.axboe@...cle.com> wrote:
> > > > 
> > > > > On Wed, Apr 02 2008, Pekka J Enberg wrote:
> > > > > > On Wed, 2 Apr 2008, Jens Axboe wrote:
> > > > > > > Good catch, I wonder why it didn't complain in my testing. I've added a
> > > > > > > patch to fix that, please see it here:
> > > > > > 
> > > > > > You probably don't have kmemcheck in your kernel ;-)
> > > > > 
> > > > > Ehm no, you are right :)
> > > > 
> > > > ... and you can get kmemcheck by testing on x86.git/latest:
> > > > 
> > > >   http://people.redhat.com/mingo/x86.git/README
> > > > 
> > > > ;-)
> > > 
> > > I will check this when I get back to some bandwidth -- but in the meantime,
> > > does kmemcheck special-case SLAB_DESTROY_BY_RCU?  It is legal to access
> > > newly-freed items in that case, as long as you did rcu_read_lock()
> > > before gaining a reference to them and don't hold the reference past
> > > the matching rcu_read_unlock().
> > 
> > I don't think it does.
> > 
> > It would have to register an call_rcu callback itself in order to mark
> > it freed - and handle the race with the object being handed out again.
> > 
> 
> I had the same problem while debugging a cfq-derived i/o scheduler,
> and I found nothing preventing the reuse of the freed memory.
> The patch below seemed to fix the logic.

Thanks, from a first look this looks like it'll fix this bad rcu slab
usage. I'll give it some closer scrutiny and testing.

-- 
Jens Axboe

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