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

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Date:	Wed, 2 Apr 2008 15:40:46 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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, Paul E. McKenney wrote:
> On Wed, Apr 02, 2008 at 01:33:27PM +0200, 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.
> 
> Looks good to me from a strictly RCU viewpoint -- I must confess great
> ignorance of the CFQ code.  :-/

That can always be rectified, given enough spare time :-)

> Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Thanks, added.

-- 
Jens Axboe

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