Re: kmemcheck caught read from freed memory (cfq_free_io_context)

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Apr 02, 2008 at 03:40:46PM +0200, Jens Axboe wrote:
> 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@chello.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@oracle.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 :-)

Hey, I am not complaining about 2007 being gone already, because I am
still wondering what the heck happened to 2005 and 2006!!!  ;-)

							Thanx, Paul

> > Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Thanks, added.
> 
> -- 
> Jens Axboe
>