Re: kmemcheck caught read from freed memory (cfq_free_io_context)

[Jens Axboe <jens.axboe@oracle.com>]

On Wed, Apr 02 2008, Jens Axboe wrote:
> On Wed, Apr 02 2008, Jens Axboe wrote:
> > On Wed, Apr 02 2008, 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.
> > 
> > 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.
> 
> (CC reinstated, sometimes mutt is really annoying and drops the person
> you are replying too :-(

(for real...)

> 
> Looks good and tests fine as well. I've applied it, on top of the
> hlist_for_each_entry_safe_rcu() fix.
> 
> http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=436151ad32b4a59e0d36165a7d6312545f126661
> 
> -- 
> Jens Axboe
> 

-- 
Jens Axboe