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

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Date:	Wed, 02 Apr 2008 13:20:12 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Pekka Enberg <penberg@...helsinki.fi>, paulmck@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>,
	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, 2008-04-02 at 13:14 +0200, Jens Axboe wrote:
> On Wed, Apr 02 2008, Peter Zijlstra wrote:
> > On Wed, 2008-04-02 at 13:07 +0200, Jens Axboe wrote:
> > > On Wed, Apr 02 2008, Pekka Enberg wrote:
> > > > Hi Paul,
> > > > 
> > > > On Wed, Apr 2, 2008 at 1:55 PM, Paul E. McKenney
> > > > <paulmck@...ux.vnet.ibm.com> wrote:
> > > > >  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().
> > > > 
> > > > No, kmemcheck is work in progress and does not know about
> > > > SLAB_DESTROY_BY_RCU yet. The reason I asked Vegard to post the warning
> > > > was because Peter, Vegard, and myself identified this particular
> > > > warning as a real problem. But yeah, kmemcheck can cause false
> > > > positives for RCU for now.
> > > 
> > > Makes sense, and to me Pauls analysis of the code looks totally correct
> > > - there's no bug there, at least related to hlist traversal and
> > > kmem_cache_free(), since we are under rcu_read_lock() and thus hold off
> > > the grace for freeing.
> > 
> > but what holds off the slab allocator re-issueing that same object and
> > someone else writing other stuff into it?
> 
> Nothing, that's how rcu destry works here. But for the validation to be
> WRONG radix_tree_lookup(..., old_key) must return cic for new_key, not
> NULL.
> 


	A				B			C

cfq_cic_lookup(cfqd_1, ioc)

  rcu_read_lock()
  cic = radix_tree_lookup(, cfqd_q);

					cfq_cic_free()
  					
								cfq_cic_link(cfqd_2, ioc,)

  rcu_read_unlock()


and now we have that:

  cic->key == cfqd_2


I'm not seeing anything stopping this from happening.

Which is also why we need hlist_for_each_safe_rcu() because as soon as
we kfree()d the thing, someone else might get the object and start
poking at the hlist pointers, wrecking out iteration.



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