Re: kmemcheck caught read from freed memory (cfq_free_io_context)

[Peter Zijlstra]

On Wed, 2008-04-02 at 13:20 +0200, Peter Zijlstra wrote:
> 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@xxxxxxxxxxxxxxxxxx> 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.

Or worse, when C doesn't happen and B free's the very last object and
the slab does get returned, any usage of cic after rcu_read_unlock()
might poke into free memory.

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