Re: kmemcheck caught read from freed memory (cfq_free_io_context)

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2008-04-02 at 13:42 +0200, Jens Axboe wrote:
> On Wed, Apr 02 2008, Peter Zijlstra wrote:
> > On Wed, 2008-04-02 at 13:32 +0200, Jens Axboe wrote:
> > > On Wed, Apr 02 2008, 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@linux.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.
> > > 
> > > I don't follow your A-B-C here, what do they refer to?
> > 
> > A does a radix_tree_lookup() of cfqd_1 (darn typos)
> > B does a kfree of the same cic found by A
> > C does an alloc and gets the same cic as freed by B and inserts it
> >   in a different location.
> > 
> > So that when we return to A, cic->key == cfqd_2 even though we did a
> > lookup for cfqd_1.
> 
> That I follow, my question was if A, B, and C refer to different
> processes but with a shared io context? I'm assuming that is correct...

Ah, yeah, whatever is needed to make this race happen :-)

> And it does look buggy. It looks my assumption of what slab rcu destroy
> did is WRONG, it should be replaced by a manual call_rcu() freeing
> instead.

Yeah, SLAB_DESTROY_BY_RCU should have a _HUGE_ comment explaining it,
I'm sure this is not the first (nor the last) time people get that
wrong.

This would be one of those things that score very low on Rusty's API
list.