Re: kmemcheck caught read from freed memory (cfq_free_io_context)

On Wed, Apr 02 2008, Peter Zijlstra wrote:
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@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.

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 :-)

The only place where you'll have multiple processes involved with this
at all is if they share io contexts. That is also why the bug isn't that
critical, since it's not possible right now (CLONE_IO flag must be
used).

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.

It should, SLAB_DESTROY_BY_RCU is definitely useful, but it is expected
to be an 'easier' way of doing the call_rcu() manually. So it definitely
needs more documentation.

--
Jens Axboe

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