Messages in this thread | | | Date | Wed, 2 Apr 2008 14:34:02 +0200 | From | Jens Axboe <> | Subject | Re: kmemcheck caught read from freed memory (cfq_free_io_context) |
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On Wed, Apr 02 2008, Peter Zijlstra wrote: > On Wed, 2008-04-02 at 13:53 +0200, Jens Axboe wrote: > > 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@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 :-) > > > > 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). > > There are 3 races here: > > 1) A continues with another object than intended > (requires CLONE_IO) > > 2) A does hlist_for_each_rcu() and races with B,C so that > we continue the iteration on a possibly unrelated list. > > 3) cic is freed after the !cic->key check. > > I'm not familiar enough with the code yet to see if 3 really is an > possibility. But from what I can see there is nothing guarding its > existence.
All 3 require CLONE_IO, because if that is not set, there's a 1:1 mapping between a process and io context (no sharing occurs).
-- Jens Axboe
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