Messages in this thread | | | Subject | Re: kmemcheck caught read from freed memory (cfq_free_io_context) | From | Peter Zijlstra <> | Date | Wed, 02 Apr 2008 13:25:38 +0200 |
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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@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. > > 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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