Messages in this thread | | | Date | Wed, 2 Apr 2008 13:43:38 +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, Fabio Checconi wrote: > > From: Peter Zijlstra <a.p.zijlstra@chello.nl> > > Date: Wed, Apr 02, 2008 12:59:21PM +0200 > > > > On Wed, 2008-04-02 at 03:55 -0700, Paul E. McKenney wrote: > > > On Wed, Apr 02, 2008 at 09:28:46AM +0200, Ingo Molnar wrote: > > > > > > > > * Jens Axboe <jens.axboe@oracle.com> wrote: > > > > > > > > > On Wed, Apr 02 2008, Pekka J Enberg wrote: > > > > > > On Wed, 2 Apr 2008, Jens Axboe wrote: > > > > > > > Good catch, I wonder why it didn't complain in my testing. I've added a > > > > > > > patch to fix that, please see it here: > > > > > > > > > > > > You probably don't have kmemcheck in your kernel ;-) > > > > > > > > > > Ehm no, you are right :) > > > > > > > > ... and you can get kmemcheck by testing on x86.git/latest: > > > > > > > > http://people.redhat.com/mingo/x86.git/README > > > > > > > > ;-) > > > > > > 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(). > > > > I don't think it does. > > > > It would have to register an call_rcu callback itself in order to mark > > it freed - and handle the race with the object being handed out again. > > > > I had the same problem while debugging a cfq-derived i/o scheduler, > and I found nothing preventing the reuse of the freed memory. > The patch below seemed to fix the logic.
Thanks, from a first look this looks like it'll fix this bad rcu slab usage. I'll give it some closer scrutiny and testing.
-- Jens Axboe
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