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    SubjectRe: kmemcheck caught read from freed memory (cfq_free_io_context)
    On Wed, Apr 02 2008, Jens Axboe wrote:
    > On Wed, Apr 02 2008, Jens Axboe wrote:
    > > 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.
    >
    > (CC reinstated, sometimes mutt is really annoying and drops the person
    > you are replying too :-(

    (for real...)

    >
    > Looks good and tests fine as well. I've applied it, on top of the
    > hlist_for_each_entry_safe_rcu() fix.
    >
    > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=436151ad32b4a59e0d36165a7d6312545f126661
    >
    > --
    > Jens Axboe
    >

    --
    Jens Axboe



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