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    SubjectRe: kmemcheck caught read from freed memory (cfq_free_io_context)
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    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.





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