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    SubjectRe: [PATCH] cfq-iosched: fix ioc_data leak
    On Fri, Apr 04 2008, Fabio Checconi wrote:
    > Hi,
    > stress testing module loading and unloading on a derived scheduler, I
    > had crashes on cfq caused by what seemed to be an error in the caching of
    > cic lookup results in the ioc_data field of io contexts.
    >
    > As what's happening is a little bit involved (at least for me), I've put
    > together some of the collected oopses with and without debug patches, the
    > patches themselves, a script used to reproduce the problem and the .config
    > used on kvm/qemu here, hoping that they can explain the problem better than
    > words:
    >
    > http://feanor.sssup.it/~fabio/linux/cfq-ioc-data/
    >
    > The patch below should fix the problem.
    >
    >
    > Subject: cfq-iosched: do not leak ioc_data across iosched switches
    >
    > When switching scheduler from cfq, cfq_exit_queue() does not clear
    > ioc->ioc_data, leaving a dangling pointer that can deceive the following
    > lookups when the iosched is switched back to cfq. The pattern that can
    > trigger that is the following:
    >
    > - elevator switch from cfq to something else;
    > - module unloading, with elv_unregister() that calls cfq_free_io_context()
    > on ioc freeing the cic (via the .trim op);
    > - module gets reloaded and the elevator switches back to cfq;
    > - reallocation of a cic at the same address as before (with a valid key).
    >
    > To fix it just assign NULL to ioc_data in __cfq_exit_single_io_context(),
    > that is called from the regular exit path and from the elevator switching
    > code. The only path that frees a cic and is not covered is the error handling
    > one, but cic's freed in this way are never cached in ioc_data.
    >
    > Signed-off-by: Fabio Checconi <fabio@gandalf.sssup.it>
    > ---
    > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
    > index 0f962ec..67cd023 100644
    > --- a/block/cfq-iosched.c
    > +++ b/block/cfq-iosched.c
    > @@ -1207,6 +1207,8 @@ static void cfq_exit_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq)
    > static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
    > struct cfq_io_context *cic)
    > {
    > + struct io_context *ioc = cic->ioc;
    > +
    > list_del_init(&cic->queue_list);
    >
    > /*
    > @@ -1216,6 +1218,9 @@ static void __cfq_exit_single_io_context(struct cfq_data *cfqd,
    > cic->dead_key = (unsigned long) cic->key;
    > cic->key = NULL;
    >
    > + if (ioc->ioc_data == cic)
    > + rcu_assign_pointer(ioc->ioc_data, NULL);
    > +
    > if (cic->cfqq[ASYNC]) {
    > cfq_exit_cfqq(cfqd, cic->cfqq[ASYNC]);
    > cic->cfqq[ASYNC] = NULL;
    > @@ -1248,8 +1253,7 @@ static void cfq_exit_single_io_context(struct io_context *ioc,
    > */
    > static void cfq_exit_io_context(struct io_context *ioc)
    > {
    > - rcu_assign_pointer(ioc->ioc_data, NULL);
    > call_for_each_cic(ioc, cfq_exit_single_io_context);
    > }
    >
    > static struct cfq_io_context *
    > @@ -1480,8 +1485,7 @@ cfq_drop_dead_cic(struct cfq_data *cfqd, struct io_context *ioc,
    >
    > spin_lock_irqsave(&ioc->lock, flags);
    >
    > - if (ioc->ioc_data == cic)
    > - rcu_assign_pointer(ioc->ioc_data, NULL);
    > + BUG_ON(ioc->ioc_data == cic);
    >
    > radix_tree_delete(&ioc->radix_root, (unsigned long) cfqd);
    > hlist_del_rcu(&cic->cic_list);

    Your analysis and fix looks correct, thanks a lot!

    --
    Jens Axboe



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