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    Subject[patch 32/33] cfq-iosched: fix alias + front merge bug
    -stable review patch.  If anyone has any objections, please let us know.

    ------------------
    From: Jens Axboe <jens.axboe@oracle.com>

    There's a really rare and obscure bug in CFQ, that causes a crash in
    cfq_dispatch_insert() due to rq == NULL. One example of that is seen
    here:

    http://lkml.org/lkml/2007/4/15/41

    Neil correctly diagnosed the situation for how this can happen, read
    that analysis here:

    http://lkml.org/lkml/2007/4/25/57

    This looks like it requires md to trigger, even though it should
    potentially be possible to due with O_DIRECT (at least if you edit the
    kernel and doctor some of the unplug calls).

    The fix is to move the ->next_rq update to when we add a request to the
    rbtree. Then we remove the possibility for a request to exist in the
    rbtree code, but not have ->next_rq correctly updated.

    Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

    ---
    block/cfq-iosched.c | 12 ++++++------
    1 file changed, 6 insertions(+), 6 deletions(-)

    --- a/block/cfq-iosched.c
    +++ b/block/cfq-iosched.c
    @@ -462,6 +462,12 @@ static void cfq_add_rq_rb(struct request

    if (!cfq_cfqq_on_rr(cfqq))
    cfq_add_cfqq_rr(cfqd, cfqq);
    +
    + /*
    + * check if this request is a better next-serve candidate
    + */
    + cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
    + BUG_ON(!cfqq->next_rq);
    }

    static inline void
    @@ -1623,12 +1629,6 @@ cfq_rq_enqueued(struct cfq_data *cfqd, s
    cfqq->meta_pending++;

    /*
    - * check if this request is a better next-serve candidate)) {
    - */
    - cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
    - BUG_ON(!cfqq->next_rq);
    -
    - /*
    * we never wait for an async request and we don't allow preemption
    * of an async request. so just return early
    */
    --
    -
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