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    SubjectRe: [stable] apparent regression (crash) - 2.6.38.6
    01.06.2011 16:34, Atsushi Nemoto wrote:
    > On Thu, 19 May 2011 07:39:27 +0400, James Bottomley <James.Bottomley@HansenPartnership.com> wrote:
    >>>>>>> [ 106.994628] BUG: unable to handle kernel NULL pointer dereference at 0000000000000048
    >>>>>>> [ 106.994755] IP: [<ffffffff811bec1b>] elv_queue_empty+0x1b/0x30
    >>>>>
    >>>>> Hmm, it's another missing elevator guard, like this patch:
    >>>>>
    >>>>> http://marc.info/?l=linux-scsi&m=130348673628282
    >>>>>
    >>>>> I think the bug here is that q->elevator is null, so dereferencing
    >>>>> elevator->ops gives the bug.
    >>>>
    >>>> Is this patch going to Linus anytime soon?
    >>>
    >>> Ping?
    >>
    >> I pinged Jens about it yesterday; he said it should be on its way to
    >> Linus.
    >
    > The patch in above URL ("block: add proper state guards to
    > __elv_next_request") is in mainline and stable-queues now, but how
    > about a similar fix for elv_queue_empty()?
    >
    > The elv_queue_empty() is removed in mainline, but it seems
    > stable-2.6.38.x and prior stable-branches still need the fix for
    > elv_queue_empty().

    Something like this? (run-tested but I haven't seen the problem
    in this place)

    commit 2e8532e0a9ee1d25b279ac78ee8ce31701e2aa15
    Author: Michael Tokarev <mjt@tls.msk.ru>
    Date: Fri Jun 3 10:50:49 2011 +0400

    block: add proper state guards to elv_queue_empty()

    Like in 0a58e077eb600d1efd7e54ad9926a75a39d7f8ae (backported to
    stable 2.6.38 as 0a58e077eb600d1efd7e54ad9926a75a39d7f8ae) which
    fixes this for __elv_next_request(), as reported by Atsushi Nemoto,
    elv_queue_empty() also needs to check for dead queue condition
    before touchin elevator.

    elv_queue_empty() has been removed upstream so this is only applicable
    for versions prior to 2.6.39, including 2.6.32-longterm.

    Signed-Off-By: Michael Tokarev <mjt@tls.msk.ru>

    diff --git a/block/elevator.c b/block/elevator.c
    index 236e93c..30cec25 100644
    --- a/block/elevator.c
    +++ b/block/elevator.c
    @@ -727,7 +727,8 @@ int elv_queue_empty(struct request_queue *q)
    if (!list_empty(&q->queue_head))
    return 0;

    - if (e->ops->elevator_queue_empty_fn)
    + if (!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags) &&
    + e->ops->elevator_queue_empty_fn)
    return e->ops->elevator_queue_empty_fn(q);

    return 1;

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