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    SubjectRe: lockdep vs rmmod loop
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    On Tue, 2008-10-14 at 19:14 +0400, Alexey Dobriyan wrote:
    > On Tue, Oct 14, 2008 at 01:51:38PM +0200, Peter Zijlstra wrote:
    > > On Mon, 2008-10-13 at 21:57 +0400, Alexey Dobriyan wrote:
    > > > This exists for quite some time, IIRC.
    > > >
    > > > # rmmod loop
    > >
    > > I tried to reproduce with -git from today but failed.
    > >
    > > v2.6.27-3976-g7591103
    > >
    > > I build a x86_64 kernel with modular loop and lockdep enabled, then I
    > > did: modprobe loop; rmmod loop
    > >
    > > Is there anything else to reproducing this?
    >
    > No, modprobe,rmmod is enough here even with minimal debugging.

    Yep can reproduce with your config.

    Its:

    static void wait_on_work(struct work_struct *work)
    {
    ...
    lock_map_acquire(&work->lockdep_map);
    lock_map_release(&work->lockdep_map);


    that triggers this. Which would suggest the work-let wasn't properly
    initialized. Now on to figuring out where it came from.

    Loop doesn't appear to have anything workqueue related in it, which
    suggests the block layer.

    And indeed, it appears blk_queue_make_request() initializes
    q->unplug_work, and by modprobe loop; rmmod loop you never get to run
    that, therefore q->unplug_work gets canceled without ever having been
    initialized -> bang!

    And indeed, the below patch fixes it:

    ---
    Subject: block: move q->unplug_work initialization
    From: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Date: Thu Oct 16 13:44:57 CEST 2008

    modprobe loop; rmmod loop effectively creates a blk_queue and destroys it
    which results in q->unplug_work being canceled without it ever being
    initialized.

    Therefore, move the initialization of q->unplug_work from
    blk_queue_make_request() to blk_alloc_queue*().

    Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    block/blk-core.c | 1 +
    block/blk-settings.c | 2 --
    2 files changed, 1 insertion(+), 2 deletions(-)

    Index: linux-2.6/block/blk-core.c
    ===================================================================
    --- linux-2.6.orig/block/blk-core.c
    +++ linux-2.6/block/blk-core.c
    @@ -501,6 +501,7 @@ struct request_queue *blk_alloc_queue_no
    init_timer(&q->unplug_timer);
    setup_timer(&q->timeout, blk_rq_timed_out_timer, (unsigned long) q);
    INIT_LIST_HEAD(&q->timeout_list);
    + INIT_WORK(&q->unplug_work, blk_unplug_work);

    kobject_init(&q->kobj, &blk_queue_ktype);

    Index: linux-2.6/block/blk-settings.c
    ===================================================================
    --- linux-2.6.orig/block/blk-settings.c
    +++ linux-2.6/block/blk-settings.c
    @@ -141,8 +141,6 @@ void blk_queue_make_request(struct reque
    if (q->unplug_delay == 0)
    q->unplug_delay = 1;

    - INIT_WORK(&q->unplug_work, blk_unplug_work);
    -
    q->unplug_timer.function = blk_unplug_timeout;
    q->unplug_timer.data = (unsigned long)q;




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