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    Subject[PATCH 4.14 109/126] dm cache: fix a crash due to incorrect work item cancelling
    Date
    From: Mikulas Patocka <mpatocka@redhat.com>

    commit 7cdf6a0aae1cccf5167f3f04ecddcf648b78e289 upstream.

    The crash can be reproduced by running the lvm2 testsuite test
    lvconvert-thin-external-cache.sh for several minutes, e.g.:
    while :; do make check T=shell/lvconvert-thin-external-cache.sh; done

    The crash happens in this call chain:
    do_waker -> policy_tick -> smq_tick -> end_hotspot_period -> clear_bitset
    -> memset -> __memset -- which accesses an invalid pointer in the vmalloc
    area.

    The work entry on the workqueue is executed even after the bitmap was
    freed. The problem is that cancel_delayed_work doesn't wait for the
    running work item to finish, so the work item can continue running and
    re-submitting itself even after cache_postsuspend. In order to make sure
    that the work item won't be running, we must use cancel_delayed_work_sync.

    Also, change flush_workqueue to drain_workqueue, so that if some work item
    submits itself or another work item, we are properly waiting for both of
    them.

    Fixes: c6b4fcbad044 ("dm: add cache target")
    Cc: stable@vger.kernel.org # v3.9
    Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/md/dm-cache-target.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/drivers/md/dm-cache-target.c
    +++ b/drivers/md/dm-cache-target.c
    @@ -2971,8 +2971,8 @@ static void cache_postsuspend(struct dm_
    prevent_background_work(cache);
    BUG_ON(atomic_read(&cache->nr_io_migrations));

    - cancel_delayed_work(&cache->waker);
    - flush_workqueue(cache->wq);
    + cancel_delayed_work_sync(&cache->waker);
    + drain_workqueue(cache->wq);
    WARN_ON(cache->tracker.in_flight);

    /*

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