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    Subject[PATCH 3.11 124/182] backing_dev: fix hung task on sync
    Date
    3.11.10.9 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Derek Basehore <dbasehore@chromium.org>

    commit 6ca738d60c563d5c6cf6253ee4b8e76fa77b2b9e upstream.

    bdi_wakeup_thread_delayed() used the mod_delayed_work() function to
    schedule work to writeback dirty inodes. The problem with this is that
    it can delay work that is scheduled for immediate execution, such as the
    work from sync_inodes_sb(). This can happen since mod_delayed_work()
    can now steal work from a work_queue. This fixes the problem by using
    queue_delayed_work() instead. This is a regression caused by commit
    839a8e8660b6 ("writeback: replace custom worker pool implementation with
    unbound workqueue").

    The reason that this causes a problem is that laptop-mode will change
    the delay, dirty_writeback_centisecs, to 60000 (10 minutes) by default.
    In the case that bdi_wakeup_thread_delayed() races with
    sync_inodes_sb(), sync will be stopped for 10 minutes and trigger a hung
    task. Even if dirty_writeback_centisecs is not long enough to cause a
    hung task, we still don't want to delay sync for that long.

    We fix the problem by using queue_delayed_work() when we want to
    schedule writeback sometime in future. This function doesn't change the
    timer if it is already armed.

    For the same reason, we also change bdi_writeback_workfn() to
    immediately queue the work again in the case that the work_list is not
    empty. The same problem can happen if the sync work is run on the
    rescue worker.

    [jack@suse.cz: update changelog, add comment, use bdi_wakeup_thread_delayed()]
    Signed-off-by: Derek Basehore <dbasehore@chromium.org>
    Reviewed-by: Jan Kara <jack@suse.cz>
    Cc: Alexander Viro <viro@zento.linux.org.uk>
    Reviewed-by: Tejun Heo <tj@kernel.org>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: "Darrick J. Wong" <darrick.wong@oracle.com>
    Cc: Derek Basehore <dbasehore@chromium.org>
    Cc: Kees Cook <keescook@chromium.org>
    Cc: Benson Leung <bleung@chromium.org>
    Cc: Sonny Rao <sonnyrao@chromium.org>
    Cc: Luigi Semenzato <semenzato@chromium.org>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Dave Chinner <david@fromorbit.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Luis Henriques <luis.henriques@canonical.com>
    ---
    fs/fs-writeback.c | 8 ++++----
    mm/backing-dev.c | 5 ++++-
    2 files changed, 8 insertions(+), 5 deletions(-)

    diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    index 52e5934..160f8e6 100644
    --- a/fs/fs-writeback.c
    +++ b/fs/fs-writeback.c
    @@ -1036,10 +1036,10 @@ void bdi_writeback_workfn(struct work_struct *work)
    trace_writeback_pages_written(pages_written);
    }

    - if (!list_empty(&bdi->work_list) ||
    - (wb_has_dirty_io(wb) && dirty_writeback_interval))
    - queue_delayed_work(bdi_wq, &wb->dwork,
    - msecs_to_jiffies(dirty_writeback_interval * 10));
    + if (!list_empty(&bdi->work_list))
    + mod_delayed_work(bdi_wq, &wb->dwork, 0);
    + else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
    + bdi_wakeup_thread_delayed(bdi);

    current->flags &= ~PF_SWAPWRITE;
    }
    diff --git a/mm/backing-dev.c b/mm/backing-dev.c
    index e04454c..4f81435e 100644
    --- a/mm/backing-dev.c
    +++ b/mm/backing-dev.c
    @@ -285,13 +285,16 @@ int bdi_has_dirty_io(struct backing_dev_info *bdi)
    * Note, we wouldn't bother setting up the timer, but this function is on the
    * fast-path (used by '__mark_inode_dirty()'), so we save few context switches
    * by delaying the wake-up.
    + *
    + * We have to be careful not to postpone flush work if it is scheduled for
    + * earlier. Thus we use queue_delayed_work().
    */
    void bdi_wakeup_thread_delayed(struct backing_dev_info *bdi)
    {
    unsigned long timeout;

    timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
    - mod_delayed_work(bdi_wq, &bdi->wb.dwork, timeout);
    + queue_delayed_work(bdi_wq, &bdi->wb.dwork, timeout);
    }

    /*
    --
    1.9.1


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