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    SubjectRe: [PATCH] backing_dev: Fix hung task on sync
    Hello,

    On Fri, Feb 14, 2014 at 08:12:17PM -0800, Derek Basehore wrote:
    > 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 from the move to the bdi workqueue design.
    >
    > 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.

    Oops.

    > For the same reason, this also changes 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.
    >
    > Signed-off-by: Derek Basehore <dbasehore@chromium.org>
    > ---
    > fs/fs-writeback.c | 5 +++--
    > mm/backing-dev.c | 2 +-
    > 2 files changed, 4 insertions(+), 3 deletions(-)
    >
    > diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
    > index e0259a1..95b7b8c 100644
    > --- a/fs/fs-writeback.c
    > +++ b/fs/fs-writeback.c
    > @@ -1047,8 +1047,9 @@ 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))
    > + if (!list_empty(&bdi->work_list))
    > + mod_delayed_work(bdi_wq, &wb->dwork, 0);
    > + else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
    > queue_delayed_work(bdi_wq, &wb->dwork,
    > msecs_to_jiffies(dirty_writeback_interval * 10));

    Can you please add some comments explaining why the specific variants
    are being used here?

    > diff --git a/mm/backing-dev.c b/mm/backing-dev.c
    > index ce682f7..3fde024 100644
    > --- a/mm/backing-dev.c
    > +++ b/mm/backing-dev.c
    > @@ -294,7 +294,7 @@ 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);

    and here?

    Hmmm.... but doesn't this create an opposite problem? Now a flush
    queued for an earlier time may be overridden by something scheduled
    later, no?

    Thanks.

    --
    tejun


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