Re: [PATCH 1/2] bdi: Fix hung task on sync

[Tejun Heo]

On Tue, Feb 25, 2014 at 11:29:13PM +0100, Jan Kara wrote:
> From: Derek Basehore <dbasehore@xxxxxxxxxxxx>
> 
> 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
> 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.
> 
> Fixes: 839a8e8660b6777e7fe4e80af1a048aebe2b5977
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Derek Basehore <dbasehore@xxxxxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>

Reviewed-by: Tejun Heo <tj@xxxxxxxxxx>

Thanks.

-- 
tejun
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