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

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Date:	Thu, 27 Feb 2014 17:13:07 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Derek Basehore <dbasehore@...omium.org>, stable@...r.kernel.org
Subject: Re: [PATCH 1/2] bdi: Fix hung task on sync

On Tue 25-02-14 17:43:30, Tejun Heo wrote:
> On Tue, Feb 25, 2014 at 11:29:13PM +0100, Jan Kara wrote:
> > From: Derek Basehore <dbasehore@...omium.org>
> > 
> > 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@...r.kernel.org
> > Signed-off-by: Derek Basehore <dbasehore@...omium.org>
> > Signed-off-by: Jan Kara <jack@...e.cz>
> 
> Reviewed-by: Tejun Heo <tj@...nel.org>
  Thanks for review. Did you have time to look into the patch 2/2?

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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