linux-kernel - Re: [PATCH] backing_dev: Fix hung task on sync

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Date:	Tue, 11 Mar 2014 21:19:02 +0100
From:	Jan Kara <jack@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.cz>,
	Derek Basehore <dbasehore@...omium.org>,
	Alexander Viro <viro@...to.linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Kees Cook <keescook@...omium.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, bleung@...omium.org, sonnyrao@...omium.org,
	semenzato@...omium.org
Subject: Re: [PATCH] backing_dev: Fix hung task on sync

On Tue 11-03-14 11:23:55, Andrew Morton wrote:
> On Wed, 19 Feb 2014 14:01:39 -0500 Tejun Heo <tj@...nel.org> wrote:
> 
> > Hello, Jan.
> > 
> > On Wed, Feb 19, 2014 at 10:27:31AM +0100, Jan Kara wrote:
> > >   You are the workqueue expert so you may know better ;) But the way I
> > > understand it is that queue_delayed_work() does nothing if the timer is
> > > already running. Since we queue flusher work to run either immediately or
> > > after dirty_writeback_interval we are safe to run queue_delayed_work()
> > > whenever we want it to run after dirty_writeback_interval and
> > > mod_delayed_work() whenever we want to run it immediately.
> > 
> > Ah, okay, so it's always mod on immediate and queue on delayed.  Yeah,
> > that should work.
> > 
> > > But it's subtle and some interface where we could say queue delayed work
> > > after no later than X would be easier to grasp.
> > 
> > Yeah, I think it'd be better if we had something like
> > mod_delayed_work_if_later().  Hmm...
> 
> The code comments which you asked for were not forthcoming.
> 
> Are you otherwise OK with merging this into 3.14 and -stable?
  I have actually added the comment when nothing happened and have a patch
with Tejun's Reviewed-by. Just noone picked it up. I'll send it your way
together with another fix in flush work handling.

								Honza


> From: Derek Basehore <dbasehore@...omium.org>
> Subject: backing_dev: fix hung task on sync
> 
> bdi_wakeup_thread_delayed() used mod_delayed_work() 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.
> 
> 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@...omium.org>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Cc: Alexander Viro <viro@...to.linux.org.uk>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: "Darrick J. Wong" <darrick.wong@...cle.com>
> Cc: Derek Basehore <dbasehore@...omium.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Benson Leung <bleung@...omium.org>
> Cc: Sonny Rao <sonnyrao@...omium.org>
> Cc: Luigi Semenzato <semenzato@...omium.org>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Dave Chinner <david@...morbit.com>
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 
>  fs/fs-writeback.c |    5 +++--
>  mm/backing-dev.c  |    2 +-
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff -puN fs/fs-writeback.c~backing_dev-fix-hung-task-on-sync fs/fs-writeback.c
> --- a/fs/fs-writeback.c~backing_dev-fix-hung-task-on-sync
> +++ a/fs/fs-writeback.c
> @@ -1039,8 +1039,9 @@ void bdi_writeback_workfn(struct work_st
>  		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));
>  
> diff -puN mm/backing-dev.c~backing_dev-fix-hung-task-on-sync mm/backing-dev.c
> --- a/mm/backing-dev.c~backing_dev-fix-hung-task-on-sync
> +++ a/mm/backing-dev.c
> @@ -294,7 +294,7 @@ void bdi_wakeup_thread_delayed(struct ba
>  	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);
>  }
>  
>  /*
> _
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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