linux-kernel - Re: Freezable workqueue blocks non-freezable workqueue during the system resume process

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Date:	Thu, 3 Mar 2016 10:33:10 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Chen <hzpeterchen@...il.com>,
	Alan Stern <stern@...land.harvard.edu>, florian@...kler.org,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	usb-storage@...ts.one-eyed-alien.net, Jan Kara <jack@...e.com>,
	jkosina@...e.cz
Subject: Re: Freezable workqueue blocks non-freezable workqueue during the
 system resume process

Hello,

On Wed 02-03-16 11:00:58, Tejun Heo wrote:
> On Fri, Feb 26, 2016 at 02:19:20PM +0800, Peter Chen wrote:
> > On Thu, Feb 25, 2016 at 05:01:12PM -0500, Tejun Heo wrote:
> > > Hello, Peter.
> > > 
> > > On Wed, Feb 24, 2016 at 03:24:30PM +0800, Peter Chen wrote:
> > > > > You might want to complain to the block-layer people about this.  I 
> > > > > don't know if anything can be done to fix it.
> > > > > 
> > > > > Or maybe flush_work and flush_delayed_work can be changed to avoid 
> > > > > blocking if the workqueue is frozen.  Tejun?
> > > > > 
> > > > 
> > > > I have a patch to show the root cause of this issue.
> > > > 
> > > > http://www.spinics.net/lists/linux-usb/msg136815.html
> > > 
> > > I don't get it.  Why would it deadlock?  Shouldn't things get rolling
> > > once the workqueues are thawed?
> > 
> > The workqueue writeback can't be thawed due to driver's resume
> > (dpm_complete) is lock nested, and can't be finished.
> 
> Ugh... that's nasty.  I wonder whether the right thing to do is making
> writeback workers non-freezable.  IOs are supposed to be blocked from
> lower layer anyway.  Jan, what do you think?

Well no, at least currently IO is not blocked in lower layers AFAIK - for
that you'd need to freeze block devices & filesystems and there are issues
with that (Jiri Kosina was the last one which was trying to make this work
AFAIR). And I think you need to stop writeback (and generally any IO) to be
generated so that it doesn't interact in a strange way with device drivers
being frozen. So IMO until suspend freezes filesystems & devices properly
you have to freeze writeback workqueue.

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