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

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Message-ID: <20160314072234.GC5213@quack.suse.cz>
Date:	Mon, 14 Mar 2016 08:22:34 +0100
From:	Jan Kara <jack@...e.cz>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jan Kara <jack@...e.cz>, 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

On Fri 11-03-16 12:56:10, Tejun Heo wrote:
> Hello, Jan.
> 
> On Thu, Mar 03, 2016 at 10:33:10AM +0100, Jan Kara wrote:
> > > 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
> 
> At least libata does and I think SCSI does too, but yeah, there
> probably are drivers which depend on block layer blocking IOs, which
> btw is a pretty fragile way to go about as upper layers might not be
> the only source of activities.
> 
> > 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.
> 
> I still think the right thing to do is plugging that block layer or
> low level drivers.  It's like we're trying to plug multiple sources
> when we can plug the point where they come together anyway.

I agree that freezing writeback workers is a workaround for real issues at
best and ideally we shouldn't have to do that. But at least for now I had
the impression that it is needed for suspend to work reasonably reliably.

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