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

[Alan Stern <stern@rowland.harvard.edu>]

On Mon, 14 Mar 2016, Jan Kara wrote:

> 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.

What do you mean by "freezes ... devices"?  Only a piece of code can be 
frozen -- not a device.

The kernel does suspend device drivers; that is, it invokes their
suspend callbacks.  But it doesn't "freeze" them in any sense.  Once a 
driver has been suspended, it assumes it won't receive any I/O requests 
until it has been resumed.  Therefore the kernel first has to prevent 
all the upper layers from generating such requests and/or sending them 
to the low-level drivers.

> > 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.

The design is not to plug low-level drivers, but instead to prevent
them from receiving any requests by plugging or freezing high-level
code.

It's pretty clear that we don't want to have ongoing I/O during a 
system suspend, right?  And that means the I/O has to be prevented (or 
"plugged", if you prefer) somewhere -- either at an upper layer or at a 
lower layer.  There was a choice to be made, and the decision was to do 
it at an upper layer.

Alan Stern