linux-kernel - Re: [linux-pm] Re: Hibernation considerations

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Message-Id: <200707230842.22121.nigel@nigel.suspend2.net>
Date:	Mon, 23 Jul 2007 08:42:17 +1000
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	nigel@...pend2.net, Jeremy Maitin-Shepard <jbms@....edu>,
	Miklos Szeredi <miklos@...redi.hu>, rjw@...k.pl,
	miltonm@....com, ying.huang@...el.com,
	linux-kernel@...r.kernel.org, david@...g.hm,
	linux-pm@...ts.linux-foundation.org
Subject: Re: [linux-pm] Re: Hibernation considerations

Hi Alan.

On Monday 23 July 2007 01:26:23 Alan Stern wrote:
> On Sun, 22 Jul 2007, Nigel Cunningham wrote:
> 
> > Hi.
> > 
> > On Sunday 22 July 2007 02:13:56 Jeremy Maitin-Shepard wrote:
> > > It seems that you could still potentially get a failure to freeze if one
> > > FUSE process depends on another, and the one that is frozen second just
> > > happens to be waiting on the one that is frozen first when it is frozen.
> > > I admit that this situation is unlikely, and perhaps acceptable.
> > > 
> > > A larger concern is that it seems that freezing FUSE processes at all
> > > _will_ generate deadlocks if a non-synchronous or memory-map-supporting
> > > filesystem is loopback mounted from a FUSE filesystem.  In that case, if
> > > you attempt to sync or free memory once FUSE is frozen, you are sure to
> > > get a deadlock.
> > 
> > Ok. So then (in response to Alan too), how about keeping a tree of mounts, 
> > akin to the device tree, and working from the deepest nodes up? (In 
> > conjunction with what I already suggested)?
> 
> Face it, Nigel, this is a losing battle.  You can try to come up with
> ever-more complex schemes to try and force FUSE into the freezer's
> framework, but it just won't fit.  Or if it does, the next filesystem
> to come along will require an even more baroque type of special-case 
> handling.

It does seem to be a losing battle, but I'm wondering whether that's really 
because it's an intractable problem, or because people have given up on it 
before its time. We are talking about a computer system, so things should be 
predictable.
 
> The general problem is that task A may be in an unfreezable state,
> waiting for task B to do something, while task B is already frozen.  
> Since there's no reasonable way to determine that A really is waiting
> for B, you're just stuck.  (To make matters worse, A may not even
> realize which task it is waiting for; it may know only that it's
> waiting for somebody to do something!)  A and B could be user tasks, 
> kernel threads, or one of each.

I guess I want to persist because all of these issues aren't utterly 
unsolvable. It's just that we don't have the infrastructure yet to figure out 
the solutions to these issues trivially. Take, for example, the locking 
issue. If we could call some function to say "What process holds this lock?", 
then task A could know that it's waiting on task B and put that information 
somewhere. We could then use the information to freeze task B before task A.

 
> The only thing to do is what Rafael has been working on: unfreeze
> things, hope the tasks sort themselves out, and try again.

That's what I'm questioning. Is there a more reliable way and we've just given 
up too quickly?

Regards,

Nigel
-- 
Nigel Cunningham
Christian Reformed Church of Cobden
103 Curdie Street, Cobden 3266, Victoria, Australia
Ph. +61 3 5595 1185 / +61 417 100 574
Communal Worship: 11 am Sunday.

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