linux-kernel - Re: [linux-pm] [PATCH] Remove process freezer from suspend to RAM pathway

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Date:	Thu, 05 Jul 2007 10:37:22 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	rjw@...k.pl
CC:	miklos@...redi.hu, stern@...land.harvard.edu, oliver@...kum.org,
	paulus@...ba.org, mjg59@...f.ucam.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [linux-pm] [PATCH] Remove process freezer from suspend to RAM pathway

> > Pro-freezers say:
> > 
> >   - don't remove the freezer, otherwise we'll have to deal with
> >     numerous problems in drivers
> 
> And these problems will generally be difficult to reproduce reliably
> and debug.

I see exactly the opposite.

With the freezer I can have very rarely occuring failures, due to
freeze ordering effects.

And without the freezer I have a 100% reproducable problem, that is
not hard to fix according to Alan Stern.  OK, I don't know what the
next problem would be, but the powermac experience shows, that it's
not nearly as bad as you and Oliver try to make it out.

> > Can this be fixed?
> > 
> > It seems to be a fundamental problem with the freezer: while it does
> > make sure that user processes are not calling into drivers during
> > suspend, it also disallows perfectly harmless non-driver calls as
> > well.
> 
> The problem is that when the freezer was designed (I didn't do that, BTW),
> there was no FUSE and similar things, so it's not prepared to cope with
> such interdependencies between user space tasks.
> 
> We had an analogous problem with vfork() and it was solved by using the
> PF_FREEZER_SKIP flag.  Perhaps we can do similar thing with FUSE.

It cannot be just worked around in fuse, as a task might be sleeping
on a number of VFS mutexes as well (i_mutex, s_vfs_rename_mutex, etc).
It would be a gigantic hack, possible at all.

Miklos
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