linux-kernel - removing refrigerator does not help with s2ram vs. fuse deadlocks (was Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway)

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Message-ID: <20070705091526.GA3084@elf.ucw.cz>
Date:	Thu, 5 Jul 2007 11:15:26 +0200
From:	Pavel Machek <pavel@....cz>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Johannes Berg <johannes@...solutions.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: removing refrigerator does not help with s2ram vs. fuse deadlocks (was Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway)

Hi!

> > The fact remains that lots of drivers would still need to be changed.  
> > In the read and write methods someone would have to add code amounting
> > to this:
> > 
> > 	if (suspend_is_under_way()) {
> > 		mutex_unlock(...);
> > 		block_until_resume();
> > 		goto restart;
> > 	}
> > 
> > Freezing userspace is a small amount of code by comparison.
> 
> Normally devices have some sort of queue of pending operations.  So
> all that is required on suspend is to stop processing the queue and
> wait for any currently-underway operations to complete.  The blocking
> then happens naturally using the normal I/O wait mechanisms.

So... instead of one big freezer (we know it is problematic), you have
100 small freezers, problematic in same way :-(.

Let's take current FUSE problems, and see if they have problem on PPC,
ok?

Let's say FUSE thread touches one of those "blocking" devices. It is
now in D state, somewhere in kernel.... exactly same way refrigerator
works.

Now, if kernel needs FUSE services for some reason (that's the problem
we hit in s2ram case, right?), we have a deadlock.

So main problem still seems to be "kernel should not depend on
userland services during suspend", refrigerator or not.
								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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