linux-kernel - Re: [RFC][PATCH -mm] Freezer: Handle uninterruptible tasks

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Date:	Sun, 8 Jul 2007 18:37:41 +0000
From:	Pavel Machek <pavel@....cz>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Oliver Neukum <oliver@...kum.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH -mm] Freezer: Handle uninterruptible tasks

Hi!

> > And then you will face the problem of a user task doing I/O during 
> > hibernate after the atomic snapshot has been made.
> 
> I don't think that this is possible in normal conditions.  It would be possible
> if, for example, the task were waiting for an unavailable resource and that
> resource became available after the hibernation image had been created.
> In that case, however, to do any damage, the task would have to cause some
> filesystem-related data to be flushed in the same syscall (ie. before returning
> to user space).
> 
> Such situations may be prevented by a mechanizm detecting if any uniterruptible
> and freezing task has been woken up after creating the image and aborting the
> hibernation in that cases.  For this purpose, we only need to add an
> appropriate condition to try_to_wake_up() and make it start to trigger after,
> for example, enabling the nonboot CPUs.

Hmm, okay, I see how you meant it. Yes, it probably could work... but
I'd say it is seriously ugly.

Imagine task waking up after complete image is written... we'd have to
invalidate the image before aborting the suspend.

Actually, we could do better: we could just refuse to run those tasks
after atomic snapshot... and hope we don't deadlock,because the
uninterruptible task holds some important lock... but I still think it
is too ugly.
							Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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