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

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Date:	Sun, 8 Jul 2007 15:55:51 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
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

On Sunday, 8 July 2007 14:09, Pavel Machek wrote:
> 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).
> 
> I agree that it is relatively unlikely to trigger (if you avoid
> freezing the tasks that were uninterruptible for long), but it will
> trigger in error cases etc.

Yes, it will.

> > 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.
> 
> I don't know how to do that mechanism... but if we knew where to trap
> filesystem writes, we could simply freeze at that point, and at that
> point only, no?

>From the image/filesystems integrity standpoint, yes, that should be
sufficient.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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