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

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Date:	Sat, 7 Jul 2007 22:44:45 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	nigel@...pend2.net, pm list <linux-pm@...ts.linux-foundation.org>,
	Alan Stern <stern@...land.harvard.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	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 Saturday, 7 July 2007 13:31, Pavel Machek wrote:
> Hi!
> 
> > > I don't think we can do that. I suspect rename looks like:
> > > 
> > > 	write directory entry in source
> > > A)	(uninterruptible wait for write)
> > > 	write directory entry in destination
> > > 	(uninterruptible wait for write)
> > > 	write something else
> > > 
> > > If we freeze some task in place "A)", we'll write to the disk when the
> > > directory write is finished :-(.
> > 
> > Renaming is a single syscall, so won't the process get frozen when the syscall 
> > finishes? 
> 
> It would be frozen when syscall finishes. But if we freeze it at A)
> point, we have a problem.

With this patch, we don't freeze it. :-)

We do something like "send a freeze request to this task, but don't care if it
doesn't freeze".

Now, IMHO, the correctness here is the question of for how long a task can be
in TASK_UNINTERRUPTIBLE it it's not stuck (eg. the resource needed is gone
indefinitely, like in NFS) and I think that's not very long.

For example, if the tasks sleeps on a mutex, then it waits for another task to
release the mutex (interrupt handlers don't use mutexes) and I don't know of
any realistic scenario in which it can last in that state throughout
freeze_processes().

Greetings,
Rafael


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