Date:	Fri, 6 Jul 2007 11:01:00 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	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>, Pavel Machek <pavel@....cz>,
	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 Fri, 6 Jul 2007, Rafael J. Wysocki wrote:

> Hi,
> 
> The main limitation of the freezer is that it cannot handle uninterruptible
> tasks.  Namely, if there are uninterruptible tasks in the system, the freezer
> returns an error, which makes it impossible to suspend the system.
> 
> This mechanism is used to prevent the situations in which the suspend process
> can deadlock with a task holding a lock needed by it from happening.  However,
> AFAICS, the probability of that happening is very small and if the freezer is
> removed from the suspend code patch, then the suspend process will be exposed to
> deadlocking in this manner anyway.
> 
> Unfortunately, this mechanism also leads to severe limitations, such as that it
> makes the freezer unable to handle systems using FUSE in a reliable way.
> 
> This patch makes the freezer skip uninterruptible user space tasks (ie. such
> that have an mm of their own) when counting the tasks to be frozen.  As a
> result, these tasks have the TIF_FREEZE and TIF_SIGPENDING flags set, but the
> freezer doesn't wait for them to enter the refrigerator.  Nevertheless, they
> will enter the refrigerator as soon as they change their state.

This is a very interesting idea.  Are you certain it is safe?  That is,
if a task is waiting for a mutex and sometime later the mutex becomes
available, will the task then enter the freezer immediately -- before
it can do any I/O or call any drivers?

If you can't make this guarantee, then you might as well simply not try 
to freeze any user task until it returns from kernel mode to user mode.  
And then you will face the problem of a user task doing I/O during 
hibernate after the atomic snapshot has been made.


I had in mind something more complicated.  My idea was to define 
certain mutexes as "freezable", with the thought that it would be okay 
for a task in the freezer to own freezable mutexes.  None of the 
mutexes needed by drivers or the PM core during a suspend transition 
should be freezable.

Presumably the VFS locks _would_ fall into the freezable category.  In 
order for this to work, we would have to guarantee the following:

	A thread blocked in the kernel waiting for some response
	from userspace is allowed to hold freezable mutexes but no
	other locks.

	When a thread tries to acquire a freezable mutex, it is allowed
	to hold other freezable mutexes but no other locks.

	When the freezer is running, any task that is blocked on a
	freezable mutex (maybe also any task that tries to acquire 
	an unlocked freezable mutex) would immediately go into the 
	freezer.  So would any task waiting for a response from
	userspace.

I think this would solve the VFS-related problems (FUSE's and others).  
But obviously Raphael's approach is preferable, if it works.

Alan Stern

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