linux-kernel - Re: [linux-pm] question on resume()

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Date:	Wed, 31 Jan 2007 10:54:39 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Pavel Machek <pavel@....cz>
cc:	Oliver Neukum <oneukum@...e.de>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Oliver Neukum <oliver@...kum.name>,
	pm list <linux-pm@...ts.osdl.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [linux-pm] question on resume()

On Wed, 31 Jan 2007, Pavel Machek wrote:

> Hi!
> 
> > > Yes, it will.  The process freezer can only return success if there are no more
> > > TASK_UNINTERRUPTIBLE tasks.  Otherwise it fails (after a timeout).
> > 
> > So, this means, on suspend():
> > 
> > 1. Don't worry about TASK_UNINTERRUPTIBLE
> > 2. Do worry about TASK_INTERRUPTIBLE
> > We have to cease IO and must not call wake_up_interruptible()
> 
> "cease IO"? No, I believe it is enough not to start new I/O. Userspace
> is frozen at that point, it can't ask you to do I/O.

There may be I/O requests sitting in a queue, already submitted by
userspace.  The suspend method should wait for existing I/O to complete
and stop processing new entries from the queue.

> > Isn't that a race until suspend() is called?
> 
> I do not think so.

The part about not calling wake_up_interruptible() is indeed a race.  We 
have:

	1. Task is frozen.
	2. Driver must not call wake_up_interruptible().
	3. Driver's suspend() method is called.

How is the driver supposed to satisfy (2) before (3) has occurred?

In fact this shouldn't matter.  There shouldn't be anything wrong with 
calling wake_up_interruptible() on a frozen task.

> > On resume():
> > 
> > 1. Don't worry about TASK_UNINTERRUPTIBLE
> > 2. Do not restart IO that may call wake_up_interruptible()
> > 
> > When do we restart such IO?
> 
> We reuse signal handling code to do that for us. It is same situation
> as when someone signals task doing I/O.

Again you misunderstood the question.  The driver must start queued I/O
when its resume() method is called.  It should then be okay for the driver
to call wake_up_interruptible(), even before tasks are unfrozen.

Alan Stern

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