linux-kernel - Re: [RFC][PATCH -mm 3/3] PM: Disable _request

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Message-ID: <Pine.LNX.4.44L0.0705281719200.10323-100000@netrider.rowland.org>
Date:	Mon, 28 May 2007 18:29:27 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Pavel Machek <pavel@....cz>
cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	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>
Subject: Re: [RFC][PATCH -mm 3/3] PM: Disable _request_firmware before
 hibernation/suspend

On Mon, 28 May 2007, Pavel Machek wrote:

> > The theoretical answer is that it behaves the way we want.  The kernel 
> > thread does selective resumes in response to device requests.  If such 
> > a request comes in while the system is asleep it will awaken the 
> > system; so it's only logical that a request coming in while the system 
> > is in the process of going to sleep should abort the suspend.
> 
> I'd say that it shows ppc being broken. User wanted to suspend the
> system, and now unrelated task did lsusb... and system will not sleep.
> 
> AFAICT it is DoS issue -- if one of your users keeps doing lsusb, root
> will not be able to suspend the system.

This is a matter of one's philosophy.  In suspend-to-RAM, should tasks
be frozen or should I/O queues be frozen?

With the USB subsystem I have followed the approach taken by the PM
core, which is that tasks are frozen.  But one can -- and Linus has on
at least one occasion -- make a good case that tasks should be left
running while only I/O is frozen.  This would require the subsystem to
distinguish between a selective device suspend and a system-wide
suspend-to-RAM, so that selective resume could be enabled on demand in
one case but not the other.

It's quite doable in principle -- it's just not the technique I used.

Alan Stern

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