linux-kernel - Re: Hibernation considerations

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Date:	Wed, 18 Jul 2007 10:29:34 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	david@...g.hm
cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"Huang, Ying" <ying.huang@...el.com>,
	Jeremy Maitin-Shepard <jbms@....edu>,
	Kyle Moffett <mrmacman_g4@....com>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	pm list <linux-pm@...ts.linux-foundation.org>,
	Al Boldi <a1426z@...ab.com>
Subject: Re: Hibernation considerations

On Tue, 17 Jul 2007 david@...g.hm wrote:

> On Tue, 17 Jul 2007, Alan Stern wrote:
> 
> > On Tue, 17 Jul 2007 david@...g.hm wrote:
> >
> >>> But what about the freezer?  The original reason for using kexec was to
> >>> avoid the need for the freezer.  With no freezer, while the original
> >>> kernel is busy powering down its devices, user tasks will be free to
> >>> carry out I/O -- which will make the memory snapshot inconsistent with
> >>> the on-disk data structures.
> >>
> >> no, user tasks just don't get scheduled during shutdown.
> >
> > But a user task may be holding a lock which is needed for putting some
> > device into low-power mode.  It can't release that lock if it doesn't
> > get scheduled.
> 
> then you can't suspend that box. if you schedule it, it could get another 
> lock (or another process gets another lock)
> 
> if you can't power down or put hardware into low-power mode without the 
> approval of userspace, you are in serious trouble.

You don't seem to appreciate the issues involved here.  Part of the 
justification for the freezer is that it doesn't need userspace 
approval and it freezes tasks at controlled points where they don't 
hold any locks.

Never mind.  It seems clear that this approach will suffer the same 
drawback as the proposal for removing the freezer from the 
suspend-to-RAM pathway.  Namely, device drivers will have to be changed 
to prevent user I/O requests from proceeding while devices are supposed 
to be quiescent or in a low-power state.

If a driver fails to handle this properly, its device could be 
reactivated in order to service a user request before the memory 
snapshot is made.  This could easily ruin the snapshot.

Alan Stern

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