linux-kernel - Re: [linux-pm] Fundamental flaw in system suspend, exposed by freezer removal

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Message-Id: <200802260116.18142.david-b@pacbell.net>
Date:	Tue, 26 Feb 2008 01:16:17 -0800
From:	David Brownell <david-b@...bell.net>
To:	David Newall <davidn@...idnewall.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-pm@...ts.linux-foundation.org,
	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: [linux-pm] Fundamental flaw in system suspend, exposed by freezer removal

On Tuesday 26 February 2008, David Newall wrote:
> 
> Hardware can be inserted and removed while we're in a suspend state; and
> there's nothing that we can do about it until we resume.  Is it fair to
> say, then, that having started suspend, we could reasonably ignore any
> device insertion and removal, and handle it on resume?

"Ignore" seems a bit strong; those events may be wakeup triggers,
which would cause the hardware to make it a very short suspend state.

"Defer handling" is more to the point, be it by hardware or software.

> Presumably we need to scan for hardware changes on resume.

Not on most busses I work with; the hardware issues notifications
whenever the devices are removable.

Which is a Good Thing ... scanning, as you suggest, is inherently
not reliable.  If a mouse is swapped, it likely doesn't matter a
lot whether it's the "same" or not.

But ... how about if some removable storage media was taken out,
updated on a different system, then restored before the resume?
Not many filesystems handle that sanely.  You *really* want to
have hardware which reports disconnect/reconnect events, or which
physically prevents them (locked case etc).

- Dave

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