linux-kernel - Re: [PATCH 0/3] PM: Do not destroy/create devices while suspended

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Message-Id: <200712261612.24583.rjw@sisk.pl>
Date:	Wed, 26 Dec 2007 16:12:23 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <lenb@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@...e.cz>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 0/3] PM: Do not destroy/create devices while suspended

On Wednesday, 26 of December 2007, Alan Stern wrote:
> On Tue, 25 Dec 2007, Rafael J. Wysocki wrote:
> 
> > > > Do we need to worry about the possibility that when the system wakes up 
> > > > from hibernation, the set of usable CPUs might be smaller than it was 
> > > > beforehand?
> > > 
> > > This is possible in error conditions.
> > > 
> > > > Is any special handling needed for this, or is it already accounted for?
> > > 
> > > Hm, well.  The cleanest thing would be to allow the drivers to remove the
> > > device objects on CPU_UP_CANCELED_FROZEN, which means that we weren't able to
> > > bring the CPU up during a resume, but still that will deadlock with
> > > gregkh-driver-pm-acquire-device-locks-prior-to-suspending.patch.
> > 
> > Hmm.  In principle, device objects may be destroyed on CPU_UP_CANCELED_FROZEN
> > without acquiring the device locks, since in fact we know these objects won't
> > be accessed concurrently at that time (the locks are already held by the PM
> > core, but the PM core is not going to actually access the devices before the
> > subsequent resume).
> 
> How about delaying the CPU_UP_CANCELED_FROZEN announcements until it's 
> really safe to send them out?  That is, after all devices have been 
> resumed and the PM core no longer holds any of their locks.  (Should 
> this be before or after tasks leave the freezer? -- I'm not sure.)
> 
> So the idea is send appropriate announcements at the usual time for
> CPUs that do come back up normally, and don't send anything right away
> for CPUs that fail to come up.  Just keep track of which ones failed,
> and then later take care of them.

However, we don't want to execute .resume() for device objects that correspond
to the "dead" CPUs, so to a minimum we should remove them from the dpm_off
list on CPU_UP_CANCELED_FROZEN.  For this purpose, we can define a
callback that will remove the device from dpm_off immediately and schedule its
destruction after all devices have been resumed.

Rafael
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