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

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Message-Id: <200712251721.42233.rjw@sisk.pl>
Date:	Tue, 25 Dec 2007 17:21:41 +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 Monday, 24 of December 2007, Alan Stern wrote:
> On Mon, 24 Dec 2007, Rafael J. Wysocki wrote:
> 
> > Hi,
> > 
> > Some device drivers register CPU hotplug notifiers and use them to destroy
> > device objects when removing the corresponding CPUs and to create these objects
> > when adding the CPUs back.
> > 
> > Unfortunately, this is not the right thing to do during suspend/hibernation,
> > since in that cases the CPU hotplug notifiers are called after suspending
> > devices and before resuming them, so the operations in question are carried
> > out on the objects representing suspended devices which shouldn't be
> > unregistered behing the PM core's back.  Although right now it usually doesn't
> > lead to any practical complications, it will predictably deadlock if
> > gregkh-driver-pm-acquire-device-locks-prior-to-suspending.patch is applied.
> > 
> > The solution is to prevent drivers from removing/adding devices from within
> > CPU hotplug notifiers during suspend/hibernation using the FROZEN bit
> > in the notifier's action argument.  The following three patches modify the
> > MSR, x86-64 MCE and cpuid drivers along these lines.
> 
> 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.

Greetings,
Rafael

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