Date: Wed, 2 Jan 2008 11:52:17 +0100
From: Ingo Molnar <mingo@elte.hu>
To: "Rafael J. Wysocki" <rjw@sisk.pl>
Cc: pm list <linux-pm@lists.linux-foundation.org>,
       ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
       Alan Stern <stern@rowland.harvard.edu>,
       Andrew Morton <akpm@linux-foundation.org>, Len Brown <lenb@kernel.org>,
       LKML <linux-kernel@vger.kernel.org>, Pavel Machek <pavel@suse.cz>,
       Greg KH <gregkh@suse.de>
Subject: Re: [PATCH 0/4] PM: Do not destroy/create devices while suspended
	(rev. 2)
Message-ID: <20080102105217.GA14731@elte.hu>
References: <200801020032.45529.rjw@sisk.pl>
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* Rafael J. Wysocki <rjw@sisk.pl> 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.  However, this has to be 
> done with care, since the devices objects related to the nonboot CPUs 
> that failed to go online during resume should not be present in the 
> system.  For this reason, it seems reasonable to introduce a mechanism 
> allowing drivers to ask the PM core to remove device objects 
> corresponding to suspended devices on their behalf.
> 
> The first patch in the series introduces such a mechanism.  The 
> remaining three patches modify the MSR, x86-64 MCE and cpuid drivers 
> in accordance with the above approach.

btw., it would be really, really cool if there was a scriptable way i 
could test suspend/resume functionality. Pavel has this /dev/rtc thing 
to set up an alarm (not sure how functional it is) - would it be 
possible to have it as a "suspend for 10 seconds then resume" debug 
functionality? That way any suspend breakage would be detectable (and 
bisectable) in automated testing - if the resume does not come back 
after 10-20 seconds then the test failed.

	Ingo
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