linux-kernel - Re: [PATCH] PM: Acquire device locks on suspend

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Date:	Mon, 7 Jan 2008 12:23:17 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Johannes Berg <johannes@...solutions.net>,
	Greg KH <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Len Brown <lenb@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	pm list <linux-pm@...ts.linux-foundation.org>
Subject: Re: [PATCH] PM: Acquire device locks on suspend

On Mon, 7 Jan 2008, Rafael J. Wysocki wrote:

> Please see the patch at: http://lkml.org/lkml/2008/1/6/298 .  It represents my
> current idea about how to do that.

It has some problems.

First, note that the list manipulations in dpm_suspend(), 
device_power_down(), and so on aren't protected by dpm_list_mtx.  So 
your patch could corrupt the list pointers.  Are you assuming that no 
other threads can be running at this time?

Note also that device_pm_destroy_suspended() does up(&dev->sem), but it 
doesn't know whether or not dev->sem was locked to begin with.

Do you want to rule out the possibility of a driver's suspend or remove 
methods calling destroy_suspended_device() on its own device?  With 
your synchronous approach, this would mean that the suspend/resume 
method would indirectly end up calling the remove method.  This is 
dangerous at best; with USB it would be a lockdep violation.  With an 
asynchronous approach, on the other hand, this wouldn't be a problem.

Alan Stern

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