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

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Message-Id: <200801071901.24926.rjw@sisk.pl>
Date:	Mon, 7 Jan 2008 19:01:23 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
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 Monday, 7 of January 2008, Alan Stern wrote:
> 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.

Yes, they need the locking.  I have overlooked that, mostly because the locking
was removed by gregkh-driver-pm-acquire-device-locks-prior-to-suspending.patch
too (because you assumed there woundn't be any need to remove a device during
a suspend, right?).

> Are you assuming that no other threads can be running at this time?

No, I'm not.

> 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 mean it might have been released already by another thread
calling device_pm_destroy_suspended() on the same device?

> 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.

Well, the asynchronous apprach has the problem that the device may end up
on a wrong list when removed by one of the .suspend() callbacks (and I don't
see how to avoid that without extra complexity).  Perhaps that's something we
can live with, though.

One more question: is there any particular reason not to call
device_pm_remove() at the beginning of device_del()?

Rafael
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