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

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Date:	Sat, 5 Jan 2008 16:41:09 -0500 (EST)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	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 Sat, 5 Jan 2008, Rafael J. Wysocki wrote:

> On Saturday, 5 of January 2008, Alan Stern wrote:
> > On Sat, 5 Jan 2008, Rafael J. Wysocki wrote:
> > 
> > > > Another thing to watch out for: Just in case somebody ends up calling
> > > > destroy_suspended_device(dev) from within dev's own resume method, you 
> > > > should interchange the resume_device() and the list_move_tail() 
> > > > calls in dpm_resume().
> > > 
> > > However, if we unregister them all at once after releasing pm_sleep_rwsem,
> > > that shouldn't be necessary, right?
> > 
> > It's still necessary, because destroy_suspended_device() still has to
> > move the device from one list to another.  You don't want it to end up 
> > on the dpm_locked list.
> 
> Hmm.  That means we'd have to do the same thing in dpm_power_up() in case
> someone calls destroy_suspended_device() from resume_device_early(dev).

Yes.

> Still, even doing that is not enough, since someone can call
> destroy_suspended_device() from a .suspend() routine and then the device
> will end up on a wrong list just as well.

That should never happen.  The whole idea of destroy_suspended_device()
is that the device couldn't be resumed and in fact should be
unregistered because it is no longer working or no longer present.  A
suspend routine won't detect this sort of thing since it doesn't try to
resume the device.

But it wouldn't hurt to mention in the kerneldoc that 
destroy_suspended_device() is meant to be called only during a system 
resume.

Alan Stern

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