linux-kernel - Re: [patch update] PM: Introduce core framework for run-time PM of I/O devices (rev. 6)

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Date:	Fri, 26 Jun 2009 21:25:49 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
cc:	Greg KH <gregkh@...e.de>, LKML <linux-kernel@...r.kernel.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [patch update] PM: Introduce core framework for run-time PM of
 I/O devices (rev. 6)

On Sat, 27 Jun 2009, Rafael J. Wysocki wrote:

> > Speaking of races, have you noticed that the way power.work_done gets 
> > used is racy?
> 
> Not really. :-)
> 
> > You can't wait for the completion before releasing the 
> > lock, but then anything could happen.
> > 
> > A safer approach would be to use a wait_queue.
> 
> I'm not sure what you mean exactly.  What's the race?

Somebody calls pm_runtime_suspend when a suspend is already in 
progress.  The routine sees that the status is RPM_SUSPENDING, so it 
prepares to wait until the suspend is finished.  It drops the lock and 
calls wait_for_completion.

But in between those last two steps, the suspend could finish and 
a resume could start up.  Then the wait_for_completion wouldn't return 
until the device was fully resumed!

Now I admit this isn't as bad as it sounds.  The same sort of thing
could happen even if there weren't two suspends going on at the same
time; a resume could occur between when the routine drops the lock and
when it returns.  So okay, forget I mentioned it.

Alan Stern

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