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

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Message-ID: <Pine.LNX.4.44L0.0906271042450.12287-100000@netrider.rowland.org>
Date:	Sat, 27 Jun 2009 10:51:33 -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?

Come to think of it, there really is a problem here.  Because the
wait_for_completion call occurs outside the spinlock, it can race with
the init_completion call.  It's not good for both of them to run at the
same time; the completion's internal spinlock and list pointers could 
get corrupted.

Therefore I stand by my original assertion: The struct completion 
should be replaced with a wait_queue.  Set the runtime_error field to 
-EINPROGRESS initially, and make other threads wait until the value 
changes.

Alan Stern

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