linux-kernel - Re: [RFC][PATCH] PM: Avoid losing wakeup events during suspend

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Date:	Mon, 21 Jun 2010 22:40:23 +0200
From:	Florian Mickler <florian@...kler.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dmitry Torokhov <dmitry.torokhov@...il.com>,
	Arve Hjønnevåg <arve@...roid.com>,
	Neil Brown <neilb@...e.de>, mark gross <640e9920@...il.com>
Subject: Re: [RFC][PATCH] PM: Avoid losing wakeup events during suspend

On Mon, 21 Jun 2010 12:54:44 -0400 (EDT)
Alan Stern <stern@...land.harvard.edu> wrote:

> On Mon, 21 Jun 2010, Florian Mickler wrote:
> 
> > > > > What happens if an event arrives just before you read
> > > > > /sys/power/wakeup_count, but the userspace consumer doesn't realize
> > > > > there is a new unprocessed event until after the power manager checks
> > > > > it?
> > > 
> > > > I think this is not the kernel's problem.  In this approach the kernel makes it
> > > > possible for the user space to avoid the race.  Whether or not the user space
> > > > will use this opportunity is a different matter.
> > > 
> > > It is _not_ possible for userspace to avoid this race.  Help from the 
> > > kernel is needed.
> > 
> > It is possible if every (relevant) userspace program implements a
> > callback for the powermanager to check if one of it's wakeup-sources
> > got activated.
> > 
> > That way the powermanager would read /sys/power/wakeup_count, then do
> > the roundtrip to all it's registered users and only then suspend. 
> 
> After further thought, there's still a race:

Ah, yes. That's what I meant in my other mail with 'how to determine
that an wake event is in flight'. 

Read this too late.
 
> Alan Stern

Cheers,
Flo
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