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

[Florian Mickler <florian@mickler.org>]

On Mon, 21 Jun 2010 12:54:44 -0400 (EDT)
Alan Stern <stern@rowland.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