Message-Id: <201006220048.10875.rjw@sisk.pl>
Date:	Tue, 22 Jun 2010 00:48:10 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Florian Mickler <florian@...kler.org>,
	"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 Tuesday, June 22, 2010, Rafael J. Wysocki wrote:
> On Tuesday, June 22, 2010, Alan Stern wrote:
> > On Mon, 21 Jun 2010, Florian Mickler wrote:
> > 
> > > > In the end you would want to have communication in both directions:  
> > > > suspend blockers _and_ callbacks.  Polling is bad if done too often.  
> > > > But I think the idea is a good one.
> > > 
> > > Actually, I'm not so shure. 
> > > 
> > > 1. you have to roundtrip whereas in the suspend_blocker scheme you have
> > > active annotations (i.e. no further action needed) 
> > 
> > That's why it's best to use both.  The normal case is that programs
> > activate and deactivate blockers by sending one-way messages to the PM
> > process.  The exceptional case is when the PM process is about to
> > initiate a suspend; that's when it does the round-trip polling.  Since
> > the only purpose of the polling is to avoid a race, 90% of the time it
> > will succeed.
> > 
> > > 2. it may not be possible for a user to determine if a wake-event is
> > > in-flight. you would have to somehow pass the wake-event-number with
> > > it, so that the userspace process could ack it properly without
> > > confusion. Or... I don't know of anything else... 
> > > 
> > > 	1. userspace-manager (UM) reads a number (42). 
> > > 
> > > 	2. it questions userspace program X: is it ok to suspend?
> > > 
> > > 	[please fill in how userspace program X determines to block
> > > 	suspend]
> > > 
> > > 	3a. UM's roundtrip ends and it proceeds to write "42" to the
> > > 	kernel [suspending]
> > > 	3b. UM's roundtrip ends and it aborts suspend, because a
> > > 	(userspace-)suspend-blocker got activated
> > > 
> > > I'm not shure how the userspace program could determine that there is a
> > > wake-event in flight. Perhaps by storing the number of last wake-event.
> > > But then you need per-wake-event-counters... :|
> > 
> > Rafael seems to think timeouts will fix this.  I'm not so sure.
> > 
> > > Do you have some thoughts about the wake-event-in-flight detection?
> > 
> > Not really, except for something like the original wakelock scheme in
> > which the kernel tells the PM core when an event is over.
> 
> But the kernel doesn't really know that, so it really can't tell the PM core
> anything useful.  What happens with suspend blockers is that a kernel suspend

s/suspend/subsyste/  (-ETOOLATE)

> cooperates with a user space consumer of the event to get the story straight.
> 
> However, that will only work if the user space is not buggy and doesn't crash,
> for example, before releasing the suspend blocker it's holding.
> 
> Apart from this, there are those events withoug user space "handoff" that
> use timeouts.
> 
> Also there are events like wake-on-LAN that can be regarded as instantaneous
> from the power manager's point of view, so they don't really need all of the
> "suspend blockers" machinery and for them we will need to use a cooldown
> timeout anyway.
> 
> And if we need to use that cooldown timeout, I don't see why not to use
> timeouts for avoiding the race you're worrying about.
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