linux-kernel - Re: [markgross@...ngar.org: [RFC] wake up notifications and suspend blocking (aka more wakelock stuff)]

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Date:	Sat, 15 Oct 2011 07:05:37 -0700
From:	mark gross <markgross@...gnar.org>
To:	mark gross <markgross@...gnar.org>
Cc:	NeilBrown <neilb@...e.de>, linux-kernel@...r.kernel.org,
	John Stultz <john.stultz@...aro.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>, arve@...roid.com,
	Alan Stern <stern@...land.harvard.edu>,
	amit.kucheria@...aro.org, farrowg@...ibm.com,
	"Dmitry Fink (Palm GBU)" <Dmitry.Fink@...m.com>,
	linux-pm@...ts.linux-foundation.org, khilman@...com,
	Magnus Damm <damm@...nsource.se>, mjg@...hat.com,
	peterz@...radead.org
Subject: Re: [markgross@...ngar.org: [RFC] wake up notifications and suspend
 blocking (aka more wakelock stuff)]

On Fri, Oct 14, 2011 at 07:01:16AM -0700, mark gross wrote:
> On Thu, Oct 13, 2011 at 04:35:26PM +1100, NeilBrown wrote:
> > On Wed, 12 Oct 2011 20:48:05 -0700 mark gross <markgross@...gnar.org> wrote:
> >

snip

> > Can you describe a race?
> > Here is the sequence as I see it.
> > 
> > 0: some user-space process is blocking suspend by telling the suspend daemon
> >    not to suspend.  There are no pending kernel wakeup events.
> >    All processes that handle wakeup events are registered with the daemon.
> > 1: last blocking user-space  process releases its "don't suspend" lock.
> > 2: suspend-daemon decides to initiate suspend.
> > 3: suspend_daemon reads wakeup_count and remembers number.
> > 4: suspend daemon tells all registered clients that suspend is imminent.
> > 5: each client executes 'poll' or 'select' or whatever and discovers that
> >    there are no events.
> > 6: each client tells daemon that it is OK to suspend
> > 7: when all votes are in, suspend daemon checks that no process is requesting
> >    that suspend be blocked.
> > 8: if that succeeds, suspend daemon writes the number back to wakeup_count
> > 9: if that succeeds, suspend daemon daemon writes 'mem' to 'state'.
> > 10: goto 1
> > 
> > If a wake_event happens after 3 and before 8, the write at 8 will fail.
> > If a wake_event happens after 8, and before 9, the suspend will abort.
> > If a wake_event happens after 9, the suspend will resume
> > If a wake_event happens before 3, one of the processes will get an event
> > notification from select or poll or whatever, and will ask the suspend
> > daemon not to suspend just now and this will be noticed at 7, so 8 and 9
> > will be skipped and we go straight to 10.
> > 
> > No race.
> 
> With the suspend daemon designed as above I see no race either.  I was
> thinking of a more trivial suspend daemon design and trying to fix it up
> in the kernel.
>

After responding to this yesterday I still feel I'm missing something
with respect to wake event acknowledgment.  After a wake source becomes
active how does it know its ok to become deactivate at the driver level?

The race I'm now worried about is say a button press going up through
the event layers.  The event manager process, or X, (registered with the
suspend daemon) is in a race to get the event and the socket based
notification handshake described above.

i.e. how is the key event delivery vrs unblocking of the suspend-daemon
read of wakeup_count in step 3 race dealt with?

Sorry for getting confused and flip flopping on you.  This is pretty
subtle stuff for me.

--mark

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