Re: [linux-pm] [PATCH 0/8] Suspend block api (version 8)

Rafael J. Wysocki
2010-06-01T22:02:31Z

On Tuesday 01 June 2010, Alan Stern wrote:
> On Tue, 1 Jun 2010, Neil Brown wrote:
>
> > > As I said before, we generally can't prevent such things from happening,
> > > because even if we handle the particular race described above, it still is
> > > possible that the event will be "lost" if it arrives just a bit later (eg.
> > > during a suspend-toggle switch). So the PRE_SUSPEND thing won't really
> > > solve the entire problem while increasing complexity.
> > >
> > > > My freerunner has a single core so without CONFIG_PREEMPT it may be that
> > > > there is no actual race-window - maybe the PRE_SUSPENDs will all run before a
> > > > soft_irq thread has a chance to finish handling of the interrupt (my
> > > > knowledge of these details is limits). But on a muilti-core device I think
> > > > there would definitely be a race-window.
> > >
> > > Yes, there always will be a race window. The only thing we can do is to
> > > narrow it, but we cannot really close it (at least not on a PC, but I'm not
> > > really sure it can be closed at all).
> >
> > Well you are the expert so I assume you are right, but I would really like to
> > understand why this is.
> >
> > I guess that if the event was delivered as an edge-triggered interrupt which
> > the interrupt controller couldn't latch, then it might get lost. Is that
> > what happens?
>
> You're barking up the wrong tree. If I understand correctly, Rafael is
> saying that there's a race involving events which are not _wakeup_
> events. If a non-wakeup event arrives shortly before a suspend, it can
> have its normal effect. If it arrives while a suspend is in progress,
> its delivery may be delayed until the system resumes. And if it
> arrives after the system is fully suspended, it may never be noticed at
> all.
>
> With wakeup events the problem isn't so bad. Wakeup events are always
> noticed, and if the system is designed properly they will either abort
> a suspend-in-progress or else cause the system to resume as soon as the
> suspend is complete. (Linux is not yet properly designed in this
> sense.)
>
> Or maybe I'm misunderstanding also, and Rafael is saying that there's
> a race involving events whose meaning changes depending on whether or
> not the system is asleep. This is obviously true and clearly
> unavoidable.

In fact I was referring to both in different places. That's why what I said
wasn't too clear, sorry about that.

Rafael