Re: [PATCH 1/8] PM: Add suspend block api.
From: Alan Stern
Date: Fri May 07 2010 - 10:22:29 EST
On Thu, 6 May 2010, Arve Hjønnevåg wrote:
> On Thu, May 6, 2010 at 12:40 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, 6 May 2010, Rafael J. Wysocki wrote:
> >
> >> > Here's a completely new issue. When using opportunistic suspends on an
> >> > SMP system, it could happen that the system gets a wakeup event and
> >> > this routine starts running again before the event's IRQ handler has
> >> > finished (or has enabled a suspend blocker). The system would
> >> > re-suspend too soon.
> >>
> >> This routine will be run from a freezable workqueue.
> >
> > But how do you know that processes won't get unfrozen until all the
> > pending IRQs have been handled? Imagine something like this:
> >
> > CPU 0 CPU 1
> > ----- -----
> > Wake up non-boot CPUs
> > Resume devices Invoke the IRQ handler
> >
> > [ CPU 0 should wait here for the handler to finish,
> > but it doesn't ]
> >
> > Defrost threads Handler running...
> > Workqueue routine runs
> > Start another suspend
> > Handler enables a suspend blocker,
> > but it's too late
>
> It is not optimal, but it is not too late. We check if any suspend
> blockers block suspend after disabling non-boot cpus so as long as
> this is done in a way that does not lose interrupts the resuspend
> attempt will not succeed.
Is it possible for the resuspend to disable CPU 1 before the IRQ
handler can enable its suspend blocker? (Probably not -- but I don't
know enough about how non-boot CPUs are enabled or disabled.)
Alan Stern
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