Re: [PATCH 1/8] PM: Add suspend block api.

Alan Stern
2010-05-07T14:22:20Z

On Thu, 6 May 2010, Arve Hjønnevåg wrote:

> On Thu, May 6, 2010 at 12:40 PM, Alan Stern <stern@rowland.harvard.edu> 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