Re: [PATCH v1 1/2] PM: runtime: Do not call __rpm_callback() from rpm_idle()

[Ulf Hansson]

On Mon, 5 Dec 2022 at 13:13, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
>
> On Mon, Dec 5, 2022 at 1:08 PM Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote:
> >
> > On Fri, 2 Dec 2022 at 15:32, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> > >
> > > From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > >
> > > Calling __rpm_callback() from rpm_idle() after adding device links
> > > support to the former is a clear mistake.
> > >
> > > Not only it causes rpm_idle() to carry out unnecessary actions, but it
> > > is also against the assumption regarding the stability of PM-runtime
> > > status accross __rpm_callback() invocations, because rpm_suspend() and
> > > rpm_resume() may run in parallel with __rpm_callback() when it is called
> > > by rpm_idle() and the device's PM-runtime status can be updated by any
> > > of them.
> >
> > Urgh, that's a nasty bug you are fixing here. Is there perhaps some
> > links to some error reports that can make sense to include here?
>
> There is a bug report, but I have no confirmation that this fix is
> sufficient to address it (even though I'm quite confident that it will
> be).
>
> > >
> > > Fixes: 21d5c57b3726 ("PM / runtime: Use device links")
> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > > ---
> > >  drivers/base/power/runtime.c |   12 +++++++++++-
> > >  1 file changed, 11 insertions(+), 1 deletion(-)
> > >
> > > Index: linux-pm/drivers/base/power/runtime.c
> > > ===================================================================
> > > --- linux-pm.orig/drivers/base/power/runtime.c
> > > +++ linux-pm/drivers/base/power/runtime.c
> > > @@ -484,7 +484,17 @@ static int rpm_idle(struct device *dev,
> > >
> > >         dev->power.idle_notification = true;
> > >
> > > -       retval = __rpm_callback(callback, dev);
> >
> > Couldn't we just extend __rpm_callback() to take another in-parameter,
> > rather than open-coding the below?
>
> I'd rather not do that.
>
> I'd prefer rpm_callback() to be used only in rpm_suspend() and
> rpm_resume() where all of the assumptions hold and rpm_idle() really
> is a special case.
>
> And there is not much open-coding here, just the locking part.

That and the actual call to the callback. Not much, but still.

>
> > Note that, __rpm_callback() already uses a "bool use_links" internal
> > variable, that indicates whether the device links should be used or
> > not.
>
> Yes, it does, but why does that matter?

It means that __rpm_callback() is already prepared to (almost) cover this case.

>
> > > +       if (dev->power.irq_safe)
> > > +               spin_unlock(&dev->power.lock);
> > > +       else
> > > +               spin_unlock_irq(&dev->power.lock);
> > > +
> > > +       retval = callback(dev);
> > > +
> > > +       if (dev->power.irq_safe)
> > > +               spin_lock(&dev->power.lock);
> > > +       else
> > > +               spin_lock_irq(&dev->power.lock);
> > >
> > >         dev->power.idle_notification = false;
> > >         wake_up_all(&dev->power.wait_queue);
> > >
> > >
> > >

Note, it's not a big deal to me, if you feel strongly that your
current approach is better, I am fine with that too.

Kind regards
Uffe