linux-kernel - Re: [PATCH v1 1/2] PM: runtime: Do not call __rpm_callback() from rpm

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Date:   Mon, 5 Dec 2022 13:51:39 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Tushar Nimkar <quic_tnimkar@...cinc.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Nitin Rawat <quic_nitirawa@...cinc.com>,
        Peter Wang <peter.wang@...iatek.com>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH v1 1/2] PM: runtime: Do not call __rpm_callback() from rpm_idle()

On Mon, Dec 5, 2022 at 1:47 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
>
> On Mon, 5 Dec 2022 at 13:13, Rafael J. Wysocki <rafael@...nel.org> wrote:
> >
> > On Mon, Dec 5, 2022 at 1:08 PM Ulf Hansson <ulf.hansson@...aro.org> wrote:
> > >
> > > On Fri, 2 Dec 2022 at 15:32, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > > >
> > > > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > >
> > > > 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@...el.com>
> > > > ---
> > > >  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 it doesn't need to check the callback pointer, though.

Moreover, IMO this code is easier to read without having to look at
__rpm_callback() and reverse engineer all of the different use cases
covered by it.

> > > 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.

Well, why does it have to cover all of the cases that are even somewhat related?

> >
> > > > +       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.

OK, thanks!