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 13:13:20 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     "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>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        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 1:08 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.

> 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?

> > +       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);
> >
> >
> >