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 13:07:59 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     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 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?

>
> 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?

Note that, __rpm_callback() already uses a "bool use_links" internal
variable, that indicates whether the device links should be used or
not.

> +       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);
>
>
>

Kind regards
Uffe