linux-kernel - Re: [PATCH v1] PM: runtime: Avoid device usage count underflows

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Message-ID: <CAHp75Vc2Lci4ewdosqX4Thm6ME7pKjKw+C+wtUsq2esRM-eXjg@mail.gmail.com>
Date:   Mon, 11 Apr 2022 18:05:37 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH v1] PM: runtime: Avoid device usage count underflows

On Wed, Apr 6, 2022 at 11:49 PM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>
> A PM-runtime device usage count underflow is potentially critical,
> because it may cause a device to be suspended when it is expected to
> be operational.
>
> For this reason, (1) make rpm_check_suspend_allowed() return an error
> when the device usage count is negative to prevent devices from being
> suspended in that case, (2) introduce rpm_drop_usage_count() that will
> detect device usage count underflows, warn about them and fix them up,
> and (3) use it to drop the usage count in a few places instead of
> atomic_dec_and_test().

...

> +               retval = rpm_drop_usage_count(dev);
> +               if (retval > 0) {
>                         trace_rpm_usage_rcuidle(dev, rpmflags);
>                         return 0;
> +               } else if (retval < 0) {
> +                       return retval;
>                 }

Can be written in a form

               if (retval < 0)
                       return retval;
               if (retval > 0) {
                       trace_rpm_usage_rcuidle(dev, rpmflags);
                       return 0;
               }

...

> +               if (retval > 0) {
>                         trace_rpm_usage_rcuidle(dev, rpmflags);
>                         return 0;
> +               } else if (retval < 0) {
> +                       return retval;
>                 }

Ditto.

-- 
With Best Regards,
Andy Shevchenko