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

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Message-ID: <CAJZ5v0hCPG0_4MUW5bt+FLtPmnFZ9NUxsEiFpd-6+wOmYxPm5A@mail.gmail.com>
Date:   Mon, 11 Apr 2022 17:17:46 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        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 Mon, Apr 11, 2022 at 5:09 PM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
>
> 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;
>                }
>

I know.

And why would it be better?