linux-kernel - Re: [PATCH v1 2/3] thermal: Drop redundant and confusing device_is

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Message-ID: <CAJZ5v0hwLcL9UKQs7WA=hb2v31eEY83rv-bQVgSv_EV9AidYZA@mail.gmail.com>
Date:   Mon, 11 Dec 2023 18:58:00 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lukasz Luba <lukasz.luba@....com>
Subject: Re: [PATCH v1 2/3] thermal: Drop redundant and confusing
 device_is_registered() checks

On Mon, Dec 11, 2023 at 6:39 PM Daniel Lezcano
<daniel.lezcano@...aro.org> wrote:
>
> On 08/12/2023 20:19, Rafael J. Wysocki wrote:
> > From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> >
> > Multiple places in the thermal subsystem (most importantly, sysfs
> > attribute callback functions) check if the given thermal zone device is
> > still registered in order to return early in case the device_del() in
> > thermal_zone_device_unregister() has run already.
> >
> > However, after thermal_zone_device_unregister() has been made wait for
> > all of the zone-related activity to complete before returning, it is
> > not necessary to do that any more, because all of the code holding a
> > reference to the thermal zone device object will be waited for even if
> > it does not do anything special to enforce this.
> >
> > Accordingly, drop all of the device_is_registered() checks that are now
> > redundant and get rid of the zone locking that is not necessary any more
> > after dropping them.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > ---
>
> [ ... ]
>
> > @@ -132,11 +120,6 @@ trip_point_temp_store(struct device *dev
> >
> >       mutex_lock(&tz->lock);
> >
> > -     if (!device_is_registered(dev)) {
> > -             ret = -ENODEV;
> > -             goto unlock;
> > -     }
> > -
> >       trip = &tz->trips[trip_id];
> >
> >       if (temp != trip->temperature) {
> > @@ -162,23 +145,12 @@ trip_point_temp_show(struct device *dev,
> >                    char *buf)
> >   {
> >       struct thermal_zone_device *tz = to_thermal_zone(dev);
> > -     int trip_id, temp;
> > +     int trip_id;
> >
> >       if (sscanf(attr->attr.name, "trip_point_%d_temp", &trip_id) != 1)
> >               return -EINVAL;
> >
> > -     mutex_lock(&tz->lock);
> > -
> > -     if (!device_is_registered(dev)) {
> > -             mutex_unlock(&tz->lock);
> > -             return -ENODEV;
> > -     }
> > -
> > -     temp = tz->trips[trip_id].temperature;
> > -
> > -     mutex_unlock(&tz->lock);
> > -
> > -     return sprintf(buf, "%d\n", temp);
> > +     return sprintf(buf, "%d\n", tz->trips[trip_id].temperature);
>
> Without the lock, could the trip_temp_store() make the value change
> while we read it?

The lock doesn't change that, because the write can occur before
dropping the lock and the printf() and reading an int is atomic on all
architectures supported by Linux.