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From:   "Rafael J. Wysocki" <rafael@kernel.org>
Date:   Thu, 30 Nov 2023 20:33:14 +0100
Message-ID: <CAJZ5v0iSvhaCeZaDShRKnk93=jwcQWnJuqfL9grWyQm4YHaAYw@mail.gmail.com>
Subject: Re: [PATCH v1 2/2] thermal: sysfs: Eliminate unnecessary zone locking
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc:     Daniel Lezcano <daniel.lezcano@linaro.org>,
        Lukasz Luba <lukasz.luba@arm.com>,
        Linux PM <linux-pm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        Zhang Rui <rui.zhang@intel.com>
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On Thu, Nov 30, 2023 at 7:38 PM Rafael J. Wysocki <rjw@rjwysocki.net> wrote:
>
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
>
> The _show() callback functions of the trip point sysfs attributes,
> temperature, hysteresis and type, need not use thermal zone locking,
> because the layout of the data structures they access does not change
> after the thermal zone registration.
>
> Namely, they all need to access a specific entry in the thermal
> zone's trips[] table that is always present for non-tripless thermal
> zones and its size cannot change after the thermal zone has been
> registered.  Thus it is always safe to access the trips[] table of a
> registered thermal zone from each of the sysfs attributes in question.
>
> Moreover, the type of a trip point does not change after registering its
> thermal zone, and while its temperature and hysteresis can change, for
> example due to a firmware-induced thermal zone update, holding the zone
> lock around reading them is pointless, because it does not prevent stale
> values from being returned to user space.  For example, a trip point
> temperature can always change ater trip_point_temp_show() has read it
> and before the function's return statement is executed, regardless of
> whether or not zone locking is used.
>
> For this reason, drop the zone locking from trip_point_type_show(),
> trip_point_temp_show(), and trip_point_hyst_show().
>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
>  drivers/thermal/thermal_sysfs.c |   60 ++++++++++++++--------------------------
>  1 file changed, 21 insertions(+), 39 deletions(-)
>
> Index: linux-pm/drivers/thermal/thermal_sysfs.c
> ===================================================================
> --- linux-pm.orig/drivers/thermal/thermal_sysfs.c
> +++ linux-pm/drivers/thermal/thermal_sysfs.c
> @@ -83,25 +83,18 @@ trip_point_type_show(struct device *dev,
>                      char *buf)
>  {
>         struct thermal_zone_device *tz = to_thermal_zone(dev);
> -       struct thermal_trip trip;
> -       int trip_id, result;
> +       int trip_id;
> +
> +       if (!device_is_registered(dev))
> +               return -ENODEV;
>
>         if (sscanf(attr->attr.name, "trip_point_%d_type", &trip_id) != 1)
>                 return -EINVAL;
>
> -       mutex_lock(&tz->lock);
> -
> -       if (device_is_registered(dev))
> -               result = __thermal_zone_get_trip(tz, trip_id, &trip);
> -       else
> -               result = -ENODEV;
> -
> -       mutex_unlock(&tz->lock);
> -
> -       if (result)
> -               return result;
> +       if (trip_id < 0 || trip_id > tz->num_trips)

An off-by-one here, it should be trip_id >= tz->num_trips (and
analogously below).

I'll send an update of this shortly.

> +               return -EINVAL;
>
> -       switch (trip.type) {
> +       switch (tz->trips[trip_id].type) {
>         case THERMAL_TRIP_CRITICAL:
>                 return sprintf(buf, "critical\n");
>         case THERMAL_TRIP_HOT:
> @@ -164,25 +157,18 @@ trip_point_temp_show(struct device *dev,
>                      char *buf)
>  {
>         struct thermal_zone_device *tz = to_thermal_zone(dev);
> -       struct thermal_trip trip;
> -       int trip_id, ret;
> +       int trip_id;
> +
> +       if (!device_is_registered(dev))
> +               return -ENODEV;
>
>         if (sscanf(attr->attr.name, "trip_point_%d_temp", &trip_id) != 1)
>                 return -EINVAL;
>
> -       mutex_lock(&tz->lock);
> -
> -       if (device_is_registered(dev))
> -               ret = __thermal_zone_get_trip(tz, trip_id, &trip);
> -       else
> -               ret = -ENODEV;
> -
> -       mutex_unlock(&tz->lock);
> -
> -       if (ret)
> -               return ret;
> +       if (trip_id < 0 || trip_id > tz->num_trips)
> +               return -EINVAL;
>
> -       return sprintf(buf, "%d\n", trip.temperature);
> +       return sprintf(buf, "%d\n", tz->trips[trip_id].temperature);
>  }
>
>  static ssize_t
> @@ -234,22 +220,18 @@ trip_point_hyst_show(struct device *dev,
>                      char *buf)
>  {
>         struct thermal_zone_device *tz = to_thermal_zone(dev);
> -       struct thermal_trip trip;
> -       int trip_id, ret;
> +       int trip_id;
> +
> +       if (!device_is_registered(dev))
> +               return -ENODEV;
>
>         if (sscanf(attr->attr.name, "trip_point_%d_hyst", &trip_id) != 1)
>                 return -EINVAL;
>
> -       mutex_lock(&tz->lock);
> -
> -       if (device_is_registered(dev))
> -               ret = __thermal_zone_get_trip(tz, trip_id, &trip);
> -       else
> -               ret = -ENODEV;
> -
> -       mutex_unlock(&tz->lock);
> +       if (trip_id < 0 || trip_id > tz->num_trips)
> +               return -EINVAL;
>
> -       return ret ? ret : sprintf(buf, "%d\n", trip.hysteresis);
> +       return sprintf(buf, "%d\n", tz->trips[trip_id].hysteresis);
>  }
>
>  static ssize_t
>
>
>
>