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Message-ID: <c68cdec6-a3ba-491d-8193-0f3d13705d5f@linaro.org>
Date:   Fri, 1 Dec 2023 10:16:20 +0100
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Subject: Re: [PATCH v1 2/2] thermal: sysfs: Eliminate unnecessary zone locking
Content-Language: en-US
To:     "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Lukasz Luba <lukasz.luba@arm.com>
Cc:     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>
References: <5754079.DvuYhMxLoT@kreacher> <2933888.e9J7NaK4W3@kreacher>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
In-Reply-To: <2933888.e9J7NaK4W3@kreacher>
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On 30/11/2023 19:37, Rafael J. Wysocki 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().

Isn't the lock used to protect against device_del() from 
thermal_zone_device_unregister() ?


> 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)
> +		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
> 
> 
> 
> 

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