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

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Date:   Mon, 11 Dec 2023 18:39:26 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>
Cc:     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 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?

[ ... ]

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