linux-kernel - Re: [PATCH v3 5/8] ACPI: thermal: Hold thermal zone lock around trip updates

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Date:   Tue, 1 Aug 2023 20:39:07 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux ACPI <linux-acpi@...r.kernel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Michal Wilczynski <michal.wilczynski@...el.com>,
        Zhang Rui <rui.zhang@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v3 5/8] ACPI: thermal: Hold thermal zone lock around trip
 updates


Hi Rafael,

On 25/07/2023 14:16, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> There is a race condition between acpi_thermal_trips_update() and
> acpi_thermal_check_fn(), because the trip points may get updated while
> the latter is running which in theory may lead to inconsistent results.
> For example, if two trips are updated together, using the temperature
> value of one of them from before the update and the temperature value
> of the other one from after the update may not lead to the expected
> outcome.
> 
> To address this, make acpi_thermal_trips_update() hold the thermal zone
> lock across the entire update of trip points.

As commented in patch 3/8, having a driver locking a thermal core 
structure is not right and goes to the opposite direction of the recent 
cleanups.

Don't we have 2 race conditions:

acpi_thermal_trips_update() + thermal_zone_device_check()

acpi_thermal_trips_update() + acpi_thermal_trips_update()

For the former, we can disable the thermal zone, update and then enable

For the latter use a driver lock ?


> While at it, change the acpi_thermal_trips_update() return data type
> to void as that function always returns 0 anyway.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> v2 -> v3: No changes.
> 
> v1 -> v2:
>     * Hold the thermal zone lock instead of thermal_check_lock around trip
>       point updates (this also helps to protect thermal_get_trend() from using
>       stale trip temperatures).
>     * Add a comment documenting the purpose of the locking.
>     * Make acpi_thermal_trips_update() void.
> 
> ---
>   drivers/acpi/thermal.c |   21 ++++++++++++++++-----
>   1 file changed, 16 insertions(+), 5 deletions(-)
> 
> Index: linux-pm/drivers/acpi/thermal.c
> ===================================================================
> --- linux-pm.orig/drivers/acpi/thermal.c
> +++ linux-pm/drivers/acpi/thermal.c
> @@ -190,7 +190,7 @@ static int acpi_thermal_get_polling_freq
>   	return 0;
>   }
>   
> -static int acpi_thermal_trips_update(struct acpi_thermal *tz, int flag)
> +static void __acpi_thermal_trips_update(struct acpi_thermal *tz, int flag)
>   {
>   	acpi_status status;
>   	unsigned long long tmp;
> @@ -398,17 +398,28 @@ static int acpi_thermal_trips_update(str
>   			ACPI_THERMAL_TRIPS_EXCEPTION(flag, tz, "device");
>   		}
>   	}
> +}
>   
> -	return 0;
> +static void acpi_thermal_trips_update(struct acpi_thermal *tz, int flag)
> +{
> +	/*
> +	 * The locking is needed here to protect thermal_get_trend() from using
> +	 * a stale passive trip temperature and to synchronize with the trip
> +	 * temperature updates in acpi_thermal_check_fn().
> +	 */
> +	thermal_zone_device_lock(tz->thermal_zone);
> +
> +	__acpi_thermal_trips_update(tz, flag);
> +
> +	thermal_zone_device_unlock(tz->thermal_zone);
>   }
>   
>   static int acpi_thermal_get_trip_points(struct acpi_thermal *tz)
>   {
> -	int i, ret = acpi_thermal_trips_update(tz, ACPI_TRIPS_INIT);
>   	bool valid;
> +	int i;
>   
> -	if (ret)
> -		return ret;
> +	__acpi_thermal_trips_update(tz, ACPI_TRIPS_INIT);
>   
>   	valid = tz->trips.critical.valid |
>   		tz->trips.hot.valid |
> 
> 
> 

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