linux-kernel - Re: [PATCH] leds: Add mutex protection in brightness

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Date:   Fri, 4 Nov 2016 12:53:17 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Jacek Anaszewski <j.anaszewski@...sung.com>,
        linux-leds@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Pavel Machek <pavel@....cz>, Andrew Lunn <andrew@...n.ch>
Subject: Re: [PATCH] leds: Add mutex protection in brightness_show()

Hi,

On 04-11-16 08:52, Jacek Anaszewski wrote:
> Initially the claim about no need for lock in brightness_show()
> was valid as the function was just returning unchanged
> LED brightness. After the addition of led_update_brightness() this
> is no longer true, as the function can change the brightness if
> a LED class driver implements brightness_get op. It can lead to
> races between led_update_brightness() and led_set_brightness(),
> resulting in overwriting new brightness with the old one before
> the former is written to the device.
>
> Signed-off-by: Jacek Anaszewski <j.anaszewski@...sung.com>
> Cc: Hans de Goede <hdegoede@...hat.com>
> Cc: Sakari Ailus <sakari.ailus@...ux.intel.com>
> Cc: Pavel Machek <pavel@....cz>
> Cc: Andrew Lunn <andrew@...n.ch>
> ---
>  drivers/leds/led-class.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
> index 731e4eb..0c2307b 100644
> --- a/drivers/leds/led-class.c
> +++ b/drivers/leds/led-class.c
> @@ -30,8 +30,9 @@ static ssize_t brightness_show(struct device *dev,
>  {
>  	struct led_classdev *led_cdev = dev_get_drvdata(dev);
>
> -	/* no lock needed for this */
> +	mutex_lock(&led_cdev->led_access);
>  	led_update_brightness(led_cdev);
> +	mutex_unlock(&led_cdev->led_access);
>
>  	return sprintf(buf, "%u\n", led_cdev->brightness);
>  }
>

I'm afraid that this fix is not enough, the led_access lock is only
held when the brightness is being updated through sysfs, not for
trigger / sw-blinking updates (which cannot take a mutex as they
may be called from non blocking contexts).

We may need to consider to add a spinlock to the led_classdev and
always lock that when calling into the driver, except for when
the driver has a brightness_set_blocking callback. Which will need
special handling.

Regards,

Hans