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

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Date:   Fri, 04 Nov 2016 17:06:56 +0100
From:   Jacek Anaszewski <j.anaszewski@...sung.com>
To:     Hans de Goede <hdegoede@...hat.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 Hans,

On 11/04/2016 12:53 PM, Hans de Goede wrote:
> 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.

led_update_brightness() currently has two users besides LED subsystem
(at least grep reports those places):

1. v4l2-flash-led-class wrapper, for which led_access mutex was
    introduced. Its purpose was to disable LED sysfs interface while
    v4l2-flash wrapper takes over control of LED class device
    (not saying that the mutex wasn't missing even without this
     use case). Now I've realized that the call to
     led_sysfs_is_disabled() is missing in this patch.
2. /drivers/platform/x86/thinkpad_acpi.c - it calls
    led_update_brightness() on suspend

I think that the best we can now do is to add
lockdep_assert_held(&led_cdev->led_access) in led_update_brightness()
and a description saying that the caller has to acquire led_access
lock before calling it. Similarly as in case of
led_sysfs_disable()/led_sysfs_disable().

-- 
Best regards,
Jacek Anaszewski