Re: [PATCH] leds: Add mutex protection in brightness_show()

[Jacek Anaszewski]

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@xxxxxxxxxxx>
> > Cc: Hans de Goede <hdegoede@xxxxxxxxxx>
> > Cc: Sakari Ailus <sakari.ailus@xxxxxxxxxxxxxxx>
> > Cc: Pavel Machek <pavel@xxxxxx>
> > Cc: Andrew Lunn <andrew@xxxxxxx>
> > ---
> >  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