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

[Hans de Goede]

Hi,

On 04-11-16 17:06, Jacek Anaszewski wrote:
> 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().

The problem is not only callers of led_update_brightness() not holding
led_cdev->led_access, the problem is also callers of led_set_brightness
not holding led_cdev->led_access and they cannot take this lock because
they may be called from a non-blocking context.

Regards,

Hans