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

[Hans de Goede]

Hi,

On 04-11-16 17:46, Hans de Goede wrote:
> 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.

Thinking more about this, using a spinlock is also not going to work
because led_cdev->brightness_set_blocking and led_cdev->brightness_get
can both block and thus cannot be called with a spinlock held.

I think that we need to just make this a problem of the led drivers
and in include/linux/leds.h document that the led-core does not do
locking and that the drivers themselves need to protect against
their brightness_set / brightness_get callbacks when necessary.

Regards,

Hans