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

From: Hans de Goede
Date: Sun Nov 06 2016 - 09:53:17 EST


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

On 04-11-16 17:06, Jacek Anaszewski wrote:
Hi Hans,

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

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);
+ 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

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.