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SubjectRe: [PATCH] leds: turn off the LED and wait for completion on unregistering LED class device
Hi Milo,

Thanks for catching this, applied.

Best Regards,
Jacek Anaszewski

On 11/20/2015 09:03 AM, Milo Kim wrote:
> Workqueue, 'set_brightness_work' is used for scheduling brightness control.
> This workqueue is canceled when the LED class device is unregistered.
> Currently, LED subsystem handles like below.
>
> cancel_work_sync(&led_cdev->set_brightness_work)
> led_set_brightness(led_cdev, LED_OFF)
>
> However, this could be a problem.
> Workqueue is going to be canceled but LED device needs to be off.
> The worst case is null pointer access due to scheduling a workqueue.
>
> LED module is loaded.
> LED driver private data is allocated by using devm_zalloc().
>
> LED module is unloaded.
> led_classdev_unregister() is called.
> cancel_work_sync()
> led_set_brightness(led_cdev, LED_OFF)
> schedule_work() if LED driver uses brightness_set_blocking()
> In the meantime, driver private data will be freed.
>
> ..scheduling..
>
> brightness_set_blocking() callback is invoked.
> For the brightness control, LED driver tries to access private
> data but resource is removed!
>
> To avoid this problem, LED subsystem should turn off the brightness first
> and wait for completion.
>
> led_set_brightness(led_cdev, LED_OFF)
> flush_work(&led_cdev->set_brightness_work)
>
> It guarantees that LED driver turns off the brightness prior to
> resource management.
>
> Cc: linux-leds@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Milo Kim <milo.kim@ti.com>
> ---
> drivers/leds/led-class.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/leds/led-class.c b/drivers/leds/led-class.c
> index d946991..14139c3 100644
> --- a/drivers/leds/led-class.c
> +++ b/drivers/leds/led-class.c
> @@ -245,12 +245,13 @@ void led_classdev_unregister(struct led_classdev *led_cdev)
> up_write(&led_cdev->trigger_lock);
> #endif
>
> - cancel_work_sync(&led_cdev->set_brightness_work);
> -
> /* Stop blinking */
> led_stop_software_blink(led_cdev);
> +
> led_set_brightness(led_cdev, LED_OFF);
>
> + flush_work(&led_cdev->set_brightness_work);
> +
> device_unregister(led_cdev->dev);
>
> down_write(&leds_list_lock);
>



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