leds: avoid races with workqueue
There are races between "main" thread and workqueue. They manifest
themselves on Thinkpad X60:
This should result in LED blinking, but it turns it off instead:
root@amd:/data/pavel# cd /sys/class/leds/tpacpi\:\:power
root@amd:/sys/class/leds/tpacpi::power# echo timer > trigger
root@amd:/sys/class/leds/tpacpi::power# echo timer > trigger
It should be possible to transition from blinking to solid on by echo
0 > brightness; echo 1 > brightness... but that does not work, either,
if done too quickly.
Synchronization of the workqueue fixes both.
Fixes:
1afcadfcd184 ("leds: core: Use set_brightness_work for the blocking op")
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>