On-board LEDs shut down on trigger=default-on on Pi 3B when quickly switched from another trigger

[enurseitov]

I'm not sure if it is related to hardware or kernel.
Bug seen on multiple RPis 3B with 4.9 and 4.14 kernels. Bug doesn't appear on 3B+.

If trigger of any of on-board LEDs is switched to default-on while it previously was controlled by some high-frequency trigger (like cpu or mmc0), there is a high chance that LED will turn OFF instead of turning on and all the consecutive echo default-on | sudo tee /sys/class/leds/led0/trigger will take no effect. To get out, you have to switch trigger to another source (like none), wait some time (itherwise it may have no effect) and switch back to default-on.

Steps to reproduce:

• echo cpu | sudo tee /sys/class/leds/led0/trigger
• load the CPU, e.g. stress -c 2
• echo default-on | sudo tee /sys/class/leds/led0/trigger

Expected behavior: ACT LED turns SOLID ON
Actual behavior: ACT LED turns SOLID OFF