The output of this command was empty:

cat /proc/$(pidof md2_raid6)/stack # md2_raid6 process was consuming 
100% cpu

Thanks for the information. I suspect the hang is waiting for md_lock().

Could you please dump the mdX_raid6 stack when the hang happens?
You can do it by:

     cat /proc/$(pidof mdX_raid6)/stack

Thanks,
Song

Regards,

Georgi Nikolov