If a process sleep in TASK_KILLABLE state, we can kill it by signal 'SIGKILL' to it.
"kill -9 pid" will set sig in 'task_struct->signal->shared_pending', then wake up the process.
But, wait_event_killable only check "task_struct->pending" to see if a fatal signal are pending.(__fatal_signal_pending)
So, kill -9 can not kill a KILLABLE process.
How to kill a TASK_KILLABLE process?