Man page 9 : wake_up_process

wake_up_process - Wake up a specific process

int wake_up_process(struct task_struct *  p );

p
    The process to be woken up.

Attempt to wake up the nominated process and move it to the set of runnable processes. Returns 1 if the process was woken up, 0 if it was already running.

It may be assumed that this function implies a write memory barrier before changing the task state if and only if any tasks are woken up.