wait_queue api in kernel

 I have been trapped under the impression that `wait_queue` is always linked with task pending. However, this is totally wrong. 

The major API of wait_queue is

1. initialization

DEFINE_WAIT_FUNC(name, function) or init_wait_queue_entry

2. add an entry to queue

void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry);

Variations:
poll_wait(...)

3. Helper functions do real wait for the schedule.

// CPU0 waker

wake_up(wq_head); 


// CPU1 waiter

prepare_to_wait(&wq_head, &wait, state);
schedule();
if (@cond) {break;}

finish_wait(&wq_head, &wait);
// sleep until a condition gets true
wait_event(wq_head, condition)	

P.S Glad to see the naming is moving to a less misleading situation. Basically the entry and head are named by their role, although the underlying struct is always list_head.

wait_queue_t -> wait_queue_entry_t

wait_queue_t::task_list -> wait_queue_entry_t::entry

wait_queue_head_t::task_list -> wait_queue_head_t::head