Linux源碼分析：completion的解讀

1. /**
2. * wait_for_completion: - waits for completion of a task
3. * @x: holds the state of this particular completion
4. *
5. * This waits to be signaled for completion of a specific task. It is NOT
6. * interruptible and there is no timeout.
7. *
8. * See also similar routines (i.e. wait_for_completion_timeout()) with timeout
9. * and interrupt capability. Also see complete().
10. */
11. void __sched wait_for_completion(struct completion *x)
12. {
13. wait_for_common(x, MAX_SCHEDULE_TIMEOUT, TASK_UNINTERRUPTIBLE);
14. }

1. static long __sched
2. wait_for_common(struct completion *x, long timeout, int state)
3. {
4. might_sleep();
6. spin_lock_irq(&x->wait.lock);
7. timeout = do_wait_for_common(x, timeout, state);
8. spin_unlock_irq(&x->wait.lock);
9. return timeout;
10. }

注意：spin_lock_irq(&x->wait.lock)和spin_unlock_irq(&x->wait.lock)並非真正對應的一對自旋鎖，因為在自旋鎖保護中是不允許休眠和調度的。與他們相對應的解鎖和上鎖操作在do_wait_for_common(x, timeout, state)函數內部。

1. static inline long __sched
2. do_wait_for_common(struct completion *x, long timeout, int state)
3. {
4. if (!x->done) {
5. DECLARE_WAITQUEUE(wait, current);
7. wait.flags |= WQ_FLAG_EXCLUSIVE;
8. __add_wait_queue_tail(&x->wait, &wait);
9. do {
10. if (signal_pending_state(state, current)) {
11. timeout = -ERESTARTSYS;
12. break;
13. }
14. __set_current_state(state);
15. spin_unlock_irq(&x->wait.lock);
16. timeout = schedule_timeout(timeout);
17. spin_lock_irq(&x->wait.lock);
18. } while (!x->done && timeout);
19. __remove_wait_queue(&x->wait, &wait);
20. if (!x->done)
21. return timeout;
22. }
23. x->done--;
24. return timeout ?: 1;
25. }