1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _LINUX_SCHED_WAKE_Q_H
3 #define _LINUX_SCHED_WAKE_Q_H
4
5 /*
6 * Wake-queues are lists of tasks with a pending wakeup, whose
7 * callers have already marked the task as woken internally,
8 * and can thus carry on. A common use case is being able to
9 * do the wakeups once the corresponding user lock as been
10 * released.
11 *
12 * We hold reference to each task in the list across the wakeup,
13 * thus guaranteeing that the memory is still valid by the time
14 * the actual wakeups are performed in wake_up_q().
15 *
16 * One per task suffices, because there's never a need for a task to be
17 * in two wake queues simultaneously; it is forbidden to abandon a task
18 * in a wake queue (a call to wake_up_q() _must_ follow), so if a task is
19 * already in a wake queue, the wakeup will happen soon and the second
20 * waker can just skip it.
21 *
22 * The DEFINE_WAKE_Q macro declares and initializes the list head.
23 * wake_up_q() does NOT reinitialize the list; it's expected to be
24 * called near the end of a function. Otherwise, the list can be
25 * re-initialized for later re-use by wake_q_init().
26 *
27 * NOTE that this can cause spurious wakeups. schedule() callers
28 * must ensure the call is done inside a loop, confirming that the
29 * wakeup condition has in fact occurred.
30 *
31 * NOTE that there is no guarantee the wakeup will happen any later than the
32 * wake_q_add() location. Therefore task must be ready to be woken at the
33 * location of the wake_q_add().
34 */
35
36 #include <linux/sched.h>
37
38 struct wake_q_head {
39 struct wake_q_node *first;
40 struct wake_q_node **lastp;
41 };
42
43 #define WAKE_Q_TAIL ((struct wake_q_node *) 0x01)
44
45 #define DEFINE_WAKE_Q(name) \
46 struct wake_q_head name = { WAKE_Q_TAIL, &name.first }
47
48 static inline void wake_q_init(struct wake_q_head *head)
49 {
50 head->first = WAKE_Q_TAIL;
51 head->lastp = &head->first;
52 }
53
54 static inline bool wake_q_empty(struct wake_q_head *head)
55 {
56 return head->first == WAKE_Q_TAIL;
57 }
58
59 extern void wake_q_add(struct wake_q_head *head, struct task_struct *task);
60 extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task);
61 extern void wake_up_q(struct wake_q_head *head);
62
63 #endif /* _LINUX_SCHED_WAKE_Q_H */