1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef _LINUX_RCUWAIT_H_
3#define _LINUX_RCUWAIT_H_
4
5#include <linux/rcupdate.h>
6
7/*
8 * rcuwait provides a way of blocking and waking up a single
9 * task in an rcu-safe manner; where it is forbidden to use
10 * after exit_notify(). task_struct is not properly rcu protected,
11 * unless dealing with rcu-aware lists, ie: find_task_by_*().
12 *
13 * Alternatively we have task_rcu_dereference(), but the return
14 * semantics have different implications which would break the
15 * wakeup side. The only time @task is non-nil is when a user is
16 * blocked (or checking if it needs to) on a condition, and reset
17 * as soon as we know that the condition has succeeded and are
18 * awoken.
19 */
20struct rcuwait {
21 struct task_struct *task;
22};
23
24#define __RCUWAIT_INITIALIZER(name) \
25 { .task = NULL, }
26
27static inline void rcuwait_init(struct rcuwait *w)
28{
29 w->task = NULL;
30}
31
32extern void rcuwait_wake_up(struct rcuwait *w);
33
34/*
35 * The caller is responsible for locking around rcuwait_wait_event(),
36 * such that writes to @task are properly serialized.
37 */
38#define rcuwait_wait_event(w, condition) \
39({ \
40 /* \
41 * Complain if we are called after do_exit()/exit_notify(), \
42 * as we cannot rely on the rcu critical region for the \
43 * wakeup side. \
44 */ \
45 WARN_ON(current->exit_state); \
46 \
47 rcu_assign_pointer((w)->task, current); \
48 for (;;) { \
49 /* \
50 * Implicit barrier (A) pairs with (B) in \
51 * rcuwait_wake_up(). \
52 */ \
53 set_current_state(TASK_UNINTERRUPTIBLE); \
54 if (condition) \
55 break; \
56 \
57 schedule(); \
58 } \
59 \
60 WRITE_ONCE((w)->task, NULL); \
61 __set_current_state(TASK_RUNNING); \
62})
63
64#endif /* _LINUX_RCUWAIT_H_ */
65