1/* SPDX-License-Identifier: GPL-2.0 */
2#ifndef _LINUX_SWAIT_H
3#define _LINUX_SWAIT_H
4
5#include <linux/list.h>
6#include <linux/stddef.h>
7#include <linux/spinlock.h>
8#include <linux/wait.h>
9#include <asm/current.h>
10
11/*
12 * BROKEN wait-queues.
13 *
14 * These "simple" wait-queues are broken garbage, and should never be
15 * used. The comments below claim that they are "similar" to regular
16 * wait-queues, but the semantics are actually completely different, and
17 * every single user we have ever had has been buggy (or pointless).
18 *
19 * A "swake_up_one()" only wakes up _one_ waiter, which is not at all what
20 * "wake_up()" does, and has led to problems. In other cases, it has
21 * been fine, because there's only ever one waiter (kvm), but in that
22 * case gthe whole "simple" wait-queue is just pointless to begin with,
23 * since there is no "queue". Use "wake_up_process()" with a direct
24 * pointer instead.
25 *
26 * While these are very similar to regular wait queues (wait.h) the most
27 * important difference is that the simple waitqueue allows for deterministic
28 * behaviour -- IOW it has strictly bounded IRQ and lock hold times.
29 *
30 * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all
31 * from IRQ disabled, and dropping the lock upon every wakeup, giving a higher
32 * priority task a chance to run.
33 *
34 * Secondly, we had to drop a fair number of features of the other waitqueue
35 * code; notably:
36 *
37 * - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue;
38 * all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right
39 * sleeper state.
40 *
41 * - the !exclusive mode; because that leads to O(n) wakeups, everything is
42 * exclusive.
43 *
44 * - custom wake callback functions; because you cannot give any guarantees
45 * about random code. This also allows swait to be used in RT, such that
46 * raw spinlock can be used for the swait queue head.
47 *
48 * As a side effect of these; the data structures are slimmer albeit more ad-hoc.
49 * For all the above, note that simple wait queues should _only_ be used under
50 * very specific realtime constraints -- it is best to stick with the regular
51 * wait queues in most cases.
52 */
53
54struct task_struct;
55
56struct swait_queue_head {
57 raw_spinlock_t lock;
58 struct list_head task_list;
59};
60
61struct swait_queue {
62 struct task_struct *task;
63 struct list_head task_list;
64};
65
66#define __SWAITQUEUE_INITIALIZER(name) { \
67 .task = current, \
68 .task_list = LIST_HEAD_INIT((name).task_list), \
69}
70
71#define DECLARE_SWAITQUEUE(name) \
72 struct swait_queue name = __SWAITQUEUE_INITIALIZER(name)
73
74#define __SWAIT_QUEUE_HEAD_INITIALIZER(name) { \
75 .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \
76 .task_list = LIST_HEAD_INIT((name).task_list), \
77}
78
79#define DECLARE_SWAIT_QUEUE_HEAD(name) \
80 struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INITIALIZER(name)
81
82extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name,
83 struct lock_class_key *key);
84
85#define init_swait_queue_head(q) \
86 do { \
87 static struct lock_class_key __key; \
88 __init_swait_queue_head((q), #q, &__key); \
89 } while (0)
90
91#ifdef CONFIG_LOCKDEP
92# define __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) \
93 ({ init_swait_queue_head(&name); name; })
94# define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \
95 struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name)
96#else
97# define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \
98 DECLARE_SWAIT_QUEUE_HEAD(name)
99#endif
100
101/**
102 * swait_active -- locklessly test for waiters on the queue
103 * @wq: the waitqueue to test for waiters
104 *
105 * returns true if the wait list is not empty
106 *
107 * NOTE: this function is lockless and requires care, incorrect usage _will_
108 * lead to sporadic and non-obvious failure.
109 *
110 * NOTE2: this function has the same above implications as regular waitqueues.
111 *
112 * Use either while holding swait_queue_head::lock or when used for wakeups
113 * with an extra smp_mb() like:
114 *
115 * CPU0 - waker CPU1 - waiter
116 *
117 * for (;;) {
118 * @cond = true; prepare_to_swait_exclusive(&wq_head, &wait, state);
119 * smp_mb(); // smp_mb() from set_current_state()
120 * if (swait_active(wq_head)) if (@cond)
121 * wake_up(wq_head); break;
122 * schedule();
123 * }
124 * finish_swait(&wq_head, &wait);
125 *
126 * Because without the explicit smp_mb() it's possible for the
127 * swait_active() load to get hoisted over the @cond store such that we'll
128 * observe an empty wait list while the waiter might not observe @cond.
129 * This, in turn, can trigger missing wakeups.
130 *
131 * Also note that this 'optimization' trades a spin_lock() for an smp_mb(),
132 * which (when the lock is uncontended) are of roughly equal cost.
133 */
134static inline int swait_active(struct swait_queue_head *wq)
135{
136 return !list_empty(&wq->task_list);
137}
138
139/**
140 * swq_has_sleeper - check if there are any waiting processes
141 * @wq: the waitqueue to test for waiters
142 *
143 * Returns true if @wq has waiting processes
144 *
145 * Please refer to the comment for swait_active.
146 */
147static inline bool swq_has_sleeper(struct swait_queue_head *wq)
148{
149 /*
150 * We need to be sure we are in sync with the list_add()
151 * modifications to the wait queue (task_list).
152 *
153 * This memory barrier should be paired with one on the
154 * waiting side.
155 */
156 smp_mb();
157 return swait_active(wq);
158}
159
160extern void swake_up_one(struct swait_queue_head *q);
161extern void swake_up_all(struct swait_queue_head *q);
162extern void swake_up_locked(struct swait_queue_head *q);
163
164extern void prepare_to_swait_exclusive(struct swait_queue_head *q, struct swait_queue *wait, int state);
165extern long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state);
166
167extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
168extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
169
170/* as per ___wait_event() but for swait, therefore "exclusive == 1" */
171#define ___swait_event(wq, condition, state, ret, cmd) \
172({ \
173 __label__ __out; \
174 struct swait_queue __wait; \
175 long __ret = ret; \
176 \
177 INIT_LIST_HEAD(&__wait.task_list); \
178 for (;;) { \
179 long __int = prepare_to_swait_event(&wq, &__wait, state);\
180 \
181 if (condition) \
182 break; \
183 \
184 if (___wait_is_interruptible(state) && __int) { \
185 __ret = __int; \
186 goto __out; \
187 } \
188 \
189 cmd; \
190 } \
191 finish_swait(&wq, &__wait); \
192__out: __ret; \
193})
194
195#define __swait_event(wq, condition) \
196 (void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \
197 schedule())
198
199#define swait_event_exclusive(wq, condition) \
200do { \
201 if (condition) \
202 break; \
203 __swait_event(wq, condition); \
204} while (0)
205
206#define __swait_event_timeout(wq, condition, timeout) \
207 ___swait_event(wq, ___wait_cond_timeout(condition), \
208 TASK_UNINTERRUPTIBLE, timeout, \
209 __ret = schedule_timeout(__ret))
210
211#define swait_event_timeout_exclusive(wq, condition, timeout) \
212({ \
213 long __ret = timeout; \
214 if (!___wait_cond_timeout(condition)) \
215 __ret = __swait_event_timeout(wq, condition, timeout); \
216 __ret; \
217})
218
219#define __swait_event_interruptible(wq, condition) \
220 ___swait_event(wq, condition, TASK_INTERRUPTIBLE, 0, \
221 schedule())
222
223#define swait_event_interruptible_exclusive(wq, condition) \
224({ \
225 int __ret = 0; \
226 if (!(condition)) \
227 __ret = __swait_event_interruptible(wq, condition); \
228 __ret; \
229})
230
231#define __swait_event_interruptible_timeout(wq, condition, timeout) \
232 ___swait_event(wq, ___wait_cond_timeout(condition), \
233 TASK_INTERRUPTIBLE, timeout, \
234 __ret = schedule_timeout(__ret))
235
236#define swait_event_interruptible_timeout_exclusive(wq, condition, timeout)\
237({ \
238 long __ret = timeout; \
239 if (!___wait_cond_timeout(condition)) \
240 __ret = __swait_event_interruptible_timeout(wq, \
241 condition, timeout); \
242 __ret; \
243})
244
245#define __swait_event_idle(wq, condition) \
246 (void)___swait_event(wq, condition, TASK_IDLE, 0, schedule())
247
248/**
249 * swait_event_idle_exclusive - wait without system load contribution
250 * @wq: the waitqueue to wait on
251 * @condition: a C expression for the event to wait for
252 *
253 * The process is put to sleep (TASK_IDLE) until the @condition evaluates to
254 * true. The @condition is checked each time the waitqueue @wq is woken up.
255 *
256 * This function is mostly used when a kthread or workqueue waits for some
257 * condition and doesn't want to contribute to system load. Signals are
258 * ignored.
259 */
260#define swait_event_idle_exclusive(wq, condition) \
261do { \
262 if (condition) \
263 break; \
264 __swait_event_idle(wq, condition); \
265} while (0)
266
267#define __swait_event_idle_timeout(wq, condition, timeout) \
268 ___swait_event(wq, ___wait_cond_timeout(condition), \
269 TASK_IDLE, timeout, \
270 __ret = schedule_timeout(__ret))
271
272/**
273 * swait_event_idle_timeout_exclusive - wait up to timeout without load contribution
274 * @wq: the waitqueue to wait on
275 * @condition: a C expression for the event to wait for
276 * @timeout: timeout at which we'll give up in jiffies
277 *
278 * The process is put to sleep (TASK_IDLE) until the @condition evaluates to
279 * true. The @condition is checked each time the waitqueue @wq is woken up.
280 *
281 * This function is mostly used when a kthread or workqueue waits for some
282 * condition and doesn't want to contribute to system load. Signals are
283 * ignored.
284 *
285 * Returns:
286 * 0 if the @condition evaluated to %false after the @timeout elapsed,
287 * 1 if the @condition evaluated to %true after the @timeout elapsed,
288 * or the remaining jiffies (at least 1) if the @condition evaluated
289 * to %true before the @timeout elapsed.
290 */
291#define swait_event_idle_timeout_exclusive(wq, condition, timeout) \
292({ \
293 long __ret = timeout; \
294 if (!___wait_cond_timeout(condition)) \
295 __ret = __swait_event_idle_timeout(wq, \
296 condition, timeout); \
297 __ret; \
298})
299
300#endif /* _LINUX_SWAIT_H */
301