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 * Simple waitqueues are semantically very different to regular wait queues
13 * (wait.h). The most important difference is that the simple waitqueue allows
14 * for deterministic behaviour -- IOW it has strictly bounded IRQ and lock hold
15 * times.
16 *
17 * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all
18 * from IRQ disabled, and dropping the lock upon every wakeup, giving a higher
19 * priority task a chance to run.
20 *
21 * Secondly, we had to drop a fair number of features of the other waitqueue
22 * code; notably:
23 *
24 * - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue;
25 * all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right
26 * sleeper state.
27 *
28 * - the !exclusive mode; because that leads to O(n) wakeups, everything is
29 * exclusive. As such swake_up_one will only ever awake _one_ waiter.
30 *
31 * - custom wake callback functions; because you cannot give any guarantees
32 * about random code. This also allows swait to be used in RT, such that
33 * raw spinlock can be used for the swait queue head.
34 *
35 * As a side effect of these; the data structures are slimmer albeit more ad-hoc.
36 * For all the above, note that simple wait queues should _only_ be used under
37 * very specific realtime constraints -- it is best to stick with the regular
38 * wait queues in most cases.
39 */
40
41 struct task_struct;
42
43 struct swait_queue_head {
44 raw_spinlock_t lock;
45 struct list_head task_list;
46 };
47
48 struct swait_queue {
49 struct task_struct *task;
50 struct list_head task_list;
51 };
52
53 #define __SWAITQUEUE_INITIALIZER(name) { \
54 .task = current, \
55 .task_list = LIST_HEAD_INIT((name).task_list), \
56 }
57
58 #define DECLARE_SWAITQUEUE(name) \
59 struct swait_queue name = __SWAITQUEUE_INITIALIZER(name)
60
61 #define __SWAIT_QUEUE_HEAD_INITIALIZER(name) { \
62 .lock = __RAW_SPIN_LOCK_UNLOCKED(name.lock), \
63 .task_list = LIST_HEAD_INIT((name).task_list), \
64 }
65
66 #define DECLARE_SWAIT_QUEUE_HEAD(name) \
67 struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INITIALIZER(name)
68
69 extern void __init_swait_queue_head(struct swait_queue_head *q, const char *name,
70 struct lock_class_key *key);
71
72 #define init_swait_queue_head(q) \
73 do { \
74 static struct lock_class_key __key; \
75 __init_swait_queue_head((q), #q, &__key); \
76 } while (0)
77
78 #ifdef CONFIG_LOCKDEP
79 # define __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name) \
80 ({ init_swait_queue_head(&name); name; })
81 # define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \
82 struct swait_queue_head name = __SWAIT_QUEUE_HEAD_INIT_ONSTACK(name)
83 #else
84 # define DECLARE_SWAIT_QUEUE_HEAD_ONSTACK(name) \
85 DECLARE_SWAIT_QUEUE_HEAD(name)
86 #endif
87
88 /**
89 * swait_active -- locklessly test for waiters on the queue
90 * @wq: the waitqueue to test for waiters
91 *
92 * returns true if the wait list is not empty
93 *
94 * NOTE: this function is lockless and requires care, incorrect usage _will_
95 * lead to sporadic and non-obvious failure.
96 *
97 * NOTE2: this function has the same above implications as regular waitqueues.
98 *
99 * Use either while holding swait_queue_head::lock or when used for wakeups
100 * with an extra smp_mb() like:
101 *
102 * CPU0 - waker CPU1 - waiter
103 *
104 * for (;;) {
105 * @cond = true; prepare_to_swait_exclusive(&wq_head, &wait, state);
106 * smp_mb(); // smp_mb() from set_current_state()
107 * if (swait_active(wq_head)) if (@cond)
108 * wake_up(wq_head); break;
109 * schedule();
110 * }
111 * finish_swait(&wq_head, &wait);
112 *
113 * Because without the explicit smp_mb() it's possible for the
114 * swait_active() load to get hoisted over the @cond store such that we'll
115 * observe an empty wait list while the waiter might not observe @cond.
116 * This, in turn, can trigger missing wakeups.
117 *
118 * Also note that this 'optimization' trades a spin_lock() for an smp_mb(),
119 * which (when the lock is uncontended) are of roughly equal cost.
120 */
121 static inline int swait_active(struct swait_queue_head *wq)
122 {
123 return !list_empty(&wq->task_list);
124 }
125
126 /**
127 * swq_has_sleeper - check if there are any waiting processes
128 * @wq: the waitqueue to test for waiters
129 *
130 * Returns true if @wq has waiting processes
131 *
132 * Please refer to the comment for swait_active.
133 */
134 static inline bool swq_has_sleeper(struct swait_queue_head *wq)
135 {
136 /*
137 * We need to be sure we are in sync with the list_add()
138 * modifications to the wait queue (task_list).
139 *
140 * This memory barrier should be paired with one on the
141 * waiting side.
142 */
143 smp_mb();
144 return swait_active(wq);
145 }
146
147 extern void swake_up_one(struct swait_queue_head *q);
148 extern void swake_up_all(struct swait_queue_head *q);
149 extern void swake_up_locked(struct swait_queue_head *q, int wake_flags);
150
151 extern void prepare_to_swait_exclusive(struct swait_queue_head *q, struct swait_queue *wait, int state);
152 extern long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state);
153
154 extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
155 extern void finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
156
157 /* as per ___wait_event() but for swait, therefore "exclusive == 1" */
158 #define ___swait_event(wq, condition, state, ret, cmd) \
159 ({ \
160 __label__ __out; \
161 struct swait_queue __wait; \
162 long __ret = ret; \
163 \
164 INIT_LIST_HEAD(&__wait.task_list); \
165 for (;;) { \
166 long __int = prepare_to_swait_event(&wq, &__wait, state);\
167 \
168 if (condition) \
169 break; \
170 \
171 if (___wait_is_interruptible(state) && __int) { \
172 __ret = __int; \
173 goto __out; \
174 } \
175 \
176 cmd; \
177 } \
178 finish_swait(&wq, &__wait); \
179 __out: __ret; \
180 })
181
182 #define __swait_event(wq, condition) \
183 (void)___swait_event(wq, condition, TASK_UNINTERRUPTIBLE, 0, \
184 schedule())
185
186 #define swait_event_exclusive(wq, condition) \
187 do { \
188 if (condition) \
189 break; \
190 __swait_event(wq, condition); \
191 } while (0)
192
193 #define __swait_event_timeout(wq, condition, timeout) \
194 ___swait_event(wq, ___wait_cond_timeout(condition), \
195 TASK_UNINTERRUPTIBLE, timeout, \
196 __ret = schedule_timeout(__ret))
197
198 #define swait_event_timeout_exclusive(wq, condition, timeout) \
199 ({ \
200 long __ret = timeout; \
201 if (!___wait_cond_timeout(condition)) \
202 __ret = __swait_event_timeout(wq, condition, timeout); \
203 __ret; \
204 })
205
206 #define __swait_event_interruptible(wq, condition) \
207 ___swait_event(wq, condition, TASK_INTERRUPTIBLE, 0, \
208 schedule())
209
210 #define swait_event_interruptible_exclusive(wq, condition) \
211 ({ \
212 int __ret = 0; \
213 if (!(condition)) \
214 __ret = __swait_event_interruptible(wq, condition); \
215 __ret; \
216 })
217
218 #define __swait_event_interruptible_timeout(wq, condition, timeout) \
219 ___swait_event(wq, ___wait_cond_timeout(condition), \
220 TASK_INTERRUPTIBLE, timeout, \
221 __ret = schedule_timeout(__ret))
222
223 #define swait_event_interruptible_timeout_exclusive(wq, condition, timeout)\
224 ({ \
225 long __ret = timeout; \
226 if (!___wait_cond_timeout(condition)) \
227 __ret = __swait_event_interruptible_timeout(wq, \
228 condition, timeout); \
229 __ret; \
230 })
231
232 #define __swait_event_idle(wq, condition) \
233 (void)___swait_event(wq, condition, TASK_IDLE, 0, schedule())
234
235 /**
236 * swait_event_idle_exclusive - wait without system load contribution
237 * @wq: the waitqueue to wait on
238 * @condition: a C expression for the event to wait for
239 *
240 * The process is put to sleep (TASK_IDLE) until the @condition evaluates to
241 * true. The @condition is checked each time the waitqueue @wq is woken up.
242 *
243 * This function is mostly used when a kthread or workqueue waits for some
244 * condition and doesn't want to contribute to system load. Signals are
245 * ignored.
246 */
247 #define swait_event_idle_exclusive(wq, condition) \
248 do { \
249 if (condition) \
250 break; \
251 __swait_event_idle(wq, condition); \
252 } while (0)
253
254 #define __swait_event_idle_timeout(wq, condition, timeout) \
255 ___swait_event(wq, ___wait_cond_timeout(condition), \
256 TASK_IDLE, timeout, \
257 __ret = schedule_timeout(__ret))
258
259 /**
260 * swait_event_idle_timeout_exclusive - wait up to timeout without load contribution
261 * @wq: the waitqueue to wait on
262 * @condition: a C expression for the event to wait for
263 * @timeout: timeout at which we'll give up in jiffies
264 *
265 * The process is put to sleep (TASK_IDLE) until the @condition evaluates to
266 * true. The @condition is checked each time the waitqueue @wq is woken up.
267 *
268 * This function is mostly used when a kthread or workqueue waits for some
269 * condition and doesn't want to contribute to system load. Signals are
270 * ignored.
271 *
272 * Returns:
273 * 0 if the @condition evaluated to %false after the @timeout elapsed,
274 * 1 if the @condition evaluated to %true after the @timeout elapsed,
275 * or the remaining jiffies (at least 1) if the @condition evaluated
276 * to %true before the @timeout elapsed.
277 */
278 #define swait_event_idle_timeout_exclusive(wq, condition, timeout) \
279 ({ \
280 long __ret = timeout; \
281 if (!___wait_cond_timeout(condition)) \
282 __ret = __swait_event_idle_timeout(wq, \
283 condition, timeout); \
284 __ret; \
285 })
286
287 #endif /* _LINUX_SWAIT_H */
288
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