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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 0 : static inline int swait_active(struct swait_queue_head *wq)
122 : {
123 0 : 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 0 : 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 0 : smp_mb();
144 0 : 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);
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 */
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