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  */
swait_active(struct swait_queue_head * wq)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  */
swq_has_sleeper(struct swait_queue_head * wq)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);
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