swait.h

  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
 41struct task_struct;
 42
 43struct swait_queue_head {
 44	raw_spinlock_t		lock;
 45	struct list_head	task_list;
 46};
 47
 48struct 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
 69extern 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 */
121static 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 */
134static 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
147extern void swake_up_one(struct swait_queue_head *q);
148extern void swake_up_all(struct swait_queue_head *q);
149extern void swake_up_locked(struct swait_queue_head *q, int wake_flags);
150
151extern void prepare_to_swait_exclusive(struct swait_queue_head *q, struct swait_queue *wait, int state);
152extern long prepare_to_swait_event(struct swait_queue_head *q, struct swait_queue *wait, int state);
153
154extern void __finish_swait(struct swait_queue_head *q, struct swait_queue *wait);
155extern 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)				\
187do {									\
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)			\
248do {									\
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 */