1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * Copyright (c) 2008 Intel Corporation
4 * Author: Matthew Wilcox <willy@linux.intel.com>
5 *
6 * This file implements counting semaphores.
7 * A counting semaphore may be acquired 'n' times before sleeping.
8 * See mutex.c for single-acquisition sleeping locks which enforce
9 * rules which allow code to be debugged more easily.
10 */
11
12/*
13 * Some notes on the implementation:
14 *
15 * The spinlock controls access to the other members of the semaphore.
16 * down_trylock() and up() can be called from interrupt context, so we
17 * have to disable interrupts when taking the lock. It turns out various
18 * parts of the kernel expect to be able to use down() on a semaphore in
19 * interrupt context when they know it will succeed, so we have to use
20 * irqsave variants for down(), down_interruptible() and down_killable()
21 * too.
22 *
23 * The ->count variable represents how many more tasks can acquire this
24 * semaphore. If it's zero, there may be tasks waiting on the wait_list.
25 */
26
27#include <linux/compiler.h>
28#include <linux/kernel.h>
29#include <linux/export.h>
30#include <linux/sched.h>
31#include <linux/sched/debug.h>
32#include <linux/sched/wake_q.h>
33#include <linux/semaphore.h>
34#include <linux/spinlock.h>
35#include <linux/ftrace.h>
36#include <trace/events/lock.h>
37#include <linux/hung_task.h>
38
39static noinline void __down(struct semaphore *sem);
40static noinline int __down_interruptible(struct semaphore *sem);
41static noinline int __down_killable(struct semaphore *sem);
42static noinline int __down_timeout(struct semaphore *sem, long timeout);
43static noinline void __up(struct semaphore *sem, struct wake_q_head *wake_q);
44
45#ifdef CONFIG_DETECT_HUNG_TASK_BLOCKER
46static inline void hung_task_sem_set_holder(struct semaphore *sem)
47{
48 WRITE_ONCE((sem)->last_holder, (unsigned long)current);
49}
50
51static inline void hung_task_sem_clear_if_holder(struct semaphore *sem)
52{
53 if (READ_ONCE((sem)->last_holder) == (unsigned long)current)
54 WRITE_ONCE((sem)->last_holder, 0UL);
55}
56
57unsigned long sem_last_holder(struct semaphore *sem)
58{
59 return READ_ONCE(sem->last_holder);
60}
61#else
62static inline void hung_task_sem_set_holder(struct semaphore *sem)
63{
64}
65static inline void hung_task_sem_clear_if_holder(struct semaphore *sem)
66{
67}
68unsigned long sem_last_holder(struct semaphore *sem)
69{
70 return 0UL;
71}
72#endif
73
74static inline void __sem_acquire(struct semaphore *sem)
75{
76 sem->count--;
77 hung_task_sem_set_holder(sem);
78}
79
80/**
81 * down - acquire the semaphore
82 * @sem: the semaphore to be acquired
83 *
84 * Acquires the semaphore. If no more tasks are allowed to acquire the
85 * semaphore, calling this function will put the task to sleep until the
86 * semaphore is released.
87 *
88 * Use of this function is deprecated, please use down_interruptible() or
89 * down_killable() instead.
90 */
91void __sched down(struct semaphore *sem)
92{
93 unsigned long flags;
94
95 might_sleep();
96 raw_spin_lock_irqsave(&sem->lock, flags);
97 if (likely(sem->count > 0))
98 __sem_acquire(sem);
99 else
100 __down(sem);
101 raw_spin_unlock_irqrestore(&sem->lock, flags);
102}
103EXPORT_SYMBOL(down);
104
105/**
106 * down_interruptible - acquire the semaphore unless interrupted
107 * @sem: the semaphore to be acquired
108 *
109 * Attempts to acquire the semaphore. If no more tasks are allowed to
110 * acquire the semaphore, calling this function will put the task to sleep.
111 * If the sleep is interrupted by a signal, this function will return -EINTR.
112 * If the semaphore is successfully acquired, this function returns 0.
113 */
114int __sched down_interruptible(struct semaphore *sem)
115{
116 unsigned long flags;
117 int result = 0;
118
119 might_sleep();
120 raw_spin_lock_irqsave(&sem->lock, flags);
121 if (likely(sem->count > 0))
122 __sem_acquire(sem);
123 else
124 result = __down_interruptible(sem);
125 raw_spin_unlock_irqrestore(&sem->lock, flags);
126
127 return result;
128}
129EXPORT_SYMBOL(down_interruptible);
130
131/**
132 * down_killable - acquire the semaphore unless killed
133 * @sem: the semaphore to be acquired
134 *
135 * Attempts to acquire the semaphore. If no more tasks are allowed to
136 * acquire the semaphore, calling this function will put the task to sleep.
137 * If the sleep is interrupted by a fatal signal, this function will return
138 * -EINTR. If the semaphore is successfully acquired, this function returns
139 * 0.
140 */
141int __sched down_killable(struct semaphore *sem)
142{
143 unsigned long flags;
144 int result = 0;
145
146 might_sleep();
147 raw_spin_lock_irqsave(&sem->lock, flags);
148 if (likely(sem->count > 0))
149 __sem_acquire(sem);
150 else
151 result = __down_killable(sem);
152 raw_spin_unlock_irqrestore(&sem->lock, flags);
153
154 return result;
155}
156EXPORT_SYMBOL(down_killable);
157
158/**
159 * down_trylock - try to acquire the semaphore, without waiting
160 * @sem: the semaphore to be acquired
161 *
162 * Try to acquire the semaphore atomically. Returns 0 if the semaphore has
163 * been acquired successfully or 1 if it cannot be acquired.
164 *
165 * NOTE: This return value is inverted from both spin_trylock and
166 * mutex_trylock! Be careful about this when converting code.
167 *
168 * Unlike mutex_trylock, this function can be used from interrupt context,
169 * and the semaphore can be released by any task or interrupt.
170 */
171int __sched down_trylock(struct semaphore *sem)
172{
173 unsigned long flags;
174 int count;
175
176 raw_spin_lock_irqsave(&sem->lock, flags);
177 count = sem->count - 1;
178 if (likely(count >= 0))
179 __sem_acquire(sem);
180 raw_spin_unlock_irqrestore(&sem->lock, flags);
181
182 return (count < 0);
183}
184EXPORT_SYMBOL(down_trylock);
185
186/**
187 * down_timeout - acquire the semaphore within a specified time
188 * @sem: the semaphore to be acquired
189 * @timeout: how long to wait before failing
190 *
191 * Attempts to acquire the semaphore. If no more tasks are allowed to
192 * acquire the semaphore, calling this function will put the task to sleep.
193 * If the semaphore is not released within the specified number of jiffies,
194 * this function returns -ETIME. It returns 0 if the semaphore was acquired.
195 */
196int __sched down_timeout(struct semaphore *sem, long timeout)
197{
198 unsigned long flags;
199 int result = 0;
200
201 might_sleep();
202 raw_spin_lock_irqsave(&sem->lock, flags);
203 if (likely(sem->count > 0))
204 __sem_acquire(sem);
205 else
206 result = __down_timeout(sem, timeout);
207 raw_spin_unlock_irqrestore(&sem->lock, flags);
208
209 return result;
210}
211EXPORT_SYMBOL(down_timeout);
212
213/**
214 * up - release the semaphore
215 * @sem: the semaphore to release
216 *
217 * Release the semaphore. Unlike mutexes, up() may be called from any
218 * context and even by tasks which have never called down().
219 */
220void __sched up(struct semaphore *sem)
221{
222 unsigned long flags;
223 DEFINE_WAKE_Q(wake_q);
224
225 raw_spin_lock_irqsave(&sem->lock, flags);
226
227 hung_task_sem_clear_if_holder(sem);
228
229 if (likely(list_empty(&sem->wait_list)))
230 sem->count++;
231 else
232 __up(sem, wake_q: &wake_q);
233 raw_spin_unlock_irqrestore(&sem->lock, flags);
234 if (!wake_q_empty(head: &wake_q))
235 wake_up_q(head: &wake_q);
236}
237EXPORT_SYMBOL(up);
238
239/* Functions for the contended case */
240
241struct semaphore_waiter {
242 struct list_head list;
243 struct task_struct *task;
244 bool up;
245};
246
247/*
248 * Because this function is inlined, the 'state' parameter will be
249 * constant, and thus optimised away by the compiler. Likewise the
250 * 'timeout' parameter for the cases without timeouts.
251 */
252static inline int __sched ___down_common(struct semaphore *sem, long state,
253 long timeout)
254{
255 struct semaphore_waiter waiter;
256
257 list_add_tail(new: &waiter.list, head: &sem->wait_list);
258 waiter.task = current;
259 waiter.up = false;
260
261 for (;;) {
262 if (signal_pending_state(state, current))
263 goto interrupted;
264 if (unlikely(timeout <= 0))
265 goto timed_out;
266 __set_current_state(state);
267 raw_spin_unlock_irq(&sem->lock);
268 timeout = schedule_timeout(timeout);
269 raw_spin_lock_irq(&sem->lock);
270 if (waiter.up) {
271 hung_task_sem_set_holder(sem);
272 return 0;
273 }
274 }
275
276 timed_out:
277 list_del(entry: &waiter.list);
278 return -ETIME;
279
280 interrupted:
281 list_del(entry: &waiter.list);
282 return -EINTR;
283}
284
285static inline int __sched __down_common(struct semaphore *sem, long state,
286 long timeout)
287{
288 int ret;
289
290 hung_task_set_blocker(lock: sem, BLOCKER_TYPE_SEM);
291
292 trace_contention_begin(lock: sem, flags: 0);
293 ret = ___down_common(sem, state, timeout);
294 trace_contention_end(lock: sem, ret);
295
296 hung_task_clear_blocker();
297
298 return ret;
299}
300
301static noinline void __sched __down(struct semaphore *sem)
302{
303 __down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
304}
305
306static noinline int __sched __down_interruptible(struct semaphore *sem)
307{
308 return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
309}
310
311static noinline int __sched __down_killable(struct semaphore *sem)
312{
313 return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
314}
315
316static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
317{
318 return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
319}
320
321static noinline void __sched __up(struct semaphore *sem,
322 struct wake_q_head *wake_q)
323{
324 struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
325 struct semaphore_waiter, list);
326 list_del(entry: &waiter->list);
327 waiter->up = true;
328 wake_q_add(head: wake_q, task: waiter->task);
329}
330

source code of linux/kernel/locking/semaphore.c