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/semaphore.h>
33 #include <linux/spinlock.h>
34 #include <linux/ftrace.h>
35 #include <trace/events/lock.h>
36
37 static noinline void __down(struct semaphore *sem);
38 static noinline int __down_interruptible(struct semaphore *sem);
39 static noinline int __down_killable(struct semaphore *sem);
40 static noinline int __down_timeout(struct semaphore *sem, long timeout);
41 static noinline void __up(struct semaphore *sem);
42
43 /**
44 * down - acquire the semaphore
45 * @sem: the semaphore to be acquired
46 *
47 * Acquires the semaphore. If no more tasks are allowed to acquire the
48 * semaphore, calling this function will put the task to sleep until the
49 * semaphore is released.
50 *
51 * Use of this function is deprecated, please use down_interruptible() or
52 * down_killable() instead.
53 */
54 void __sched down(struct semaphore *sem)
55 {
56 unsigned long flags;
57
58 might_sleep();
59 raw_spin_lock_irqsave(&sem->lock, flags);
60 if (likely(sem->count > 0))
61 sem->count--;
62 else
63 __down(sem);
64 raw_spin_unlock_irqrestore(&sem->lock, flags);
65 }
66 EXPORT_SYMBOL(down);
67
68 /**
69 * down_interruptible - acquire the semaphore unless interrupted
70 * @sem: the semaphore to be acquired
71 *
72 * Attempts to acquire the semaphore. If no more tasks are allowed to
73 * acquire the semaphore, calling this function will put the task to sleep.
74 * If the sleep is interrupted by a signal, this function will return -EINTR.
75 * If the semaphore is successfully acquired, this function returns 0.
76 */
77 int __sched down_interruptible(struct semaphore *sem)
78 {
79 unsigned long flags;
80 int result = 0;
81
82 might_sleep();
83 raw_spin_lock_irqsave(&sem->lock, flags);
84 if (likely(sem->count > 0))
85 sem->count--;
86 else
87 result = __down_interruptible(sem);
88 raw_spin_unlock_irqrestore(&sem->lock, flags);
89
90 return result;
91 }
92 EXPORT_SYMBOL(down_interruptible);
93
94 /**
95 * down_killable - acquire the semaphore unless killed
96 * @sem: the semaphore to be acquired
97 *
98 * Attempts to acquire the semaphore. If no more tasks are allowed to
99 * acquire the semaphore, calling this function will put the task to sleep.
100 * If the sleep is interrupted by a fatal signal, this function will return
101 * -EINTR. If the semaphore is successfully acquired, this function returns
102 * 0.
103 */
104 int __sched down_killable(struct semaphore *sem)
105 {
106 unsigned long flags;
107 int result = 0;
108
109 might_sleep();
110 raw_spin_lock_irqsave(&sem->lock, flags);
111 if (likely(sem->count > 0))
112 sem->count--;
113 else
114 result = __down_killable(sem);
115 raw_spin_unlock_irqrestore(&sem->lock, flags);
116
117 return result;
118 }
119 EXPORT_SYMBOL(down_killable);
120
121 /**
122 * down_trylock - try to acquire the semaphore, without waiting
123 * @sem: the semaphore to be acquired
124 *
125 * Try to acquire the semaphore atomically. Returns 0 if the semaphore has
126 * been acquired successfully or 1 if it cannot be acquired.
127 *
128 * NOTE: This return value is inverted from both spin_trylock and
129 * mutex_trylock! Be careful about this when converting code.
130 *
131 * Unlike mutex_trylock, this function can be used from interrupt context,
132 * and the semaphore can be released by any task or interrupt.
133 */
134 int __sched down_trylock(struct semaphore *sem)
135 {
136 unsigned long flags;
137 int count;
138
139 raw_spin_lock_irqsave(&sem->lock, flags);
140 count = sem->count - 1;
141 if (likely(count >= 0))
142 sem->count = count;
143 raw_spin_unlock_irqrestore(&sem->lock, flags);
144
145 return (count < 0);
146 }
147 EXPORT_SYMBOL(down_trylock);
148
149 /**
150 * down_timeout - acquire the semaphore within a specified time
151 * @sem: the semaphore to be acquired
152 * @timeout: how long to wait before failing
153 *
154 * Attempts to acquire the semaphore. If no more tasks are allowed to
155 * acquire the semaphore, calling this function will put the task to sleep.
156 * If the semaphore is not released within the specified number of jiffies,
157 * this function returns -ETIME. It returns 0 if the semaphore was acquired.
158 */
159 int __sched down_timeout(struct semaphore *sem, long timeout)
160 {
161 unsigned long flags;
162 int result = 0;
163
164 might_sleep();
165 raw_spin_lock_irqsave(&sem->lock, flags);
166 if (likely(sem->count > 0))
167 sem->count--;
168 else
169 result = __down_timeout(sem, timeout);
170 raw_spin_unlock_irqrestore(&sem->lock, flags);
171
172 return result;
173 }
174 EXPORT_SYMBOL(down_timeout);
175
176 /**
177 * up - release the semaphore
178 * @sem: the semaphore to release
179 *
180 * Release the semaphore. Unlike mutexes, up() may be called from any
181 * context and even by tasks which have never called down().
182 */
183 void __sched up(struct semaphore *sem)
184 {
185 unsigned long flags;
186
187 raw_spin_lock_irqsave(&sem->lock, flags);
188 if (likely(list_empty(&sem->wait_list)))
189 sem->count++;
190 else
191 __up(sem);
192 raw_spin_unlock_irqrestore(&sem->lock, flags);
193 }
194 EXPORT_SYMBOL(up);
195
196 /* Functions for the contended case */
197
198 struct semaphore_waiter {
199 struct list_head list;
200 struct task_struct *task;
201 bool up;
202 };
203
204 /*
205 * Because this function is inlined, the 'state' parameter will be
206 * constant, and thus optimised away by the compiler. Likewise the
207 * 'timeout' parameter for the cases without timeouts.
208 */
209 static inline int __sched ___down_common(struct semaphore *sem, long state,
210 long timeout)
211 {
212 struct semaphore_waiter waiter;
213
214 list_add_tail(&waiter.list, &sem->wait_list);
215 waiter.task = current;
216 waiter.up = false;
217
218 for (;;) {
219 if (signal_pending_state(state, current))
220 goto interrupted;
221 if (unlikely(timeout <= 0))
222 goto timed_out;
223 __set_current_state(state);
224 raw_spin_unlock_irq(&sem->lock);
225 timeout = schedule_timeout(timeout);
226 raw_spin_lock_irq(&sem->lock);
227 if (waiter.up)
228 return 0;
229 }
230
231 timed_out:
232 list_del(&waiter.list);
233 return -ETIME;
234
235 interrupted:
236 list_del(&waiter.list);
237 return -EINTR;
238 }
239
240 static inline int __sched __down_common(struct semaphore *sem, long state,
241 long timeout)
242 {
243 int ret;
244
245 trace_contention_begin(sem, 0);
246 ret = ___down_common(sem, state, timeout);
247 trace_contention_end(sem, ret);
248
249 return ret;
250 }
251
252 static noinline void __sched __down(struct semaphore *sem)
253 {
254 __down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
255 }
256
257 static noinline int __sched __down_interruptible(struct semaphore *sem)
258 {
259 return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
260 }
261
262 static noinline int __sched __down_killable(struct semaphore *sem)
263 {
264 return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
265 }
266
267 static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
268 {
269 return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
270 }
271
272 static noinline void __sched __up(struct semaphore *sem)
273 {
274 struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
275 struct semaphore_waiter, list);
276 list_del(&waiter->list);
277 waiter->up = true;
278 wake_up_process(waiter->task);
279 }
280
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