xref: /linux/kernel/locking/semaphore.c (revision e5c86679d5e864947a52fb31e45a425dea3e7fa9)
1 /*
2  * Copyright (c) 2008 Intel Corporation
3  * Author: Matthew Wilcox <willy@linux.intel.com>
4  *
5  * Distributed under the terms of the GNU GPL, version 2
6  *
7  * This file implements counting semaphores.
8  * A counting semaphore may be acquired 'n' times before sleeping.
9  * See mutex.c for single-acquisition sleeping locks which enforce
10  * rules which allow code to be debugged more easily.
11  */
12 
13 /*
14  * Some notes on the implementation:
15  *
16  * The spinlock controls access to the other members of the semaphore.
17  * down_trylock() and up() can be called from interrupt context, so we
18  * have to disable interrupts when taking the lock.  It turns out various
19  * parts of the kernel expect to be able to use down() on a semaphore in
20  * interrupt context when they know it will succeed, so we have to use
21  * irqsave variants for down(), down_interruptible() and down_killable()
22  * too.
23  *
24  * The ->count variable represents how many more tasks can acquire this
25  * semaphore.  If it's zero, there may be tasks waiting on the wait_list.
26  */
27 
28 #include <linux/compiler.h>
29 #include <linux/kernel.h>
30 #include <linux/export.h>
31 #include <linux/sched.h>
32 #include <linux/sched/debug.h>
33 #include <linux/semaphore.h>
34 #include <linux/spinlock.h>
35 #include <linux/ftrace.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 down(struct semaphore *sem)
55 {
56 	unsigned long flags;
57 
58 	raw_spin_lock_irqsave(&sem->lock, flags);
59 	if (likely(sem->count > 0))
60 		sem->count--;
61 	else
62 		__down(sem);
63 	raw_spin_unlock_irqrestore(&sem->lock, flags);
64 }
65 EXPORT_SYMBOL(down);
66 
67 /**
68  * down_interruptible - acquire the semaphore unless interrupted
69  * @sem: the semaphore to be acquired
70  *
71  * Attempts to acquire the semaphore.  If no more tasks are allowed to
72  * acquire the semaphore, calling this function will put the task to sleep.
73  * If the sleep is interrupted by a signal, this function will return -EINTR.
74  * If the semaphore is successfully acquired, this function returns 0.
75  */
76 int down_interruptible(struct semaphore *sem)
77 {
78 	unsigned long flags;
79 	int result = 0;
80 
81 	raw_spin_lock_irqsave(&sem->lock, flags);
82 	if (likely(sem->count > 0))
83 		sem->count--;
84 	else
85 		result = __down_interruptible(sem);
86 	raw_spin_unlock_irqrestore(&sem->lock, flags);
87 
88 	return result;
89 }
90 EXPORT_SYMBOL(down_interruptible);
91 
92 /**
93  * down_killable - acquire the semaphore unless killed
94  * @sem: the semaphore to be acquired
95  *
96  * Attempts to acquire the semaphore.  If no more tasks are allowed to
97  * acquire the semaphore, calling this function will put the task to sleep.
98  * If the sleep is interrupted by a fatal signal, this function will return
99  * -EINTR.  If the semaphore is successfully acquired, this function returns
100  * 0.
101  */
102 int down_killable(struct semaphore *sem)
103 {
104 	unsigned long flags;
105 	int result = 0;
106 
107 	raw_spin_lock_irqsave(&sem->lock, flags);
108 	if (likely(sem->count > 0))
109 		sem->count--;
110 	else
111 		result = __down_killable(sem);
112 	raw_spin_unlock_irqrestore(&sem->lock, flags);
113 
114 	return result;
115 }
116 EXPORT_SYMBOL(down_killable);
117 
118 /**
119  * down_trylock - try to acquire the semaphore, without waiting
120  * @sem: the semaphore to be acquired
121  *
122  * Try to acquire the semaphore atomically.  Returns 0 if the semaphore has
123  * been acquired successfully or 1 if it it cannot be acquired.
124  *
125  * NOTE: This return value is inverted from both spin_trylock and
126  * mutex_trylock!  Be careful about this when converting code.
127  *
128  * Unlike mutex_trylock, this function can be used from interrupt context,
129  * and the semaphore can be released by any task or interrupt.
130  */
131 int down_trylock(struct semaphore *sem)
132 {
133 	unsigned long flags;
134 	int count;
135 
136 	raw_spin_lock_irqsave(&sem->lock, flags);
137 	count = sem->count - 1;
138 	if (likely(count >= 0))
139 		sem->count = count;
140 	raw_spin_unlock_irqrestore(&sem->lock, flags);
141 
142 	return (count < 0);
143 }
144 EXPORT_SYMBOL(down_trylock);
145 
146 /**
147  * down_timeout - acquire the semaphore within a specified time
148  * @sem: the semaphore to be acquired
149  * @timeout: how long to wait before failing
150  *
151  * Attempts to acquire the semaphore.  If no more tasks are allowed to
152  * acquire the semaphore, calling this function will put the task to sleep.
153  * If the semaphore is not released within the specified number of jiffies,
154  * this function returns -ETIME.  It returns 0 if the semaphore was acquired.
155  */
156 int down_timeout(struct semaphore *sem, long timeout)
157 {
158 	unsigned long flags;
159 	int result = 0;
160 
161 	raw_spin_lock_irqsave(&sem->lock, flags);
162 	if (likely(sem->count > 0))
163 		sem->count--;
164 	else
165 		result = __down_timeout(sem, timeout);
166 	raw_spin_unlock_irqrestore(&sem->lock, flags);
167 
168 	return result;
169 }
170 EXPORT_SYMBOL(down_timeout);
171 
172 /**
173  * up - release the semaphore
174  * @sem: the semaphore to release
175  *
176  * Release the semaphore.  Unlike mutexes, up() may be called from any
177  * context and even by tasks which have never called down().
178  */
179 void up(struct semaphore *sem)
180 {
181 	unsigned long flags;
182 
183 	raw_spin_lock_irqsave(&sem->lock, flags);
184 	if (likely(list_empty(&sem->wait_list)))
185 		sem->count++;
186 	else
187 		__up(sem);
188 	raw_spin_unlock_irqrestore(&sem->lock, flags);
189 }
190 EXPORT_SYMBOL(up);
191 
192 /* Functions for the contended case */
193 
194 struct semaphore_waiter {
195 	struct list_head list;
196 	struct task_struct *task;
197 	bool up;
198 };
199 
200 /*
201  * Because this function is inlined, the 'state' parameter will be
202  * constant, and thus optimised away by the compiler.  Likewise the
203  * 'timeout' parameter for the cases without timeouts.
204  */
205 static inline int __sched __down_common(struct semaphore *sem, long state,
206 								long timeout)
207 {
208 	struct semaphore_waiter waiter;
209 
210 	list_add_tail(&waiter.list, &sem->wait_list);
211 	waiter.task = current;
212 	waiter.up = false;
213 
214 	for (;;) {
215 		if (signal_pending_state(state, current))
216 			goto interrupted;
217 		if (unlikely(timeout <= 0))
218 			goto timed_out;
219 		__set_current_state(state);
220 		raw_spin_unlock_irq(&sem->lock);
221 		timeout = schedule_timeout(timeout);
222 		raw_spin_lock_irq(&sem->lock);
223 		if (waiter.up)
224 			return 0;
225 	}
226 
227  timed_out:
228 	list_del(&waiter.list);
229 	return -ETIME;
230 
231  interrupted:
232 	list_del(&waiter.list);
233 	return -EINTR;
234 }
235 
236 static noinline void __sched __down(struct semaphore *sem)
237 {
238 	__down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
239 }
240 
241 static noinline int __sched __down_interruptible(struct semaphore *sem)
242 {
243 	return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
244 }
245 
246 static noinline int __sched __down_killable(struct semaphore *sem)
247 {
248 	return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
249 }
250 
251 static noinline int __sched __down_timeout(struct semaphore *sem, long timeout)
252 {
253 	return __down_common(sem, TASK_UNINTERRUPTIBLE, timeout);
254 }
255 
256 static noinline void __sched __up(struct semaphore *sem)
257 {
258 	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
259 						struct semaphore_waiter, list);
260 	list_del(&waiter->list);
261 	waiter->up = true;
262 	wake_up_process(waiter->task);
263 }
264