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