Root/kernel/semaphore.c

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
36static noinline void __down(struct semaphore *sem);
37static noinline int __down_interruptible(struct semaphore *sem);
38static noinline int __down_killable(struct semaphore *sem);
39static noinline int __down_timeout(struct semaphore *sem, long jiffies);
40static 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 */
53void 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}
64EXPORT_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 */
75int 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}
89EXPORT_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 */
101int 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}
115EXPORT_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 */
130int 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}
143EXPORT_SYMBOL(down_trylock);
144
145/**
146 * down_timeout - acquire the semaphore within a specified time
147 * @sem: the semaphore to be acquired
148 * @jiffies: 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 */
155int down_timeout(struct semaphore *sem, long jiffies)
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, jiffies);
165    raw_spin_unlock_irqrestore(&sem->lock, flags);
166
167    return result;
168}
169EXPORT_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 */
178void 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}
189EXPORT_SYMBOL(up);
190
191/* Functions for the contended case */
192
193struct semaphore_waiter {
194    struct list_head list;
195    struct task_struct *task;
196    int 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 */
204static 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 = 0;
213
214    for (;;) {
215        if (signal_pending_state(state, task))
216            goto interrupted;
217        if (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
236static noinline void __sched __down(struct semaphore *sem)
237{
238    __down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
239}
240
241static noinline int __sched __down_interruptible(struct semaphore *sem)
242{
243    return __down_common(sem, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
244}
245
246static noinline int __sched __down_killable(struct semaphore *sem)
247{
248    return __down_common(sem, TASK_KILLABLE, MAX_SCHEDULE_TIMEOUT);
249}
250
251static noinline int __sched __down_timeout(struct semaphore *sem, long jiffies)
252{
253    return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);
254}
255
256static 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 = 1;
262    wake_up_process(waiter->task);
263}
264

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