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Linux/kernel/semaphore.c

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  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 jiffies);
 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  * @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  */
155 int 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 }
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         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  */
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 = 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 
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 jiffies)
252 {
253         return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);
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 = 1;
262         wake_up_process(waiter->task);
263 }
264 

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