信号量在Linux中的实现





在Linux 3.13中，普通信号量的定义如下

/*  2  * Copyright (c) 2008 Intel Corporation  3  * Author: Matthew Wilcox <[e-mail]>  4  *  5  * Distributed under the terms of the GNU GPL, version 2  6  *  7  * Please see kernel/semaphore.c for documentation of these functions  8  */  9 #ifndef __LINUX_SEMAPHORE_H 10 #define __LINUX_SEMAPHORE_H 11  12 #include <linux/list.h> 13 #include <linux/spinlock.h> 14  15 /* Please don't access any members of this structure directly */ 16 struct semaphore { 17         raw_spinlock_t          lock; 18         unsigned int            count; 19         struct list_head        wait_list; 20 }; 21  22 #define __SEMAPHORE_INITIALIZER(name, n)                                \ 23 {                                                                       \ 24         .lock           = __RAW_SPIN_LOCK_UNLOCKED((name).lock),        \ 25         .count          = n,                                            \ 26         .wait_list      = LIST_HEAD_INIT((name).wait_list),             \ 27 } 28  29 #define DEFINE_SEMAPHORE(name)  \ 30         struct semaphore name = __SEMAPHORE_INITIALIZER(name, 1) 31  32 static inline void sema_init(struct semaphore *sem, int val) 33 { 34         static struct lock_class_key __key; 35         *sem = (struct semaphore) __SEMAPHORE_INITIALIZER(*sem, val); 36         lockdep_init_map(&sem->lock.dep_map, "semaphore->lock", &__key, 0); 37 } 38  39 extern void down(struct semaphore *sem); 40 extern int __must_check down_interruptible(struct semaphore *sem); 41 extern int __must_check down_killable(struct semaphore *sem); 42 extern int __must_check down_trylock(struct semaphore *sem); 43 extern int __must_check down_timeout(struct semaphore *sem, long jiffies); 44 extern void up(struct semaphore *sem); 45  46 #endif /* __LINUX_SEMAPHORE_H */




count<0, 其绝对值表示在等待队列中的进程个数；count>0，表示可以同时进入临界区域的进程个数。down操作对count减1，如果count<0，则进程进入等待队列；up操作对count加1，如果count>=0，这唤醒一个等待队列上的进程。

sleepers 对当前临界资源的进程个数的辅助计数。

 

在linux中对信号量的操作有up(),down()和down_interuptible()。down（）和down_interuptible唯一的区别，在等待过程中，down_interuptible()可以被其他信号中断，而down（）不可以。

up函数的调用次序是：先调用up_wakeup(),在调用__up()。而down（）函数的次序是：先调用down_failed(),再调用__down().

函数实现

/** 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         else110                 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 waiting119  * @sem: the semaphore to be acquired120  *121  * Try to acquire the semaphore atomically.  Returns 0 if the semaphore has122  * been acquired successfully or 1 if it it cannot be acquired.123  *124  * NOTE: This return value is inverted from both spin_trylock and125  * 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 time147  * @sem: the semaphore to be acquired148  * @jiffies: how long to wait before failing149  *150  * Attempts to acquire the semaphore.  If no more tasks are allowed to151  * 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         else164                 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 semaphore173  * @sem: the semaphore to release174  *175  * Release the semaphore.  Unlike mutexes, up() may be called from any176  * 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         else186                 __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 be201  * constant, and thus optimised away by the compiler.  Likewise the202  * '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 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 = true;262         wake_up_process(waiter->task);

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