mutex即互斥，用于控制多线程间同步、互斥访问资源。
相关的结构体。
代码语言：javascript复制/* Mutexes (not abstract because of PTHREAD_MUTEX_INITIALIZER). */
typedef struct
{
// 自旋锁
int m_spinlock; /* Spin lock to guarantee mutual exclusion. */
// 用于递归加锁，即某个线程多次获取了该互斥变量。m_count记录了次数
int m_count; /* 0 if free, > 0 if taken. */
// 记录谁获取了该互斥变量，在递归加锁的时候会使用这个字段
pthread_t m_owner; /* Owner of mutex (for recursive mutexes) */
// 互斥变量的类型，递归或非递归
int m_kind; /* Kind of mutex */
// 等待该互斥变量的线程队列
struct _pthread_queue m_waiting; /* Threads waiting on this mutex. */
} pthread_mutex_t;
// 初始化互斥变量，类型是递归或非递归
#define PTHREAD_MUTEX_INITIALIZER
{0, 0, 0, PTHREAD_MUTEX_FAST_NP, {0, 0}}
#define PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP
{0, 0, 0, PTHREAD_MUTEX_RECURSIVE_NP, {0, 0}}下面是实现的代码。
代码语言：javascript复制/* Linuxthreads - a simple clone()-based implementation of Posix */
/* threads for Linux. */
/* Copyright (C) 1996 Xavier Leroy (Xavier.Leroy@inria.fr) */
/* */
/* This program is free software; you can redistribute it and/or */
/* modify it under the terms of the GNU Library General Public License */
/* as published by the Free Software Foundation; either version 2 */
/* of the License, or (at your option) any later version. */
/* */
/* This program is distributed in the hope that it will be useful, */
/* but WITHOUT ANY WARRANTY; without even the implied warranty of */
/* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the */
/* GNU Library General Public License for more details. */
/* Mutexes */
#include <errno.h>
#include <sched.h>
#include <stddef.h>
#include "pthread.h"
#include "internals.h"
#include "spinlock.h"
#include "queue.h"
#include "restart.h"
// 利用属性结构体初始化mutex节点
int __pthread_mutex_init(pthread_mutex_t * mutex,
const pthread_mutexattr_t * mutex_attr)
{
mutex->m_spinlock = 0;
mutex->m_count = 0;
mutex->m_owner = NULL;
mutex->m_kind =
mutex_attr == NULL ? PTHREAD_MUTEX_FAST_NP : mutex_attr->mutexkind;
queue_init(&mutex->m_waiting);
return 0;
}
weak_alias (__pthread_mutex_init, pthread_mutex_init)
// 销毁互斥锁
int __pthread_mutex_destroy(pthread_mutex_t * mutex)
{
int count;
acquire(&mutex->m_spinlock);
count = mutex->m_count;
release(&mutex->m_spinlock);
// 正在被使用
if (count > 0) return EBUSY;
return 0;
}
weak_alias (__pthread_mutex_destroy, pthread_mutex_destroy)
// 非阻塞式获取锁
int __pthread_mutex_trylock(pthread_mutex_t * mutex)
{
pthread_t self;
acquire(&mutex->m_spinlock);
switch(mutex->m_kind) {
case PTHREAD_MUTEX_FAST_NP:
// 还没有被使用，则使用数加一，返回成功
if (mutex->m_count == 0) {
mutex->m_count = 1;
release(&mutex->m_spinlock);
return 0;
}
break;
// 递归获取互斥变量
case PTHREAD_MUTEX_RECURSIVE_NP:
self = thread_self();
// 等于0则说明还没有被获取过，可以直接获取，或者已经被当前线程获取了，则次数加一
if (mutex->m_count == 0 || mutex->m_owner == self) {
mutex->m_count ;
mutex->m_owner = self;
release(&mutex->m_spinlock);
return 0;
}
break;
default:
return EINVAL;
}
release(&mutex->m_spinlock);
return EBUSY;
}
weak_alias (__pthread_mutex_trylock, pthread_mutex_trylock)
// 阻塞式获取互斥变量
int __pthread_mutex_lock(pthread_mutex_t * mutex)
{
pthread_t self;
while(1) {
acquire(&mutex->m_spinlock);
switch(mutex->m_kind) {
case PTHREAD_MUTEX_FAST_NP:
if (mutex->m_count == 0) {
mutex->m_count = 1;
release(&mutex->m_spinlock);
return 0;
}
self = thread_self();
break;
case PTHREAD_MUTEX_RECURSIVE_NP:
self = thread_self();
// 等于0或者本线程已经获得过该互斥锁，则可以重复获得，m_count累加
if (mutex->m_count == 0 || mutex->m_owner == self) {
mutex->m_count ;
// 标记该互斥锁已经被本线程获取
mutex->m_owner = self;
release(&mutex->m_spinlock);
return 0;
}
break;
default:
return EINVAL;
}
/* Suspend ourselves, then try again */
// 获取失败，需要阻塞，把当前线程插入该互斥锁的等待队列
enqueue(&mutex->m_waiting, self);
release(&mutex->m_spinlock);
// 挂起等待唤醒
suspend(self); /* This is not a cancellation point */
}
}
weak_alias (__pthread_mutex_lock, pthread_mutex_lock)
int __pthread_mutex_unlock(pthread_mutex_t * mutex)
{
pthread_t th;
acquire(&mutex->m_spinlock);
switch (mutex->m_kind) {
case PTHREAD_MUTEX_FAST_NP:
mutex->m_count = 0;
break;
case PTHREAD_MUTEX_RECURSIVE_NP:
mutex->m_count--;
if (mutex->m_count > 0) {
release(&mutex->m_spinlock);
return 0;
}
mutex->m_count = 0; /* so that excess unlocks do not break everything */
break;
default:
return EINVAL;
}
// 取出一个被阻塞的线程（如果有的话），唤醒他
th = dequeue(&mutex->m_waiting);
release(&mutex->m_spinlock);
if (th != NULL) restart(th);
return 0;
}
weak_alias (__pthread_mutex_unlock, pthread_mutex_unlock)
int __pthread_mutexattr_init(pthread_mutexattr_t *attr)
{
attr->mutexkind = PTHREAD_MUTEX_FAST_NP;
return 0;
}
weak_alias (__pthread_mutexattr_init, pthread_mutexattr_init)
int __pthread_mutexattr_destroy(pthread_mutexattr_t *attr)
{
return 0;
}
weak_alias (__pthread_mutexattr_destroy, pthread_mutexattr_destroy)
int __pthread_mutexattr_setkind_np(pthread_mutexattr_t *attr, int kind)
{
if (kind != PTHREAD_MUTEX_FAST_NP && kind != PTHREAD_MUTEX_RECURSIVE_NP)
return EINVAL;
attr->mutexkind = kind;
return 0;
}
weak_alias (__pthread_mutexattr_setkind_np, pthread_mutexattr_setkind_np)
int __pthread_mutexattr_getkind_np(const pthread_mutexattr_t *attr, int *kind)
{
*kind = attr->mutexkind;
return 0;
}
weak_alias (__pthread_mutexattr_getkind_np, pthread_mutexattr_getkind_np)
// 保存init_routine只执行一次
int pthread_once(pthread_once_t * once_control, void (*init_routine)(void))
{
if (testandset(once_control) == 0) init_routine();
return 0;
}
