linux多线程之自旋锁

小鱼儿 2022-09-25 05:22 239阅读 0赞

**基本概念：**

何谓自旋锁？它是为实现保护共享资源而提出一种锁机制。其实，自旋锁与互斥锁比较类似，它们都是为了解决对某项资源的互斥使用。无论是互斥锁，还是自旋锁，在任何时刻，最多只能有一个保持者，也就说，在任何时刻最多只能有一个执行单元获得锁。但是两者在调度机制上略有不同。对于互斥锁，如果资源已经被占用，资源申请者只能进入睡眠状态。但是自旋锁不会引起调用者睡眠，如果自旋锁已经被别的执行单元保持，调用者就一直循环在那里看是否该自旋锁的保持者已经释放了锁，"自旋"一词就是因此而得名。  
  
  
**一、初始化和销毁锁**

PTHREAD_SPIN_DESTROY(P)    POSIX Programmer's Manual   PTHREAD_SPIN_DESTROY(P)
    
    NAME
           pthread_spin_destroy,  pthread_spin_init - destroy or initialize a spin
           lock object (ADVANCED REALTIME THREADS)
    
    SYNOPSIS
           #include <pthread.h>
    
           int pthread_spin_destroy(pthread_spinlock_t *lock);
           int pthread_spin_init(pthread_spinlock_t *lock, int pshared);

**两个函数的返回值：若成功，返回0；否则，返回错误编号**  
关于pthread\_spin\_init函数的参数pshard，单进程可以设置成PTHREAD\_PROCESS\_SHARED

**二、加锁与解锁**

PTHREAD_SPIN_LOCK(P)       POSIX Programmer's Manual      PTHREAD_SPIN_LOCK(P)
    
    NAME
           pthread_spin_lock,  pthread_spin_trylock  -  lock  a  spin  lock object
           (ADVANCED REALTIME THREADS)
    
    SYNOPSIS
           #include <pthread.h>
    
           int pthread_spin_lock(pthread_spinlock_t *lock);
           int pthread_spin_trylock(pthread_spinlock_t *lock);
           int pthread_spin_unlock(pthread_spinlock_t *lock);

**两个函数的返回值：若成功，返回0；否则，返回错误编号**

**例子1，互斥锁的耗时， gcc pthread\_mutex.c -pthread -o mutex：**

#include <pthread.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <errno.h>
    #include <sys/timeb.h>
    
    static int num = 0;
    static int count = 10000000;
    static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    
    void Perror(const char *s)
    {
        perror(s);
        exit(EXIT_FAILURE);
    }
    
    long long getSystemTime() {
        struct timeb t;
        ftime(&t);
        return 1000 * t.time + t.millitm;
    }
    
    void* fun2(void *arg)
    {
        pthread_t thread_id = pthread_self();
        printf("the thread2 id is %ld\n", (long)thread_id);
        int i = 1;
        for (; i<=count; ++i) {
            pthread_mutex_lock(&mutex);
            num += 1;
            pthread_mutex_unlock(&mutex);
        }
    }
    
    int main()
    {
        int err;
        pthread_t thread1;
        pthread_t thread2;
    
        thread1 = pthread_self();
        printf("the thread1 id is %ld\n", (long)thread1);
    
        long long start = getSystemTime();
    
        // Create thread
        err = pthread_create(&thread2, NULL, fun2, NULL);
        if (err != 0) {
            Perror("can't create thread2\n");
        }
    
        int i = 1;
        for (; i<=count; ++i) {
            pthread_mutex_lock(&mutex);
            num += 1;
            pthread_mutex_unlock(&mutex);
        }
    
        pthread_join(thread2, NULL);
        long long end = getSystemTime();
    
        printf("The num is %d, pay %lld ms\n", num, (end-start));
    
        return 0;
    }

**例子2，自旋锁的耗时，gcc pthread\_spin.c -pthread -o spin：**

#include <pthread.h>
    #include <stdio.h>
    #include <unistd.h>
    #include <stdlib.h>
    #include <errno.h>
    #include <sys/timeb.h>
    
    static int num = 0;
    static int count = 10000000;
    static pthread_spinlock_t spin;
    
    void Perror(const char *s)
    {
        perror(s);
        exit(EXIT_FAILURE);
    }
    
    long long getSystemTime() {
        struct timeb t;
        ftime(&t);
        return 1000 * t.time + t.millitm;
    }
    
    void* fun2(void *arg)
    {
        pthread_t thread_id = pthread_self();
        printf("the thread2 id is %ld\n", (long)thread_id);
        int i = 1;
        for (; i<=count; ++i) {
            pthread_spin_lock(&spin);
            num += 1;
            pthread_spin_unlock(&spin);
        }
    }
    
    int main()
    {
        int err;
        pthread_t thread1;
        pthread_t thread2;
    
        pthread_spin_init(&spin, PTHREAD_PROCESS_PRIVATE);
    
        thread1 = pthread_self();
        printf("the thread1 id is %ld\n", (long)thread1);
    
        long long start = getSystemTime();
    
        // Create thread
        err = pthread_create(&thread2, NULL, fun2, NULL);
        if (err != 0) {
            Perror("can't create thread2\n");
        }
    
        int i = 1;
        for (; i<=count; ++i) {
            pthread_spin_lock(&spin);
            num += 1;
            pthread_spin_unlock(&spin);
        }
    
        pthread_join(thread2, NULL);
        long long end = getSystemTime();
    
        printf("The num is %d, pay %lld ms\n", num, (end-start));
        pthread_spin_destroy(&spin);
        return 0;
    }

**运行结果，可以看到某些条件下，自旋锁是比较快的：**

![20160814001658157][]

参考：《unix环境高级编程》·第三版

End;

[20160814001658157]: /images/20220718/0ec8ca01d1044b6f83cb619090b0db7a.png