linux多线程之自旋锁

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：

参考：《unix环境高级编程》·第三版

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