OpenHarmony 内核源码分析(用户态锁篇) | 如何使用快锁Futex(上)

//futex_demo.c
#define _GNU_SOURCE
#include <stdio.h>
#include <errno.h>
#include <stdatomic.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <sys/time.h>
#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                        } while (0)
static uint32_t *futex1, *futex2, *iaddr;
/// 快速系统调用
static int futex(uint32_t *uaddr, int futex_op, uint32_t val,
        const struct timespec *timeout, uint32_t *uaddr2, uint32_t val3)
{
    return syscall(SYS_futex, uaddr, futex_op, val,
                    timeout, uaddr2, val3);
}
/// 申请快锁
static void fwait(uint32_t *futexp)
{
    long s;
    while (1) {
        const uint32_t one = 1;
        if (atomic_compare_exchange_strong(futexp, &one, 0))
            break; //申请快锁成功
        //申请快锁失败,需等待
        s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
        if (s == -1 && errno != EAGAIN)
            errExit("futex-FUTEX_WAIT");
    }
}
/// 释放快锁
static void fpost(uint32_t *futexp)
{
    long s;
    const uint32_t zero = 0;
    if (atomic_compare_exchange_strong(futexp, &zero, 1)) {//释放快锁成功
        s = futex(futexp, FUTEX_WAKE, 1, NULL, NULL, 0);//唤醒等锁 进程/线程
        if (s  == -1)
            errExit("futex-FUTEX_WAKE");
    }
}
/// 父子进程竞争快锁
int main(int argc, char *argv[])
{
    pid_t childPid;
    int nloops;
    setbuf(stdout, NULL);
    nloops = (argc > 1) ? atoi(argv[1]) : 3;
    iaddr = mmap(NULL, sizeof(*iaddr) * 2, PROT_READ | PROT_WRITE,
                MAP_ANONYMOUS | MAP_SHARED, -1, 0);//创建可读可写匿名共享内存
    if (iaddr == MAP_FAILED)
        errExit("mmap");
    futex1 = &iaddr[0]; //绑定锁一地址
    futex2 = &iaddr[1]; //绑定锁二地址
    *futex1 = 0; // 锁一不可申请 
    *futex2 = 1; // 锁二可申请
    childPid = fork();
    if (childPid == -1)
        errExit("fork");
    if (childPid == 0) {//子进程返回
        for (int j = 0; j < nloops; j++) {
            fwait(futex1);//申请锁一
            printf("子进程  (%jd) %d\n", (intmax_t) getpid(), j);
            fpost(futex2);//释放锁二
        }
        exit(EXIT_SUCCESS);
    }
    // 父进程返回执行
    for (int j = 0; j < nloops; j++) {
        fwait(futex2);//申请锁二
        printf("父进程 (%jd) %d\n", (intmax_t) getpid(), j);
        fpost(futex1);//释放锁一
    }
    wait(NULL);
    exit(EXIT_SUCCESS);
}

root@DESKTOP-5PBPDNG:/home/turing# gcc ./futex_demo.c -o futex_demo
root@DESKTOP-5PBPDNG:/home/turing# ./futex_demo
父进程 (283) 0
子进程 (284) 0
父进程 (283) 1
子进程 (284) 1
父进程 (283) 2
子进程 (284) 2

    futex1 = &iaddr[0]; //绑定锁一地址
    futex2 = &iaddr[1]; //绑定锁二地址
    *futex1 = 0; // 锁一不可申请 
    *futex2 = 1; // 锁二可申请

    /// 申请快锁
    static void fwait(uint32_t *futexp)
    {
        long s;
        while (1) {
            const uint32_t one = 1;
            if (atomic_compare_exchange_strong(futexp, &one, 0))
                break; //申请快锁成功
            //申请快锁失败,需等待
            s = futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0);
            if (s == -1 && errno != EAGAIN)
                errExit("futex-FUTEX_WAIT");
        }
    }

    在头文件<stdatomic.h>中定义
    _Bool atomic_compare_exchange_strong（volatile A * obj，C * expected，C desired）;

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.......

    futex(futexp, FUTEX_WAIT, 0, NULL, NULL, 0) //执行一个等锁的系统调用

    if (childPid == 0) {//子进程返回
        for (int j = 0; j < nloops; j++) {
            fwait(futex1);
            printf("子进程  (%jd) %d\n", (intmax_t) getpid(), j);
            fpost(futex2);
        }
        exit(EXIT_SUCCESS);
    }

    // 父进程返回执行
    for (int j = 0; j < nloops; j++) {
        fwait(futex2);
        printf("父进程 (%jd) %d\n", (intmax_t) getpid(), j);
        fpost(futex1);
    }
    wait(NULL);
    exit(EXIT_SUCCESS);

      父进程 (283) 0
      子进程 (284) 0
      父进程 (283) 1
      子进程 (284) 1
      父进程 (283) 2
      子进程 (284) 2