如何在Linux驱动中使用信号量进行同步？

在Linux驱动中，信号量是很常用的工具，可以保证数据安全。下面的例子实现使用信号量模拟自旋锁，保证不同线程安全的访问临界空间数据。

驱动代码

// simple.c

#include <linux/delay.h>
#include <linux/init.h>
#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/sched.h>
#include <linux/semaphore.h>

static int t1 = 1, t2 = 2;
static struct semaphore sema;
static struct task_struct *kthread_1;
static struct task_struct *kthread_2;

static int _cb(void *data) {
    const int nr = *(int *)data;
    static unsigned int counter = 0;

    while (!kthread_should_stop()) {
        int ret = down_trylock(&sema);
        if (ret) {
            continue;
        }

        counter++;
        up(&sema);

        pr_info("thread [%d] counter: %u\n", nr, counter);
        msleep(1000);
    }

    return 0;
}

static int __init simple_init(void) {
    pr_info("simple_init\n");

    sema_init(&sema, 1);

    kthread_1 = kthread_create(_cb, &t1, "kthread_1");
    if (!kthread_1) {
        pr_err("kthread_create threa1 error\n");
        return -1;
    } else {
        wake_up_process(kthread_1);
    }

    kthread_2 = kthread_run(_cb, &t2, "kthread_2");
    if (!kthread_2) {
        kthread_stop(kthread_1);
        pr_err("kthread_create thread2 error\n");
        return -1;
    }

    return 0;
}

static void __exit simple_exit(void) {
    pr_info("simple_exit\n");

    kthread_stop(kthread_1);
    kthread_stop(kthread_2);
}

module_init(simple_init);
module_exit(simple_exit);

MODULE_LICENSE("GPL");
MODULE_AUTHOR("heng30");
MODULE_VERSION("v0.0.1");
MODULE_DESCRIPTION("A simple kernel module");

编译脚本

#!/bin/sh

top-dir = $(shell pwd)
kernel-version = $(shell uname -r)
kernel-dir ?= /lib/modules/$(kernel-version)/build

obj-m += simple.o

all:
        make -C $(kernel-dir) modules M=$(top-dir)

clean:
        rm -f *.o *.ko *.mod *.mod.c *.order *.symvers *.dtbo
        make -C $(kernel-dir) clean m=$(top-dir)

测试

• 编译程序：make

• 安装驱动：insmod simple.ko

• 查看输出：dmesg

[15892.481073] simple_init
[15892.481303] thread [1] counter: 1
[15892.481421] thread [2] counter: 2
[15893.519899] thread [1] counter: 3
...
[15909.008125] thread [1] counter: 34
[15910.036302] thread [1] counter: 35
[15910.036463] thread [2] counter: 36
[15910.060482] simple_exit

• 移除驱动：rmmod simple.ko