如何在Linux驱动中使用等待队列同步线程？

线程同步在编程中是很常见的功能。在Linux内核中也提供了等待队列同步机制，可以很方便进行线程同步。下面就给出了完整的例子，写入操作触发等待队列，等待队列中的等待者会得到执行的机会。

实例代码

// simple_cdev.c

#include <linux/cdev.h>
#include <linux/device.h>
#include <linux/device/class.h>
#include <linux/err.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/minmax.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/types.h>
#include <linux/uaccess.h>

#define BUFFER_SIZE 1024
#define DEV_MINOR_COUNTS 1
#define DEV_NAME "simple_cdev"

typedef struct {
    struct cdev *cdev;   // 字符设备
    struct class *class; // 设备类
    struct device *dev;  // /dev目录下的设备

    struct wait_queue_head wait_queue; // 等待队列头
    bool condition;                    // 条件变量

    char buffer[BUFFER_SIZE]; // 缓冲区
} simple_cdev_t;

simple_cdev_t my_cdev = {0};

static int _open(struct inode *inode, struct file *file) {
    pr_info("simple_cdev_open\n");
    return 0;
}

static ssize_t _read(struct file *file, char __user *userbuf, size_t size,
                     loff_t *offset) {
    // 非阻塞，等待队列准备好
    if (file->f_flags & O_NONBLOCK) {
        if (!my_cdev.condition) {
            return -EAGAIN;
        }
    } else {
        // 阻塞，等待队列准备好
        wait_event_interruptible(my_cdev.wait_queue, my_cdev.condition);
    }
    my_cdev.condition = false;

    pr_info("simple_cdev_read\n");

    size = min(size, BUFFER_SIZE);
    unsigned long remain_len =
        copy_to_user(userbuf, my_cdev.buffer, size);

    if (remain_len > 0) {
        pr_err("copy_to_user failed\n");
        return -EIO;
    }

    return size;
}

static ssize_t _write(struct file *file, const char __user *userbuf,
                      size_t size, loff_t *offset) {
    pr_info("simple_cdev_write\n");

    size = min(size, BUFFER_SIZE);
    unsigned long remain_len =
        copy_from_user(my_cdev.buffer, userbuf, size);

    if (remain_len > 0) {
        pr_err("copy_from_user failed\n");
        return -EIO;
    }

    // 唤醒等待队列
    my_cdev.condition = true;
    wake_up_interruptible(&my_cdev.wait_queue);

    return size;
}

static int _release(struct inode *inode, struct file *file) {
    pr_info("simple_cdev_release\n");
    return 0;
}

static struct file_operations fops = {
    .open = _open,
    .read = _read,
    .write = _write,
    .release = _release,
};

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

    my_cdev.cdev = cdev_alloc();
    if (!my_cdev.cdev) {
        pr_err("simple_cdev cdev_alloc failed!\n");
        return -ENOMEM;
    }

    // 初始化字符设备
    cdev_init(my_cdev.cdev, &fops);

    // 申请设备号, 由系统分配主设备号和一个从设备号
    int ret =
        alloc_chrdev_region(&my_cdev.cdev->dev, 0, DEV_MINOR_COUNTS, DEV_NAME);
    if (ret) {
        pr_err("simple_cdev alloc_chrdev_region failed!\n");
        return ret;
    }

    pr_info("simple_cdev major = %d\n", MAJOR(my_cdev.cdev->dev));

    // 添加到内核
    ret = cdev_add(my_cdev.cdev, my_cdev.cdev->dev, DEV_MINOR_COUNTS);
    if (ret) {
        pr_err("simple_cdev cdev_add failed!\n");
        return ret;
    }

    // 申请设备类, 会在/sys/class目录下常见一个simple_cdev的目录
    my_cdev.class = class_create("simple_cdev");
    if (IS_ERR(my_cdev.class)) {
        pr_err("simple_cdev class_create failed\n");
        return PTR_ERR(my_cdev.class);
    }

    // 申请设备对象，会在/dev目录下创建设备文件simple_cdev,
    // 并且在/sys/class/simple_cdev目录下常见一个simple_cdev的设备节点
    my_cdev.dev =
        device_create(my_cdev.class, NULL, my_cdev.cdev->dev, NULL, DEV_NAME);
    if (IS_ERR(my_cdev.dev)) {
        pr_err("simple_cdev device_create failed\n");
        return PTR_ERR(my_cdev.dev);
    }

    // 初始化等待队列和条件变量
    init_waitqueue_head(&my_cdev.wait_queue);
    my_cdev.condition = false;

    return 0;
}

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

    device_destroy(my_cdev.class, my_cdev.cdev->dev);
    class_destroy(my_cdev.class);
    cdev_del(my_cdev.cdev);
    unregister_chrdev_region(my_cdev.cdev->dev, DEV_MINOR_COUNTS);
    kfree(my_cdev.cdev);
}

module_init(simple_cdev_init);
module_exit(simple_cdev_exit);

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

Makefile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_cdev.o

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

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

测试

• 安装驱动：insmod simple_cdev.ko

• 移除驱动：rmmod simple_cdev.ko

• 打开两个终端窗口：

  • 第1个窗口运行：cat /dev/simple_cdev。cat命令会一直等待。
  • 第2个窗口运行：echo "hello" > /dev/simple_cdev。第1个窗口会马上输出hello字符串。在第2个窗口不断地输入内容，第1个窗口会输出相同的内容，从而实现线程同步。