等待队列

#include <linux/module.h>
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/kdev_t.h>
#include <linux/cdev.h>
#include <linux/wait.h>
#include <linux/string.h>
#include <asm/uaccess.h>
#include <asm/semaphore.h>

#define DEVICE_NAME "chardev"

static ssize_t chardev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos);
static ssize_t chardev_write(struct file *filp, const char __user *buf, size_t count, loff_t *f_pos);

static char buffer[1024];

static struct file_operations chardev_fops = {
        .read = chardev_read,
        .write = chardev_write,
};

static struct cdev *cdevp;
static dev_t devno;

static struct semaphore sem;
static wait_queue_head_t wqueue;
static int flag = 0;

static ssize_t chardev_read(struct file *filp, char __user *buf, size_t count, loff_t *f_pos)
{
        int ret;

        if (wait_event_interruptible(wqueue, flag != 0))
                goto err;

        if (down_interruptible(&sem))
                goto err;

        flag = 0;
        ret = copy_to_user(buf, buffer, strlen(buffer));

        up(&sem);
        return (!ret) ? (strlen(buffer)) : (-EFAULT);
err:
        return -ERESTARTSYS;
}

static ssize_t chardev_write(struct file *filp, const char __user *buf, size_t count, loff_t *f_pos)
{
        int ret;

        if (down_interruptible(&sem))
                goto down_err;

        memset(buffer, '\0', sizeof(buffer));
        ret = copy_from_user(buffer, buf, count);

        up(&sem);
        flag = 1;
        wake_up_interruptible(&wqueue);

        return (!ret) ? (count) : (-EFAULT);
down_err:
        return -ERESTARTSYS;
}

static int __init chardev_init(void)
{
        int ret;

        ret = alloc_chrdev_region(&devno, 0, 1, DEVICE_NAME);
        if (ret < 0)
                goto out;

        cdevp = cdev_alloc();
        if (!cdevp)
                goto alloc_err;

        init_MUTEX(&sem);
        init_waitqueue_head(&wqueue);

        cdev_init(cdevp, &chardev_fops);
        ret = cdev_add(cdevp, devno, 1);
        if (!ret)
                goto out;

        cdev_del(cdevp);
alloc_err:
        unregister_chrdev_region(devno, 1);
out:
        return ret;
}

static void __exit chardev_exit(void)
{
        cdev_del(cdevp);

        unregister_chrdev_region(devno, 1);
}

MODULE_LICENSE("GPL");

module_init(chardev_init);
module_exit(chardev_exit);