Sleeping mechanism in character driver in linux

#include <linux/module.h>
#include <linux/fs.h>  //various structures(fops)
#include <linux/major.h>
#include <linux/capability.h>
#include <asm/uaccess.h> //copy_to/from_user()
#include <linux/cdev.h>  //cdev
#include <linux/wait.h>  //wait queues
#include <linux/sched.h> //Task states (TASK_INTERRUPTIBLE etc)

wait_queue_head_t queue;
int flag = 0;

static ssize_t sample_char_read(struct file * file, char __user * buf,size_t count, loff_t *ppos)
{
  flag = 0;
  wait_event_interruptible(queue, flag == 1);
  printk("sample_char_read size(%ld)\n", count);
 return 0;
}

static ssize_t sample_char_write(struct file *filp, const char *buf,size_t size, loff_t *offp)
{
  flag = 1;
  wake_up_interruptible(&queue);
  printk("sample_char_write size(%ld)\n", size);
        return size;

}

int sample_char_open(struct inode *inode, struct file *filp)
{
       printk("sample_char_open\n");
       return 0;
}

int sample_char_release(struct inode *inode, struct file *filp)
{

       printk("sample_char_release\n");
       return 0;
}

static struct file_operations sample_char_fops =
{
       .read=          sample_char_read,
       .write=          sample_char_write,
       .open=          sample_char_open,
       .release=        sample_char_release,
};

#define sample_major_number 249
#define max_minors 1

static struct cdev char_cdev;
static dev_t dev;

int init_module(void)
{
  int ret = 0;

  dev = MKDEV(sample_major_number, 0);
  printk("\nLoading the sample char device driver\n");

  ret = register_chrdev_region(dev, max_minors, "sample_char");
  if (ret)
   {
     printk("register_chrdev_region Error\n");
   goto error;
   }
  cdev_init(&char_cdev, &sample_char_fops);
  ret = cdev_add(&char_cdev, dev, max_minors);
  if (ret) {
     printk("cdev_add Error\n");
          goto error_region;
  }

  init_waitqueue_head(&queue);
  return 0;

 error_region:
  unregister_chrdev_region(dev, max_minors);
 error:
  return ret;
}

void cleanup_module(void)
{
  cdev_del(&char_cdev);
        unregister_chrdev_region(dev, max_minors);
        printk("\nUnloading the sample char device driver\n");

}