static int kcrash_open(struct inode *inode, 
		struct file *filp)
{
	printk(KERN_INFO "kcrash: open\n");

	if(down_interruptible(&kdev->sem)){
		printk(KERN_INFO "kcrash: sema-open\n");
		return -ERESTARTSYS;
	}
	if(kdev->count > 0)
		return -EFAULT;
	
	kdev->count++;
	up(&kdev->sem);
	filp->private_data = NULL;

	return 0;
}