scull_s.c

static atomic_t scull_s_available = ATOMIC_INIT(1);

static int scull_s_open(struct inode *inode, struct file *filp)
{
    struct scull_dev *dev = &scull_s_device; /* device information */

    if (!atomic_dec_and_test(&scull_s_available)) {
	atomic_inc(&scull_s_available);
	return -EBUSY; /* already open */
    }

    /* then, everything else is copied from the bare scull device */
    if ( (filp->f_flags & O_ACCMODE) == O_WRONLY)
	scull_trim(dev);

    // Update private data for further use
    filp->private_data = dev;

    return 0;
}

static int scull_s_release(struct inode *inode, struct file *filp)
{
    atomic_inc(&scull_s_available); /* release the device */
    return 0;
}