blkfront: Fix blkfront backend switch race (bdev release)

	if (!err)

		++info->users;

	unlock_kernel();

out:

	unlock_kernel();

	return err;

}

static int blkif_release(struct gendisk *disk, fmode_t mode)

{

	struct blkfront_info *info = disk->private_data;

	struct block_device *bdev;

	struct xenbus_device *xbdev;

	lock_kernel();

	info->users--;

	if (info->users == 0) {

		/* Check whether we have been instructed to close.  We will

		   have ignored this request initially, as the device was

		   still mounted. */

		struct xenbus_device *dev = info->xbdev;

		if (!dev) {

			xlvbd_release_gendisk(info);

			kfree(info);

		} else if (xenbus_read_driver_state(dev->otherend)

			   == XenbusStateClosing && info->is_ready) {

	if (--info->users)

		goto out;

	bdev = bdget_disk(disk, 0);

	bdput(bdev);

	/*

	 * Check if we have been instructed to close. We will have

	 * deferred this request, because the bdev was still open.

	 */

	mutex_lock(&info->mutex);

	xbdev = info->xbdev;

	if (xbdev && xbdev->state == XenbusStateClosing) {

		/* pending switch to state closed */

		xlvbd_release_gendisk(info);

			xenbus_frontend_closed(dev);

		xenbus_frontend_closed(info->xbdev);

 	}

	mutex_unlock(&info->mutex);

	if (!xbdev) {

		/* sudden device removal */

		xlvbd_release_gendisk(info);

		disk->private_data = NULL;

		kfree(info);

	}

out:

	unlock_kernel();

	return 0;

}