blkfront: Fix blkfront backend switch race (bdev open)

We need not mind if users grab a late handle on a closing disk. We
probably even should not. But we have to make sure it's not a dead
one already

Let the bdev deal with a gendisk deleted under its feet. Takes the
info mutex to decide a race against backend closing.
Signed-off-by: Daniel Stodden <daniel.stodden@citrix.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

drivers/block/xen-blkfront.c

@@ -1118,16 +1118,33 @@ static int blkfront_is_ready(struct xenbus_device *dev)
 
 static int blkif_open(struct block_device *bdev, fmode_t mode)
 {
-	struct blkfront_info *info = bdev->bd_disk->private_data;
-
-	if (!info->xbdev)
-		return -ENODEV;
+	struct gendisk *disk = bdev->bd_disk;
+	struct blkfront_info *info;
+	int err = 0;
 
 	lock_kernel();
-	info->users++;
-	unlock_kernel();
 
-	return 0;
+	info = disk->private_data;
+	if (!info) {
+		/* xbdev gone */
+		err = -ERESTARTSYS;
+		goto out;
+	}
+
+	mutex_lock(&info->mutex);
+
+	if (!info->gd)
+		/* xbdev is closed */
+		err = -ERESTARTSYS;
+
+	mutex_unlock(&info->mutex);
+
+	if (!err)
+		++info->users;
+
+	unlock_kernel();
+out:
+	return err;
 }
 
 static int blkif_release(struct gendisk *disk, fmode_t mode)