rbd: expose the correct size of the device in sysfs

If an image was mapped to a snapshot, the size of the head version
would be shown. Protect capacity with header_rwsem, since it may
change.

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>
Reviewed-by: Alex Elder <elder@inktank.com>

drivers/block/rbd.c

@@ -1724,6 +1724,8 @@ static int __rbd_refresh_header(struct rbd_device *rbd_dev)
 	if (ret < 0)
 		return ret;
 
+	down_write(&rbd_dev->header_rwsem);
+
 	/* resized? */
 	if (rbd_dev->snap_id == CEPH_NOSNAP) {
 		sector_t size = (sector_t) h.image_size / SECTOR_SIZE;
@@ -1732,8 +1734,6 @@ static int __rbd_refresh_header(struct rbd_device *rbd_dev)
 		set_capacity(rbd_dev->disk, size);
 	}
 
-	down_write(&rbd_dev->header_rwsem);
-
 	snap_seq = rbd_dev->header.snapc->seq;
 	if (rbd_dev->header.total_snaps &&
 	    rbd_dev->header.snapc->snaps[0] == snap_seq)
@@ -1853,8 +1853,13 @@ static ssize_t rbd_size_show(struct device *dev,
 			     struct device_attribute *attr, char *buf)
 {
 	struct rbd_device *rbd_dev = dev_to_rbd_dev(dev);
+	sector_t size;
+
+	down_read(&rbd_dev->header_rwsem);
+	size = get_capacity(rbd_dev->disk);
+	up_read(&rbd_dev->header_rwsem);
 
-	return sprintf(buf, "%llu\n", (unsigned long long)rbd_dev->header.image_size);
+	return sprintf(buf, "%llu\n", (unsigned long long) size * SECTOR_SIZE);
 }
 
 static ssize_t rbd_major_show(struct device *dev,