rbd: always set read-only flag in rbd_add()

Hold off setting the read-only flag in rbd_add() for an image being
mapped until we have successfully probed the image.  At that point
we know whether it's a snapshot mapping or not, so we can set the
read-only flag in that one place rather than doing so (for
snapshots) in rbd_dev_mapping_set().  To do this, pass a flag to the
image probe routine indicating whether we want a read-only mapping.
Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>

drivers/block/rbd.c

@@ -359,7 +359,7 @@ static ssize_t rbd_add(struct bus_type *bus, const char *buf,
 		       size_t count);
 static ssize_t rbd_remove(struct bus_type *bus, const char *buf,
 			  size_t count);
-static int rbd_dev_image_probe(struct rbd_device *rbd_dev);
+static int rbd_dev_image_probe(struct rbd_device *rbd_dev, bool read_only);
 
 static struct bus_attribute rbd_bus_attrs[] = {
 	__ATTR(add, S_IWUSR, NULL, rbd_add),
@@ -951,11 +951,6 @@ static int rbd_dev_mapping_set(struct rbd_device *rbd_dev)
 	rbd_dev->mapping.size = size;
 	rbd_dev->mapping.features = features;
 
-	/* If we are mapping a snapshot it must be marked read-only */
-
-	if (snap_id != CEPH_NOSNAP)
-		rbd_dev->mapping.read_only = true;
-
 	return 0;
 }
 
@@ -963,7 +958,6 @@ static void rbd_dev_mapping_clear(struct rbd_device *rbd_dev)
 {
 	rbd_dev->mapping.size = 0;
 	rbd_dev->mapping.features = 0;
-	rbd_dev->mapping.read_only = true;
 }
 
 static const char *rbd_segment_name(struct rbd_device *rbd_dev, u64 offset)
@@ -4620,7 +4614,7 @@ static int rbd_dev_probe_parent(struct rbd_device *rbd_dev)
 	if (!parent)
 		goto out_err;
 
-	ret = rbd_dev_image_probe(parent);
+	ret = rbd_dev_image_probe(parent, true);
 	if (ret < 0)
 		goto out_err;
 	rbd_dev->parent = parent;
@@ -4743,7 +4737,7 @@ static void rbd_dev_image_release(struct rbd_device *rbd_dev)
  * device.  For format 2 images this includes determining the image
  * id.
  */
-static int rbd_dev_image_probe(struct rbd_device *rbd_dev)
+static int rbd_dev_image_probe(struct rbd_device *rbd_dev, bool read_only)
 {
 	int ret;
 	int tmp;
@@ -4778,6 +4772,12 @@ static int rbd_dev_image_probe(struct rbd_device *rbd_dev)
 	if (ret)
 		goto err_out_probe;
 
+	/* If we are mapping a snapshot it must be marked read-only */
+
+	if (rbd_dev->spec->snap_id != CEPH_NOSNAP)
+		read_only = true;
+	rbd_dev->mapping.read_only = read_only;
+
 	ret = rbd_dev_probe_parent(rbd_dev);
 	if (!ret)
 		return 0;
@@ -4811,6 +4811,7 @@ static ssize_t rbd_add(struct bus_type *bus,
 	struct rbd_spec *spec = NULL;
 	struct rbd_client *rbdc;
 	struct ceph_osd_client *osdc;
+	bool read_only;
 	int rc = -ENOMEM;
 
 	if (!try_module_get(THIS_MODULE))
@@ -4820,6 +4821,9 @@ static ssize_t rbd_add(struct bus_type *bus,
 	rc = rbd_add_parse_args(buf, &ceph_opts, &rbd_opts, &spec);
 	if (rc < 0)
 		goto err_out_module;
+	read_only = rbd_opts->read_only;
+	kfree(rbd_opts);
+	rbd_opts = NULL;	/* done with this */
 
 	rbdc = rbd_get_client(ceph_opts);
 	if (IS_ERR(rbdc)) {
@@ -4850,11 +4854,7 @@ static ssize_t rbd_add(struct bus_type *bus,
 	rbdc = NULL;		/* rbd_dev now owns this */
 	spec = NULL;		/* rbd_dev now owns this */
 
-	rbd_dev->mapping.read_only = rbd_opts->read_only;
-	kfree(rbd_opts);
-	rbd_opts = NULL;	/* done with this */
-
-	rc = rbd_dev_image_probe(rbd_dev);
+	rc = rbd_dev_image_probe(rbd_dev, read_only);
 	if (rc < 0)
 		goto err_out_rbd_dev;