rbd: don't acquire exclusive lock for read-only mappings

A read-only mapping should be usable with read-only OSD caps, so
neither the header lock nor the object map lock can be acquired.
Unfortunately, this means that images mapped read-only lose the
advantage of the object map.

Snapshots, however, can take advantage of the object map without
any exclusionary locks, so if the object map is desired, snapshot
the image and map the snapshot instead of the image.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>
Reviewed-by: Dongsheng Yang <dongsheng.yang@easystack.cn>

drivers/block/rbd.c

@@ -1850,6 +1850,17 @@ static u8 rbd_object_map_get(struct rbd_device *rbd_dev, u64 objno)
 
 static bool use_object_map(struct rbd_device *rbd_dev)
 {
+	/*
+	 * An image mapped read-only can't use the object map -- it isn't
+	 * loaded because the header lock isn't acquired.  Someone else can
+	 * write to the image and update the object map behind our back.
+	 *
+	 * A snapshot can't be written to, so using the object map is always
+	 * safe.
+	 */
+	if (!rbd_is_snap(rbd_dev) && rbd_is_ro(rbd_dev))
+		return false;
+
 	return ((rbd_dev->header.features & RBD_FEATURE_OBJECT_MAP) &&
 		!(rbd_dev->object_map_flags & RBD_FLAG_OBJECT_MAP_INVALID));
 }
@@ -3573,7 +3584,7 @@ static bool need_exclusive_lock(struct rbd_img_request *img_req)
 	if (!(rbd_dev->header.features & RBD_FEATURE_EXCLUSIVE_LOCK))
 		return false;
 
-	if (rbd_is_snap(rbd_dev))
+	if (rbd_is_ro(rbd_dev))
 		return false;
 
 	rbd_assert(!test_bit(IMG_REQ_CHILD, &img_req->flags));
@@ -6653,7 +6664,7 @@ static int rbd_add_acquire_lock(struct rbd_device *rbd_dev)
 		return -EINVAL;
 	}
 
-	if (rbd_is_snap(rbd_dev))
+	if (rbd_is_ro(rbd_dev))
 		return 0;
 
 	rbd_assert(!rbd_is_lock_owner(rbd_dev));