diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 0ce93ac..ddb4bc1 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2453,10 +2453,10 @@
 	if (!fi_args)
 		return -ENOMEM;
 
+	mutex_lock(&fs_devices->device_list_mutex);
 	fi_args->num_devices = fs_devices->num_devices;
 	memcpy(&fi_args->fsid, root->fs_info->fsid, sizeof(fi_args->fsid));
 
-	mutex_lock(&fs_devices->device_list_mutex);
 	list_for_each_entry_safe(device, next, &fs_devices->devices, dev_list) {
 		if (device->devid > fi_args->max_id)
 			fi_args->max_id = device->devid;