//  [c]
static noinline struct btrfs_device *device_list_add(const char *path,
			   struct btrfs_super_block *disk_super,
			   bool *new_device_added)
{
	struct btrfs_device *device;
	struct btrfs_fs_devices *fs_devices = NULL;
	struct rcu_string *name;
	u64 found_transid = btrfs_super_generation(disk_super);
	u64 devid = btrfs_stack_device_id(&disk_super->dev_item);
	bool has_metadata_uuid = (btrfs_super_incompat_flags(disk_super) &
		BTRFS_FEATURE_INCOMPAT_METADATA_UUID);
	bool fsid_change_in_progress = (btrfs_super_flags(disk_super) &
					BTRFS_SUPER_FLAG_CHANGING_FSID_V2);

	if (fsid_change_in_progress) {
		if (!has_metadata_uuid) {
			/*
			 * When we have an image which has CHANGING_FSID_V2 set
			 * it might belong to either a filesystem which has
			 * disks with completed fsid change or it might belong
			 * to fs with no UUID changes in effect, handle both.
			 */
			fs_devices = find_fsid_inprogress(disk_super);
			if (!fs_devices)
				fs_devices = find_fsid(disk_super->fsid, NULL);
		} else {
			fs_devices = find_fsid_changed(disk_super);
		}
	} else if (has_metadata_uuid) {
		fs_devices = find_fsid(disk_super->fsid,
				       disk_super->metadata_uuid);
	} else {
		fs_devices = find_fsid(disk_super->fsid, NULL);
	}


	if (!fs_devices) {
		if (has_metadata_uuid)
			fs_devices = alloc_fs_devices(disk_super->fsid,
						      disk_super->metadata_uuid);
		else
			fs_devices = alloc_fs_devices(disk_super->fsid, NULL);

		if (IS_ERR(fs_devices))
			return ERR_CAST(fs_devices);

		fs_devices->fsid_change = fsid_change_in_progress;

		mutex_lock(&fs_devices->device_list_mutex);
		list_add(&fs_devices->fs_list, &fs_uuids);

		device = NULL;
	} else {
		mutex_lock(&fs_devices->device_list_mutex);
		device = find_device(fs_devices, devid,
				disk_super->dev_item.uuid);

		/*
		 * If this disk has been pulled into an fs devices created by
		 * a device which had the CHANGING_FSID_V2 flag then replace the
		 * metadata_uuid/fsid values of the fs_devices.
		 */
		if (has_metadata_uuid && fs_devices->fsid_change &&
		    found_transid > fs_devices->latest_generation) {
			memcpy(fs_devices->fsid, disk_super->fsid,
					BTRFS_FSID_SIZE);
			memcpy(fs_devices->metadata_uuid,
					disk_super->metadata_uuid, BTRFS_FSID_SIZE);

			fs_devices->fsid_change = false;
		}
	}

	if (!device) {
		if (fs_devices->opened) {
			mutex_unlock(&fs_devices->device_list_mutex);
			return ERR_PTR(-EBUSY);
		}

		device = btrfs_alloc_device(NULL, &devid,
					    disk_super->dev_item.uuid);
		if (IS_ERR(device)) {
			mutex_unlock(&fs_devices->device_list_mutex);
			/* we can safely leave the fs_devices entry around */
			return device;
		}

		name = rcu_string_strdup(path, GFP_NOFS);
		if (!name) {
			btrfs_free_device(device);
			mutex_unlock(&fs_devices->device_list_mutex);
			return ERR_PTR(-ENOMEM);
		}
		rcu_assign_pointer(device->name, name);

		list_add_rcu(&device->dev_list, &fs_devices->devices);
		fs_devices->num_devices++;

		device->fs_devices = fs_devices;
		*new_device_added = true;

		if (disk_super->label[0])
			pr_info("BTRFS: device label %s devid %llu transid %llu %s\n",
				disk_super->label, devid, found_transid, path);
		else
			pr_info("BTRFS: device fsid %pU devid %llu transid %llu %s\n",
				disk_super->fsid, devid, found_transid, path);

	} else if (!device->name || strcmp(device->name->str, path)) {
		/*
		 * When FS is already mounted.
		 * 1. If you are here and if the device->name is NULL that
		 *    means this device was missing at time of FS mount.
		 * 2. If you are here and if the device->name is different
		 *    from 'path' that means either
		 *      a. The same device disappeared and reappeared with
		 *         different name. or
		 *      b. The missing-disk-which-was-replaced, has
		 *         reappeared now.
		 *
		 * We must allow 1 and 2a above. But 2b would be a spurious
		 * and unintentional.
		 *
		 * Further in case of 1 and 2a above, the disk at 'path'
		 * would have missed some transaction when it was away and
		 * in case of 2a the stale bdev has to be updated as well.
		 * 2b must not be allowed at all time.
		 */

		/*
		 * For now, we do allow update to btrfs_fs_device through the
		 * btrfs dev scan cli after FS has been mounted.  We're still
		 * tracking a problem where systems fail mount by subvolume id
		 * when we reject replacement on a mounted FS.
		 */
		if (!fs_devices->opened && found_transid < device->generation) {
			/*
			 * That is if the FS is _not_ mounted and if you
			 * are here, that means there is more than one
			 * disk with same uuid and devid.We keep the one
			 * with larger generation number or the last-in if
			 * generation are equal.
			 */
			mutex_unlock(&fs_devices->device_list_mutex);
			return ERR_PTR(-EEXIST);
		}

		/*
		 * We are going to replace the device path for a given devid,
		 * make sure it's the same device if the device is mounted
		 */
		if (device->bdev) {
			struct block_device *path_bdev;

			path_bdev = lookup_bdev(path);
			if (IS_ERR(path_bdev)) {
				mutex_unlock(&fs_devices->device_list_mutex);
				return ERR_CAST(path_bdev);
			}

			if (device->bdev != path_bdev) {
				bdput(path_bdev);
				mutex_unlock(&fs_devices->device_list_mutex);
				btrfs_warn_in_rcu(device->fs_info,
			"duplicate device fsid:devid for %pU:%llu old:%s new:%s",
					disk_super->fsid, devid,
					rcu_str_deref(device->name), path);
				return ERR_PTR(-EEXIST);
			}
			bdput(path_bdev);
			btrfs_info_in_rcu(device->fs_info,
				"device fsid %pU devid %llu moved old:%s new:%s",
				disk_super->fsid, devid,
				rcu_str_deref(device->name), path);
		}

		name = rcu_string_strdup(path, GFP_NOFS);
		if (!name) {
			mutex_unlock(&fs_devices->device_list_mutex);
			return ERR_PTR(-ENOMEM);
		}
		rcu_string_free(device->name);
		rcu_assign_pointer(device->name, name);
		if (test_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state)) {
			fs_devices->missing_devices--;
			clear_bit(BTRFS_DEV_STATE_MISSING, &device->dev_state);
		}
	}

	/*
	 * Unmount does not free the btrfs_device struct but would zero
	 * generation along with most of the other members. So just update
	 * it back. We need it to pick the disk with largest generation
	 * (as above).
	 */
	if (!fs_devices->opened) {
		device->generation = found_transid;
		fs_devices->latest_generation = max_t(u64, found_transid,
						fs_devices->latest_generation);
	}

	fs_devices->total_devices = btrfs_super_num_devices(disk_super);

	mutex_unlock(&fs_devices->device_list_mutex);
	return device;
}