I got this error message from initramfs on an unbootable btrfs as root machine: "request_module fs-btrfs succeeded, but still no fs?". note: at that moment raid6_pq seems not fully finished its benchmarking.
fixed by add "grep "btrfs" /proc/filesystems >/dev/null || (insmod /lib/modules/$(uname -r)/kernel/fs/btrfs/btrfs.ko)" at /usr/share/initramfs-tools/scripts/local-premount/btrfs, one line above "/bin/btrfs device scan". and added related modules manually.