init boot mount: mounting /root on /root.ro failed: Invalid argument, aufs

I am trying to set up a read-only filesystem at /root directory on my embedded device. I have following in code in my boot init command file /sbin/init-overlay: rootmnt=/root ro_mount_point="${rootmnt%/}.ro" rw_mount_point="${rootmnt%/}.rw" # For local system rearranged from init /bin/mkdir -p "${ro_mount_point}" "${rw_mount_point}" # Move the already-mounted root filesystem to the ro mount point: /bin/mount --move ${rootmnt} ${ro_mount_point} # Mount the read/write filesystem: /bin/mount -t tmpfs root.rw "${rw_mount_point}" # Mount the union: /bin/mount -t aufs -o dirs=${rw_mount_point}=rw:${ro_mount_point}=ro aufs ${rootmnt} # Correct the permissions of /: /bin/chmod 755 "${rootmnt}" # Make sure the individual ro and rw mounts are accessible from within the root # once the union is assumed as /. This makes it possible to access the # component filesystems individually. /bin/mkdir "${rootmnt}/ro" "${rootmnt}/rw" /bin/mount --bind "${ro_mount_point}" "${rootmnt}/ro" /bin/mount --bind "${rw_mount_point}" "${rootmnt}/rw" This is my boot command: console=ttyS0,115200 earlyprintk root=/dev/mmcblk0p2 rootfstype=ext4 rootwait fsck.repair=${fsck.repair} panic=10 ${extra} fbcon=${fbcon} rw init=/sbin/init-overlay But while booting, I am getting the following error: mount: mounting /root on /root.ro failed: Invalid argument Can anyone point-out what's wrong here?