raid6 grow "hangs" on 2.6.27 with mounted FS

[roma1390]

i have 2.6.27
trying thease command

# create raid6 with 4 block devices
mdadm --create /dev/md2 --level=6 --raid-devices=4 /dev/loop{1,2,3,4}
# wait to sync
sleep 3
# create and mount FS
mkfs.ext2 /dev/md2
mount /dev/md2 -t ext2 /mnt

# add block device and grow, wait for resync
mdadm --add /dev/md2  /dev/loop5
mdadm --grow /dev/md2 --raid-devices=5
sleep 3

# dmesg prints: VFS: busy inodes on changed media.
# umount hangs ...
umount /dev/md2
# also: "mdadm --query /dev/md2" hangs in D state.

attached: qemu script (start.sh), init script (mini), .config (config)

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Attachment: start.sh
Description: Bourne shell script

Attachment: mini
Description: Binary data

Attachment: config
Description: Binary data