Cannot add device "is mounted" for unmounted drive that used to be in raidset that is mounted

[Marc MERLIN <marc@merlins.org>]

I found out that a drive that used to be part of a raid system that is mounted
and running without it, btrfs apparently decides that the drive is part of the mounted
raidset and in use.
As a result, I had to eventually dd 0's over it, btrfs device scan, and finally
I was able to use it again.

btrfs should probably improve its check to see that the drive is not really used
and let me format it.


Longer details:
On a running system, I re-added a drive, it showed up as /dev/sdm1
I decrypted it
cryptsetup luksOpen /dev/sdm1 crypt_sdm1

But I can't add it:
polgara:/mnt/btrfs_backupcopy# btrfs device add -f /dev/mapper/crypt_sdm1 .
/dev/mapper/crypt_sdm1 is mounted

however, I can format it as ext4, mount it, write to it, unmount it, and
I still get the same error:
polgara:/mnt/btrfs_backupcopy# mke2fs -t ext4 /dev/mapper/crypt_sdm1
polgara:/mnt/btrfs_backupcopy# mount /dev/mapper/crypt_sdm1 /mnt/mnt
polgara:/mnt/btrfs_backupcopy# Mar 23 10:48:31 polgara kernel: [38645.955263] EXT4-fs (dm-10): mounted filesystem with ordered data mode. Opts: (null)
polgara:/mnt/btrfs_backupcopy# umount /mnt/mnt
polgara:/mnt/btrfs_backupcopy# btrfs device add -f /dev/mapper/crypt_sdm1 .
/dev/mapper/crypt_sdm1 is mounted

polgara:/mnt/btrfs_backupcopy# fuser -v /dev/mapper/crypt_sdm1
polgara:/mnt/btrfs_backupcopy# lsof -n | grep -E '(dm-10|sdm)'
polgara:/mnt/btrfs_backupcopy# mkfs.btrfs -f /dev/mapper/crypt_sdm1 
Error: /dev/mapper/crypt_sdm1 is mounted
polgara:/mnt/btrfs_backupcopy# 

In the end, I had to run
dd if=/dev/zero of=/dev/mapper/crypt_sdm1 bs=1M
for btrfs to stop telling me the filesystem was mounted.

Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/                         | PGP 1024R/763BE901