Comment # 19 on bug 1020030 from Randy Wright

I just observed something quite similar running the SLES12SP3 kotd
4.4.79-2.g7fe2b65-default.   I was testing crashdump, and as makedumpfile was
completing successfully, I observed this on the serial console:

[   51.673736] ------------[ cut here ]------------
[   51.673784] WARNING: CPU: 0 PID: 325 at ../fs/btrfs/qgroup.c:2466
btrfs_qgroup_free_refroot+0x14d/0x180 [btrfs]()
[   51.673812] Modules linked in: btrfs xor hid_generic usbhid uas ...
[   51.673813] Supported: Yes
[   51.673818] CPU: 0 PID: 325 Comm: umount Not tainted
4.4.79-2.g7fe2b65-default #1
[   51.673819] Hardware name: HPE ProLiant....
[   51.673824]  0000000000000000 ffffffff81320810 0000000000000000
ffffffffa03839e9
[   51.673827]  ffffffff8107e061 ffff8800624fc908 0000000000050000
ffff88027788ad80
[   51.673829]  ffff8800624fc880 ffff88027788a000 ffffffffa036b26d
0000000000000103
[   51.673830] Call Trace:
[   51.673853]  [<ffffffff81019b19>] dump_trace+0x59/0x310
[   51.673862]  [<ffffffff81019eba>] show_stack_log_lvl+0xea/0x170
[   51.673868]  [<ffffffff8101ac41>] show_stack+0x21/0x40
[   51.673875]  [<ffffffff81320810>] dump_stack+0x5c/0x7c
[   51.673884]  [<ffffffff8107e061>] warn_slowpath_common+0x81/0xb0
umount: /kdump/mnt0: not mounted
umount[   51.673923]  [<ffffffffa036b26d>]
btrfs_qgroup_free_refroot+0x14d/0x180 [btrfs]

If you want more detail from my sighting, I will need to create a confidental
bugzilla, as this was seen on an as-yet-unreleased HPE system.   I can relate
that I have run many previous crashdump tests on this system and never
previously seen a warning from btrfs_qgroup_free_refroot, so I can offer no
clues for reproduction.