Balance + Ctrl-C = forced readonly

[Hans van Kranenburg <hans@knorrie.org>]

Hi,

This is Linux kernel 5.7.6 (the Debian package, 5.7.6-1).

So, I wanted to try out this new quicker balance interrupt thing, and
the result was that I could crash the fs at my very first try using it,
which was simply doing balance, and then pressing Ctrl-C.

Recipe to reproduce: Start balance, wait a few seconds, then press
Ctrl-C. For me here, ~ 5 out of 10 times, it ends up exploding:

-# btrfs balance start --full /btrfs/
^C

[41190.572977] BTRFS info (device xvdb): balance: start -d -m -s
[41190.573035] BTRFS info (device xvdb): relocating block group
73001861120 flags metadata
[41205.409600] BTRFS info (device xvdb): found 12236 extents, stage:
move data extents
[41205.509316] BTRFS info (device xvdb): relocating block group
71928119296 flags data
[41205.695319] BTRFS info (device xvdb): found 3 extents, stage: move
data extents
[41205.723009] BTRFS info (device xvdb): found 3 extents, stage: update
data pointers
[41205.750590] BTRFS info (device xvdb): relocating block group
60922265600 flags metadata
[41208.183424] BTRFS: error (device xvdb) in btrfs_drop_snapshot:5505:
errno=-4 unknown
[41208.183450] BTRFS info (device xvdb): forced readonly
[41208.183469] BTRFS info (device xvdb): balance: ended with status: -4

Boom, readonly FS.

Hans