Hi,

I recently started a balance on a BTRFS filesystem.
At least one process failed to write files before or during the
balance due to a supposed out-of-space condition.
Yes, I know it's not good to allow that to happen.

Here is the relevant dmesg output:
[694828.101677] BTRFS info (device sdh1): balance: start -dusage=60
[694828.103543] BTRFS info (device sdh1): relocating block group
2933242462208 flags data
[694883.648385] BTRFS info (device sdh1): found 76 extents

At that point, disk I/O according to iotop dropped to almost zero.
After waiting a while, I attempted to cancel the balance, but that
just resulted in a hung cancel process.

Now, any process that attempts to write to or read from the filesystem hangs.
I suppose that means there's a kernel deadlock.
The output of dmesg after running echo w >/proc/sysrq-trigger is attached.

kernel: 5.0.3-arch1-1-ARCH x86_64
btrfs-progs version: v4.20.2

The data that was being written to the filesystem is not important.

- Jared