Segfault on Btrfs balance after adding device to 2 disk RAID 1 with one failed drive

[Scott & Chrystie Hansen <scottandchrystie@gmail.com>]

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Greetings,

Yesterday I had a drive failure (well...I broke the SATA power plug on the
drive) in a 2x1TB disk btrfs RAID 1 array. I removed the failed drive,
rebooted and mounted the array with '-o degraded'. I thought at first I could
use the 'btrfs device delete missing' but then realized I actually had to
_add_ a device first, even though the broken hard drive was already removed.
I added an empty 2TB disk to the array (just to get the remaining 1TB drive
mirrored again) and started a 'btrfs balance start /home'. It ran for several
hours, then got a segfault (systemd journal: http://sprunge.us/RKeM. Segfault
is at the end of the page).

This morning I tried a 'btrfs balance cancel /home' and it's been stuck for
over 12 hours now without cancelling. There's nothing in htop or glances
that's obvious except the load average (i3 CPU) has been sitting at around
2.0 (normally .1-.3) since the attempted cancel.

All the other services running on the server seem to be operating normally.
Backups are up-to-date! I'm a little afraid to reboot for fear of borking the
existing 1Tb drive, but I'm not sure what else to try at this point. Any
ideas?

Thanks!
Scott

uname -a: Linux homeserver 4.0.4-2-ARCH #1 SMP PREEMPT Fri May 22 03:05:23 UTC 2015 x86_64 GNU/Linux

btrfs --version: btrfs-progs v4.0

btrfs fi show:

    Label: 'ROOT'  uuid: 59046c5c-4b32-44dd-9e82-41a215e69bd3
            Total devices 1 FS bytes used 1.63GiB
            devid    1 size 107.79GiB used 4.04GiB path /dev/sdc3

    Label: 'DATA'  uuid: 8d170909-3031-47d5-b07a-8f4743c8c70c
            Total devices 3 FS bytes used 820.28GiB
            devid    2 size 931.51GiB used 839.03GiB path /dev/sdb
            devid    3 size 1.82TiB used 239.00GiB path /dev/sda
            *** Some devices missing

    btrfs-progs v4.0

btrfs fi df /home:

    Data, RAID1: total=836.00GiB, used=818.77GiB
    System, RAID1: total=32.00MiB, used=176.00KiB
    Metadata, RAID1: total=3.00GiB, used=1.50GiB
    GlobalReserve, single: total=512.00MiB, used=0.00B

dmesg.log is attached (or http://sprunge.us/RKeM for journalctl output)

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