btrfs subvolume snapshot hung in btrfs_commit_transaction

["Ian! D. Allen" <idallen@idallen.ca>]

I've been exercising btrfs doing a continuous loop of:

    - delete an old snapshot to keep disk space about the same
    - create snapshot from previous snapshot
    - rsync root into new snapshot

I have room for 150 snapshots on disk.  I delete the oldest, create
the newest, do the rsync into the newest, repeat.  It hung today on
snapshot 564:

$ ps uww 24575
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root     24575  0.0  0.0   6224   332 pts/10   DN   07:35   0:00 btrfs subvolume snapshot /mnt/sde1/snap564 /mnt/sde1/snap565

$ ps lww 24575
F   UID   PID  PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
4     0 24575 27716  35   -   6224   332 btrfs_ DN   pts/10     0:00 btrfs subvolume snapshot /mnt/sde1/snap564 /mnt/sde1/snap565

$ ps -o wchan 24575
WCHAN
btrfs_commit_transaction

No messages in "dmesg" or kernel log.  Anyone want me to run some other
debug tests to find out what is wrong?  Anything that tries to access
anything inside the btrfs file system /dev/sde1 hangs uninterruptably:

1     0  1863     2  20   0      0     0 wait_f D    ?          0:29 [btrfs-transacti]
4     0  4933  4925  20   0  26524  2864 lookup D+   pts/10     0:02 /bin/bash
1   777 27995  7318  20   0  26576  1784 vfs_re D+   pts/52     0:00 bash
0   777 29395  7284  20   0  21856   688 vfs_re D    pts/51     0:00 ls -abp --color=auto /mnt/sde1
0   777 29510  7284  20   0  21856   692 vfs_re D    pts/51     0:00 /bin/ls /mnt/sde1

$ ps -o wchan 1863
WCHAN
wait_for_commit

$ ps -o wchan 27995
WCHAN
vfs_readdir

-- 
| Ian! D. Allen  -  idallen@idallen.ca  -  Ottawa, Ontario, Canada
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