ubuntu@ubuntu:~$ uname -r
4.15.0-56-generic
ubuntu@ubuntu:~$ cat /proc/version
Linux version 4.15.0-56-generic (arighi@kathleen) (gcc version 7.4.0 (Ubuntu 7.4.0-1ubuntu1~18.04.1)) #62~lp1796292 SMP Thu Aug 1 07:45:21 UTC 2019

This failed on the second install while running bcache-super-show /dev/vdg
Hung task.

[  259.150347] bcache: run_cache_set() invalidating existing data
[  259.158038] bcache: register_cache() registered cache device nvme1n1p2
[  259.251093] bcache: register_bdev() registered backing device vdg
[  259.379809] bcache: bch_cached_dev_attach() Caching vdg as bcache3 on set 084505ad-5f6c-4666-9e3e-4f1650e8b015
[  259.411486] bcache: register_bcache() error /dev/vdg: device already registered (emitting change event)
[  259.537070] bcache: register_bcache() error /dev/vdg: device already registered (emitting change event)
[  259.797830] bcache: register_bcache() error /dev/vdg: device already registered (emitting change event)
[  259.900392] bcache: register_bcache() error /dev/vdg: device already registered (emitting change event)
[  271.682662] md: md0: resync done.
[  484.525529] INFO: task python3:11257 blocked for more than 120 seconds.
[  484.528933]       Tainted: P           O     4.15.0-56-generic #62~lp1796292
[  484.532221] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  484.535936] python3         D    0 11257   7974 0x00000000
[  484.535941] Call Trace:
[  484.535952]  __schedule+0x291/0x8a0
[  484.535957]  schedule+0x2c/0x80
[  484.535977]  bch_bucket_alloc+0x1fa/0x350 [bcache]
[  484.535984]  ? wait_woken+0x80/0x80
[  484.535993]  __bch_bucket_alloc_set+0xfe/0x150 [bcache]
[  484.536002]  bch_bucket_alloc_set+0x4e/0x70 [bcache]
[  484.536014]  __uuid_write+0x59/0x150 [bcache]
[  484.536025]  ? __write_super+0x137/0x170 [bcache]
[  484.536035]  bch_uuid_write+0x16/0x40 [bcache]
[  484.536046]  __cached_dev_store+0x1d8/0x8a0 [bcache]
[  484.536057]  bch_cached_dev_store+0x39/0xc0 [bcache]
[  484.536061]  sysfs_kf_write+0x3c/0x50
[  484.536064]  kernfs_fop_write+0x125/0x1a0
[  484.536069]  __vfs_write+0x1b/0x40
[  484.536071]  vfs_write+0xb1/0x1a0
[  484.536075]  SyS_write+0x5c/0xe0
[  484.536081]  do_syscall_64+0x73/0x130
[  484.536085]  entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[  484.536088] RIP: 0033:0x7f2ac7aa7154
[  484.536090] RSP: 002b:00007ffff157b628 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[  484.536093] RAX: ffffffffffffffda RBX: 0000000000000008 RCX: 00007f2ac7aa7154
[  484.536095] RDX: 0000000000000008 RSI: 0000000001c96340 RDI: 0000000000000003
[  484.536096] RBP: 00007f2ac7fa26c0 R08: 0000000000000000 R09: 0000000000000000
[  484.536098] R10: 0000000000000100 R11: 0000000000000246 R12: 0000000000000003
[  484.536099] R13: 0000000000000000 R14: 0000000001c96340 R15: 00000000019c7db0


I'll note that, I can run bcache-super-show on the device while this was hung.

$ sudo bash
root@ubuntu:~# bcache-super-show /dev/vdg 
sb.magic                ok
sb.first_sector         8 [match]
sb.csum                 C71A896B52F1C486 [match]
sb.version              1 [backing device]

dev.label               osddata5
dev.uuid                11df8370-fb64-4bb0-8171-dadabb47f6b1
dev.sectors_per_block   1
dev.sectors_per_bucket  1024
dev.data.first_sector   16
dev.data.cache_mode     1 [writeback]
dev.data.cache_state    1 [clean]

cset.uuid               084505ad-5f6c-4666-9e3e-4f1650e8b015