Comment 0 for bug 1784665

$ cat /proc/version_signature
Ubuntu 4.15.0-29.31-generic 4.15.18

$ lsb_release -rd
Description: Ubuntu Cosmic Cuttlefish (development branch)
Release: 18.10

$ apt-cache policy linux-image-`uname -r`
linux-image-4.15.0-29-generic:
  Installed: 4.15.0-29.31
  Candidate: 4.15.0-29.31
  Version table:
 *** 4.15.0-29.31 500
        500 http://archive.ubuntu.com/ubuntu cosmic/main amd64 Packages
        100 /var/lib/dpkg/status

3) mkfs.ext4 /dev/bcache0 returns successful creating an ext4 filesystem on top of a bcache device

4) mkfs.ext4 doesn't return and kernel prints hung process info

[ 58.018099] cloud-init[920]: Running command ['mkfs.ext4', '-F', '-L', 'root-fs', '-U', 'f01aec97-9457-11e8-b8d6-525400123401', '/dev/bcache0'] with allowed return codes [0] (capture=True)
[ 242.652018] INFO: task kworker/u4:0:5 blocked for more than 120 seconds.
[ 242.653767] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 242.655391] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 242.657397] INFO: task kworker/0:2:410 blocked for more than 120 seconds.
[ 242.659126] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 242.660980] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 242.663000] INFO: task bcache_allocato:2326 blocked for more than 120 seconds.
[ 242.664807] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 242.666516] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 242.668503] INFO: task bcache_writebac:2345 blocked for more than 120 seconds.
[ 242.670301] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 242.671936] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 242.673909] INFO: task mkfs.ext4:2803 blocked for more than 120 seconds.
[ 242.675414] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 242.677038] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 363.483998] INFO: task kworker/u4:0:5 blocked for more than 120 seconds.
[ 363.488441] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 363.489598] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 363.491043] INFO: task kworker/0:2:410 blocked for more than 120 seconds.
[ 363.492252] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 363.494085] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 363.495659] INFO: task bcache_allocato:2326 blocked for more than 120 seconds.
[ 363.496957] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 363.498454] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 363.499866] INFO: task bcache_writebac:2345 blocked for more than 120 seconds.
[ 363.501156] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 363.502597] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[ 363.504048] INFO: task mkfs.ext4:2803 blocked for more than 120 seconds.
[ 363.505505] Tainted: P O 4.15.0-29-generic #31-Ubuntu
[ 363.506677] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.

System has two virtio block devices. bcache was created like so:

make-bcache -C /dev/vdb
make-bcache -B /dev/vda2

resulting in /dev/bcache0

ProblemType: Bug
DistroRelease: Ubuntu 18.10
Package: linux-image-4.15.0-29-generic 4.15.0-29.31
ProcVersionSignature: User Name 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
AlsaDevices:
 total 0
 crw-rw---- 1 root audio 116, 1 Jul 31 15:52 seq
 crw-rw---- 1 root audio 116, 33 Jul 31 15:52 timer
AplayDevices: Error: [Errno 2] No such file or directory: 'aplay': 'aplay'
ApportVersion: 2.20.10-0ubuntu7
Architecture: amd64
ArecordDevices: Error: [Errno 2] No such file or directory: 'arecord': 'arecord'
AudioDevicesInUse: Error: command ['fuser', '-v', '/dev/snd/seq', '/dev/snd/timer'] failed with exit code 1:
CRDA: N/A
Date: Tue Jul 31 15:53:56 2018
IwConfig: Error: [Errno 2] No such file or directory: 'iwconfig': 'iwconfig'
Lsusb: Bus 001 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
PciMultimedia:

ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
ProcFB:

ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-29-generic root=LABEL=cloudimg-rootfs ro console=tty1 console=ttyS0
RelatedPackageVersions:
 linux-restricted-modules-4.15.0-29-generic N/A
 linux-backports-modules-4.15.0-29-generic N/A
 linux-firmware N/A
RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill'
SourcePackage: linux
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: 1.11.1-1
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-i440fx-bionic
dmi.modalias: dmi:bvnSeaBIOS:bvr1.11.1-1:bd04/01/2014:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-bionic:cvnQEMU:ct1:cvrpc-i440fx-bionic:
dmi.product.name: Standard PC (i440FX + PIIX, 1996)
dmi.product.version: pc-i440fx-bionic
dmi.sys.vendor: QEMU