Hi Mauricio,

thanks for your support. I have attached the requested information to
the bug. I had to pseudomize some of the logical volume names because
they are considered sensitive, but I did so in a consistent way so the
structure should still be clear.

So that you don’t have to look through all details, here is a rough
overview:

There are six physical disks: 4 HDDs (sda, sdb, sdc, sdd) and 2 SSDs
(sde, sdf).

There are three DM RAID devices:

- md0 is a RAID 1 that uses /dev/sda1, /dev/sdb1, /dev/sdc1, and /dev/sdd1. 
This device stores the boot partition.
- md1 is a RAID 6 that uses /dev/sda2, /dev/sdb2, /dev/sdc2, and /dev/sdd2. 
This device is the only physical volume backing volume group vg0.
- md2 is a RAID 1 that uses /dev/sde1 and /dev/sdf1. This device is used as the 
cache device (read/write) for Bcache.

There are three LVM volume groups:

- vg0 is backed by /dev/md0 and stores the root FS, the swap partition, a few 
LVs that are used for VMs, and an LV called vg2-backend, that is used by Bcache.
- vg1 is backed by /dev/md2 and only has an LV called vg2-cache that is used by 
Bcache.
- vg2 is backed by /dev/bcache0. It only contains LVs that are used for VMs.

There is one Bcache device:

- bcache0 is backed by /dev/vg0/vg2-backend (backend device) and
/dev/vg1/vg2-cache (cache device). This device is used as the PV for
vg2.

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https://bugs.launchpad.net/bugs/1867916

Title:
  Regression in kernel 4.15.0-91 causes kernel panic with Bcache

Status in linux package in Ubuntu:
  Confirmed

Bug description:
  After upgrading from kernel 4.15.0-88 to 4.15.0-91 one of our systems
  does not boot any longer. It always crashes during boot with a kernel
  panic.

  I suspect that this crash might be related to Bcache because this is
  the only one of our systems where we use Bcache and the kernel panic
  appears right after Bcache initialization.

  I already checked that this bug still exists in the 4.15.0-92.93
  kernel from proposed.

  Unfortunately, I cannot do a bisect because this is a critical
  production system and we do not have any other system with a similar
  configuration.

  I attached a screenshot with the trace of the kernel panic.

  The last message that appears before the kernel panic (or rather the
  last one that I can see - there is a rather long pause between that
  message and the panic and I cannot scroll up far enough to ensure that
  there are no other messages in between) is:

  bcache: register_bcache() error /dev/dm-0: device already registered

  When booting with kernel 4.15.0-88 that does not have this problem,
  the next message is

  bcache: register_bcache() error /dev/dm-12: device already registered
  (emitting change event)

  After that the next message is:

  Begin: Loading essential drivers ... done

  This message also appears after the kernel panic, but the boot process
  stalls and the system can only be recovered by doing a hardware reset.

  ProblemType: Bug
  DistroRelease: Ubuntu 18.04
  Package: linux-image-4.15.0-88-generic 4.15.0-88.88
  ProcVersionSignature: Ubuntu 4.15.0-88.88-generic 4.15.18
  Uname: Linux 4.15.0-88-generic x86_64
  AlsaDevices:
   total 0
   crw-rw---- 1 root audio 116,  1 Mar 17 21:08 seq
   crw-rw---- 1 root audio 116, 33 Mar 17 21:08 timer
  AplayDevices: Error: [Errno 2] No such file or directory: 'aplay': 'aplay'
  ApportVersion: 2.20.9-0ubuntu7.11
  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:
  Date: Wed Mar 18 12:55:18 2020
  HibernationDevice: RESUME=UUID=40512ea2-9fce-40f5-8362-5daf955cc26a
  InstallationDate: Installed on 2013-07-02 (2450 days ago)
  InstallationMedia: Ubuntu-Server 12.04.2 LTS "Precise Pangolin" - Release 
amd64 (20130214)
  MachineType: HP ProLiant DL160 G6
  PciMultimedia:
   
  ProcFB: 0 mgadrmfb
  ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.15.0-88-generic 
root=/dev/mapper/vg0-root ro nosmt nomdmonddf nomdmonisw nomdmonddf nomdmonisw 
nomdmonddf nomdmonisw nomdmonddf nomdmonisw nomdmonddf nomdmonisw
  RelatedPackageVersions:
   linux-restricted-modules-4.15.0-88-generic N/A
   linux-backports-modules-4.15.0-88-generic  N/A
   linux-firmware                             1.173.16
  RfKill: Error: [Errno 2] No such file or directory: 'rfkill': 'rfkill'
  SourcePackage: linux
  UpgradeStatus: Upgraded to bionic on 2018-09-23 (541 days ago)
  dmi.bios.date: 11/06/2009
  dmi.bios.vendor: HP
  dmi.bios.version: O33
  dmi.chassis.asset.tag: 0191525
  dmi.chassis.type: 23
  dmi.chassis.vendor: HP
  dmi.modalias: 
dmi:bvnHP:bvrO33:bd11/06/2009:svnHP:pnProLiantDL160G6:pvr:cvnHP:ct23:cvr:
  dmi.product.name: ProLiant DL160 G6
  dmi.sys.vendor: HP

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