systemd trying to unmount non-existent file system

[Charles Meo]

This output is copied from here: https://github.com/systemd/systemd/issues/20516
because I have not been able to capture the error messages. However what I am seeing is identical.

sd-umoun[2406]: Failed to unmount /oldroot: Device or resource busy
 sd-umoun[2407]: Failed to unmount /oldroot/dev/pts: Device or resource busy
 sd-umoun[2408]: Failed to unmount /oldroot/dev: Device or resource busy
 shutdown[1]: Failed to finalize file systems, ignoring

This has happened consistently since I upgraded to Ubuntu 20.04 two weeks ago, and is really annoying. Is there an actual problem or is it just noise that can be safely ignored?

The only other thing referencing oldroot in its config was snap, which I have completely uninstalled to isolate the problem.

/lib/systemd/systemd-shutdown also contains this string:
# strings systemd-shutdown |grep old
/oldroot
sigprocmask(SIG_BLOCK, &mask, &oldmask) == 0
sigprocmask(SIG_SETMASK, &oldmask, NULL) == 0

I have never had an /oldroot mount and there isn't one now. I gather it is something the upgrade process might create, or it could perhaps be something broken in finalrd.

Bin Li, who posted the above error to the systemd bug portal, got short shrift there and was told to contact the distro. So I am, as my version of systemd is 245, same as his. This problem is 100% repeateable and I see it every time I shutdown or restart.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: systemd 245.4-4ubuntu3.11
ProcVersionSignature: Ubuntu 5.4.0-81.91-generic 5.4.128
Uname: Linux 5.4.0-81-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.18
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: Unity:Unity7:ubuntu
Date: Sat Aug 28 14:28:03 2021
InstallationDate: Installed on 2017-05-26 (1554 days ago)
InstallationMedia: Ubuntu-Server 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
Lsusb:
 Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
 Bus 001 Device 004: ID 5986:0708 Acer, Inc Integrated Camera
 Bus 001 Device 002: ID 046d:c077 Logitech, Inc. M105 Optical Mouse
 Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Lsusb-t:
 /: Bus 02.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/6p, 5000M
 /: Bus 01.Port 1: Dev 1, Class=root_hub, Driver=xhci_hcd/12p, 480M
     |__ Port 3: Dev 2, If 0, Class=Human Interface Device, Driver=usbhid, 1.5M
     |__ Port 7: Dev 4, If 0, Class=Video, Driver=uvcvideo, 480M
     |__ Port 7: Dev 4, If 1, Class=Video, Driver=uvcvideo, 480M
MachineType: LENOVO 20EVCTO1WW
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-5.4.0-81-generic root=UUID=3b7777e3-50c9-41cc-94b5-eab06cff1d17 ro quiet splash
SourcePackage: systemd
UpgradeStatus: Upgraded to focal on 2021-08-05 (22 days ago)
dmi.bios.date: 12/01/2016
dmi.bios.vendor: LENOVO
dmi.bios.version: R00ET49W (1.24 )
dmi.board.asset.tag: Not Available
dmi.board.name: 20EVCTO1WW
dmi.board.vendor: LENOVO
dmi.board.version: SDK0J40709 WIN
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: None
dmi.modalias: dmi:bvnLENOVO:bvrR00ET49W(1.24):bd12/01/2016:svnLENOVO:pn20EVCTO1WW:pvrThinkPadE560:rvnLENOVO:rn20EVCTO1WW:rvrSDK0J40709WIN:cvnLENOVO:ct10:cvrNone:
dmi.product.family: ThinkPad E560
dmi.product.name: 20EVCTO1WW
dmi.product.sku: LENOVO_MT_20EV_BU_Think_FM_ThinkPad E560
dmi.product.version: ThinkPad E560
dmi.sys.vendor: LENOVO

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.