[Bug 1928200] [NEW] Prompt error message "Failed to unmount /oldroot" when shutdown or reboot

[Bin Li]

Public bug reported:

On ubuntu 20.04, with latest kernel and systemd, it will show error on
ThinkPad X1.

sd-umount[1334]: Failed to unmount /oldroot: Device or resource busy

systemd 245.4-4ubuntu3.6 
kernel: 5.8.0-53

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New

** Attachment added: "error.jpg"
   https://bugs.launchpad.net/bugs/1928200/+attachment/5496620/+files/error.jpg

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Title:
  Prompt error message "Failed to unmount /oldroot" when shutdown or
  reboot

Status in systemd package in Ubuntu:
  New

Bug description:
  On ubuntu 20.04, with latest kernel and systemd, it will show error on
  ThinkPad X1.

  sd-umount[1334]: Failed to unmount /oldroot: Device or resource busy

  systemd 245.4-4ubuntu3.6 
  kernel: 5.8.0-53

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