[HELP] tmp/work/debian-bullseye-arm64/isar-image-base-qemuarm64/1.0-r0/mnt/rootfs: target is busy

["cedric.hombourger@siemens.com" <cedric.hombourger@siemens.com>]

Hello,

Seeing some sporadic failures with the test suite (but also with builds
of our Isar-based product) when rootfs_install_sstate_prepare gets
executed:

DEBUG: Executing shell function rootfs_install_sstate_prepare
umount: /home/sutlej/isar/build/tmp/work/debian-bullseye-arm64/isar-
image-base-qemuarm64/1.0-r0/mnt/rootfs: target is busy.
WARNING: exit code 32 from a shell command. 

What's special about the machine I am running the builds on is that
/proc/cpuinfo reports 64 processors hence builds get massively
parallelized

I wasn't able to get to the bottom of this issue and understand why the
bind mount is busy since rootfs_install_sstate_prepare creates, uses
and removes that bind mount in the same function.

As a work-around, a lazy umount could be used but that annoys me as I'd
like to understand what could cause this. Any ideas?

Cedric