Although I have
[03:22:30 /tmp]# grep Post-Invoke /etc/apt/apt.conf
Post-Invoke {"mount -o remount,ro /usr";};
[03:22:48 /tmp]#
There are times that this remounting is not done because /usr is left busy
after apt-get dist-upgrade. There for, I wonder if and how lsof can be used in
order to find the offending files?