Comment 8 for bug 1261730

In 14.10 amd64, mountall keeps running, causing init to keep /var/log/upstart/mountall.log open for writing, which prevents root from being remounted read-only.

These things are easy to examine by adding a /bin/sh line to /etc/init.d/reboot in do_stop() before "reboot -d -f -i". Then when you reboot you can switch to virtual console 1 and use lsof and other commands to explore what is going on. For example this is the line in lsof output showing the problem:
init 1 root 8w REG 8,5 613 1062101 /var/log/upstart/mountall.log

I added a
/usr/bin/killall mountall
line to /etc/init.d/umountroot before root is supposed to be remounted read-only. That seems to fix the problem. You need the full path because /usr/bin isn't in PATH, and don't use -9.