I have a number of machines that have a /usr partition mounted read-only upon startup. In order to apply patches to them I have to remount the partition read-write. After applying patches I then wish to remount read-only. This final step causes the system to become unresponsive.
# mount -o rw -u /usr
# freebsd-update fetch
# freebsd-update install
# mount -o ro -u /usr
My ssh session ceases to respond and I am unable to establish a new ssh session. Breaking out using Ctrl+Z and putting the command into the background does not allow me to use the existing session.
However, remotely, the machine can still be pinged.
Try to obtain to a kernel stack trace of the mount process by using procstat -kk <pid>.