Somewhere in the mists of time, I arranged that my /usr partition be mounted read-only. On the occasions when I need to update something on /usr, I have a script that does "mount -u -o rw /usr; <the command from the script arguments>; mount -u -o ro /usr". Until a few years ago, this worked without issue. Then, the "mount -o ro" started--sometimes--failing with "device busy". (I don't recall which kernel version.) But there are no files opened for write on /usr (according to fstat) and the data in the file system itself doesn't change. I have not attempted to add -f to the "mount -u -o ro" because I have no idea what I might break. Any suggestions as to how I might track down what's going on before I issue what might be a fatal command?