Everybody, thank you very-very much!! Very helpful comments!!

I do not have root access and cant even cat /etc/sudoers.

After reading afoken's comment, I tried running "sudo umount" without providing a password and got "sudo password for www-data: sudo: pam_authenticate: Conversation error", which tells me that /etc/sudoers is currently not set up to allow umount from www-data with no password.

I will give the instructions to appropriate people to add www-data to /etc/sudoers with no pass.

If all else fails, i will go with pemungkah's suggestion to use a semaphore and umount via another script.

Again, thank you so much for all your answers!