I do not know why anyone would want to unmount /proc.
If memory serves me right from the good ole' days of OS classes:
/proc is a special filesystems that really is not a filesystem. /proc is the
memory image at any given moment. /proc is set up to LOOK like a real
filesystem so that useful utilities such as ps and other processes that
need current system information can do so in the usual *nix manner:
everything looks like a file to *nix.
Now if one were to unmount the current process running in core it will of
course tell you taht it is busy as it attempts to commit suicide and grant
your wish of unmounting all currently running process.
Well I hope this information is helpful to some.
Thankss for listening.