How would mkdir not be atomic? It is a logical operation that only encapsulates a single physical operation.

Rename is an interesting case, because it encapsulates two operations - a copy and a delete. Doing two operations in one step is interesting atomicity. Doing one operation in one step (mkdir) is not as interesting.

mkdir can fail atomicity if non-local storage is involved: http://www.mail-archive.com/freebsd-hackers@freebsd.org/msg2...

I still use it for the quick-and-dirty lockfile case despite this.