nformation in this thread that mkdir is in fact atomic

I don't believe that atomicity of mkdir was ever in doubt as far as local filesystems are concerned.

The question, I thought, was about whether it remained atomic for NFS mounted directories.

My point in posting was -- on ancient HP/UX, where mkdir was tested and found atomic on NFS-mounted drives -- not just that that simple form of loop was sufficient to obtain a unique directory, but actually that it is far more likely to be reliable than the two-stage mechanisms that create files first, then directories.

Tendered with the clear caveat of the ancient state of my knowledge, I fail to see that creating a file -- just another name in the directory space -- is any more likely to be atomic than creating a directory.

But ... that is based on logic alone; not current, OS-specific knowledge.