So the SGI owned code, that I don't have, did page level locking. There two kinds of locks, rd/wr on the directory, and rd/wr on a page. If you are inserting a key you get a read lock on the directory and a write lock on the page. If it fits in the page then you are done. So you can have lots of concurrent writers until a page is full and you have to split it. Bob Mende did that work I think, you might track him down for details.