Simplest open source database to make lock-free

What open source databases would be good candidates to reimplement as lock-free (or mostly so)?

I was looking at tdb (trivial database). It's used in Samba. There's a standalone version on sourceforge but it's back level. The current version in Samba has write transactions which will make converting to lock-free a little more interesting than I had orginally anticipated but still doable.

One of the things I don't like about tdb is it uses flocks for locking which implies processes instead of threads within a process. I'd have to rework some lock-free algorithms to work with processes rather than threads.

Also it's hard to tell what all the performance sensitive paths are. tdb has a way to iterate through the entire database. I can make that lock-free but I have no way to tell if doing queries in tdb that way are common enough to warrant the effort.

Anyway I was wondering if there are better candidates out there for this sort of thing.

-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.