User-space RCU

FWIW, I found the original non-RCU bonsai trees a little hard to google for (you keep getting stuff about arboriculture, imagine that!).

The original paper seems to be <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1....>. Basically a pointer-compressed tree with child pointers encoded with respect to their parents (making interior node removals and possibly some tree rotations expensive, and insertions relatively expensive compared to lookups). Probably more interesting now than when it was written in '93, since pointers have doubled in size...

As with many hash-based schemes, it looks rather dcache-unfriendly. I guess we need to mix it up with the cache-friendliness of Judy arrays somehow...

I'm not sure about the notation used, though (probably just my ignorance showing, heck it's probably something I've seen many times and then forgotten just as often). wtf is 'η' when it's at home?