lockless pagecache prep round 1

Hi List,

Following this are some prep patches from my lockless pagecache
patch stack, though they are nice patches that stand by themselves.
I would be interested in getting them merged soon, they have
survived quite a lot of stress testing here. Reviews and Acks from
interested parties would be helpful.

First is the generic atomic_cmpxchg stuff. These are really useful
primitives to have in general as can be seen by their subsequent
application. I've tried to do lots of compile testing, but if this
causes failures, then it is exposing bugs already in the code that
need fixing.

Second is some radix tree improvements and cleanups. This patchset
does introduce an "unused" radix tree API (lookup_slot), however
I thought it was appropriate to include this patch here because
there seem to be a number of users interested in this functionality
(lockless pagecache, reiser4, adaptive readahead), and I don't want
to see 3 different implementations!

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.

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