You should have a look to this thread http://archives.postgresql.org/pgsql-hackers/2005-02/msg00263.php
Take a look at this paper about "lock-free parallel hash table" http://www.cs.rug.nl/~wim/mechver/hashtable/
Is this relevant? Hash indexes are on-disk data structures, so ISTM lock-free algorithms aren't really applicable.
(BTW, is poor concurrency really the biggest issue with hash indexes? If so, there is some low-hanging fruit that I noticed a few years ago, but never got around to fixing: _hash_doinsert() write-locks the hash metapage on every insertion merely to increment a tuple counter. This could be improved by just acquiring the lock with probability 1/k, and incrementing the counter k times -- or some similar statistical approximation. IMHO there are bigger issues with hash indexes, like the lack of WAL safety, the very slow index build times, and their relatively poor performance -- i.e. no better than btree for any workload I've seen. If someone wants to step up to the plate and fix some of that, I'll improve hash index concurrency -- any takers? :-) )
-Neil
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