There is patch from Google to improve SMP performance , but for some workloads it showed for us reverse scalability.
E.g. update_key benchmark from sysbench. There are also results with Yasufumi's rw_locks (http://bugs.mysql.com/bug.php?id=26442)
results are in tps, more are better
(result in tps, more is better)
The benchmark was made on 8cores box, and as you see for 4+ threads we have worse results than for 2 threads. That is was main reason why we did not