Attached is an updated patch that also updates the upgrade part. All boost::thread unit-tests still pass, and I think the changes makes shared_mutex easier to understand.

On first sight, it might appear like some of the upgrade-related methods becomes less efficient due to state_data members being modified several times redundantly (e.g. in try_unlock_shared_and_lock_upgrade), but that is something the compiler should be able to optimize away. Also, I might have introduced a slight change in behavior in unlock_upgrade_and_lock, due to state.upgrade now being reset before upgrade_cond.wait(lk) (as opposed to before), but my impression is that it shouldn't matter.

Please review the patch, and let me know if you have any questions.