Howard Hinnant wrote: ... void read_write(rw_mutex& m, which_t w) { sharable_lock<rw_mutex, true> read_lock(m); // May need access to many releated which_t's if (compute_expensive_result(w)) { // Upgrade (blocks upcoming readers while pending) scoped_lock<rw_mutex> write_lock(upgrade(read_lock)); if (read_lock.atomic_upgrade()) { modify_state(w); if (write_lock.upgrade_pending()) register_change(w); } else if (!computation_invalidated(w) || // check registry compute_expensive_result(w)) { modify_state(w); write_lock.upgrade_pending() ? register_change(w) : clear_registry(); } else if (!write_lock.upgrade_pending()) { clear_registry(); } } } Oder (verbosity aside for a moment)? regards, alexander.