As it is common with all systems, the guiding principle for making such decisions will be "make the common case fast".
-- JaeWoong Chung, Hassan Chafi, Austen McDonald, Chi Cao Minh, Brian D. Carlstrom, Christos Kozyrakis, and Kunle Olukotun, The Common Case Transactional Behavior of Multithreaded Programs, Proceedings of the 12th Intl. Conference on High Performance Computer Architecture, February 2006