双重锁：
http://www.cs.wustl.edu/~schmidt/PDF/DC-Locking.pdf
ACCU上的讨论：
http://accu.org/index.php/journals/1328
http://accu.org/index.php/journals/1306
ACE与Boost中singleton的不同实现：
http://www.cppblog.com/Fox/archive/2009/09/22/96898.html