memory barrier

参考链接
http://www.unixresources.net/linux/clf/linuxK/archive/00/00/65/37/653778.html#article653778

http://bbs.pediy.com/archive/index.php?t-84326.html

http://www.answers.com/topic/memory-barrier

http://lxr.linux.no/linux+v3.0.1/Documentation/memory-barriers.txt#L42