work queue

use example: http://nano-chicken.blogspot.tw/2010/12/linux-modules73-work-queue.html

ref.1: http://blog.csdn.net/angle_birds/article/details/8448070

ref.2 : http://guojing.me/linux-kernel-architecture/posts/work-queue/

ref.3 : http://www.cnblogs.com/wwang/archive/2010/10/27/1862202.html