:"=&r"(val)
:"r"(1),"r"(lock):
"&" : Means that this operand is an earlyclobber operand, which is modified before the instruction is finished using the input operands. Therefore, this operand may not lie in a register that is used as an input operand or as part of any memory address.
可以理解为：'&'表示该输出操作（'&'只用于修饰输出）完成于其他输入操作完成之前。因此需要确保对应寄存器中的值不能被其他的输入操作篡改。（怎样确保呢？输入输出使用不同的寄存器就行了。实际上'&'约束就是这个意思。）
http://hi.baidu.com/_kouu/blog/item/d4430a81516d16de9123d9f9.html