__builtin_return_address && _RET_IP_

　　以前看互斥锁自旋锁实现的时候，看过RET_IP，只是了解到其为当前函数的返回地址；今天系统的看下这几个宏

• __builtin_return_address(0)的含义是，得到当前函数返回地址，即此函数被别的函数调用返回时的地址。
• __builtin_return_address(1)的含义是，得到当前函数的调用者的返回地址。

#define _RET_IP_ (unsigned long)__builtin_return_address(0)
#define _THIS_IP_ ({ __label__ __here; __here: (unsigned long)&&__here; })

/* All archs should have this, but we define it for consistency */
#ifndef ftrace_return_address0
# define ftrace_return_address0 __builtin_return_address(0)
#endif

/* Archs may use other ways for ADDR1 and beyond */
#ifndef ftrace_return_address
# ifdef CONFIG_FRAME_POINTER
#  define ftrace_return_address(n) __builtin_return_address(n)
# else
#  define ftrace_return_address(n) 0UL
# endif
#endif

#define CALLER_ADDR0 ((unsigned long)ftrace_return_address0)
#define CALLER_ADDR1 ((unsigned long)ftrace_return_address(1))
#define CALLER_ADDR2 ((unsigned long)ftrace_return_address(2))
#define CALLER_ADDR3 ((unsigned long)ftrace_return_address(3))
#define CALLER_ADDR4 ((unsigned long)ftrace_return_address(4))
#define CALLER_ADDR5 ((unsigned long)ftrace_return_address(5))
#define CALLER_ADDR6 ((unsigned long)ftrace_return_address(6))

_THIS_IP_ ：当前进程的地址；可以看到其实现使用了label 以及&& 

& in && is necessary to make GCC lookup the name as a label, instead of as a variable.

{ __label__ __here;
 __here: 
(unsigned long) &&__here; 
}

CALLER_ADDR5   __builtin_return_address (5) 这是什么鬼？

 　　实际上内建函数 __builtin_return_address返回当前函数或其调用者的返回地址，参数LEVEL指定在栈上搜索farme栈的个数 ；

其具体需要编写代码试一下