内核原有函数如printk可以设置断点，但调试自己写的模块时一直设置不成功，求大神指教

环境：
系统：CentOS6.4
调试内核：CentOS6.4内核源码
VMware+串口调试

操作如下：
(gdb) add-symbol-file /root/hellomod/hellomod.ko 0xf7e8c000
add symbol table from file "/root/hellomod/hellomod.ko" at
.text_addr = 0xf7e8c000
(y or n) y
Reading symbols from /root/hellomod/hellomod.ko...done.

(gdb) b hello_exit
Breakpoint 5 at 0x24: file /root/hellomod/hellomod.c, line 20.
(gdb) c
Continuing.
Warning:
Cannot insert breakpoint 5.
Error accessing memory address 0x24: Unknown error 4294967295.