using kernel_thread function in a module
 

Hi,
I have written a device driver module which needs a thread. for implementing a thread I use kernel_thread function. it is compiled successfully, but when I want to load it with insmod, it says: unknown symbol kernel_thread. why? should I link my module in a especial way? why it doesn't know kernel_thread. kernel_thread is defined in process.c at kernel source.
please help!!