in kernel/process.c [131:149]
unsigned long __get_wchan(struct task_struct *p)
{
unsigned long fp, pc;
unsigned long stack_page;
int count = 0;
stack_page = (unsigned long)p;
fp = ((struct pt_regs *)p->thread.ksp)->er6;
do {
if (fp < stack_page+sizeof(struct thread_info) ||
fp >= 8184+stack_page)
return 0;
pc = ((unsigned long *)fp)[1];
if (!in_sched_functions(pc))
return pc;
fp = *(unsigned long *) fp;
} while (count++ < 16);
return 0;
}