unsigned long get_wchan(struct task_struct *p)
{
	unsigned long start, bottom, top, sp, fp, ip, ret = 0;
	int count = 0;

	if (p == current || p->state == TASK_RUNNING)
		return 0;

	if (!try_get_task_stack(p))
		return 0;

	start = (unsigned long)task_stack_page(p);
	if (!start)
		goto out;

	/*
	 * Layout of the stack page:
	 *
	 * ----------- topmax = start + THREAD_SIZE - sizeof(unsigned long)
	 * PADDING
	 * ----------- top = topmax - TOP_OF_KERNEL_STACK_PADDING
	 * stack
	 * ----------- bottom = start
	 *
	 * The tasks stack pointer points at the location where the
	 * framepointer is stored. The data on the stack is:
	 * ... IP FP ... IP FP
	 *
	 * We need to read FP and IP, so we need to adjust the upper
	 * bound by another unsigned long.
	 */
	top = start + THREAD_SIZE - TOP_OF_KERNEL_STACK_PADDING;
	top -= 2 * sizeof(unsigned long);
	bottom = start;

	sp = READ_ONCE(p->thread.sp);
	if (sp < bottom || sp > top)
		goto out;

	fp = READ_ONCE_NOCHECK(((struct inactive_task_frame *)sp)->bp);
	do {
		if (fp < bottom || fp > top)
			goto out;
		ip = READ_ONCE_NOCHECK(*(unsigned long *)(fp + sizeof(unsigned long)));
		if (!in_sched_functions(ip)) {
			ret = ip;
			goto out;
		}
		fp = READ_ONCE_NOCHECK(*(unsigned long *)fp);
	} while (count++ < 16 && p->state != TASK_RUNNING);

out:
	put_task_stack(p);
	return ret;
}
Unknown