quoted

+static void rtnl_print_btrace(struct timer_list *unused)
+{
+	pr_info("----------- %s start -----------\n", __func__);
+	pr_info("%s[%d][%c] hold rtnl_lock more than 2 sec, start time: %llu\n",
+		rtnl_instance.task->comm,
+		rtnl_instance.pid,
+		task_state_to_char(rtnl_instance.task),
+		rtnl_instance.start_time);
+	stack_trace_print(rtnl_instance.addrs, rtnl_instance.nr_entries, 0);

Do we want to print same traces every 2 seconds? 

Since it is possible to stall for e.g. 30 seconds, printing either only upon
first call to rtnl_print_btrace() for each stalling duration or only upon
end of stalling duration (i.e. from rtnl_relase_btrace()) is better?

Thanks for your review, Tetsuo.

rtnl_chk_timer is a one-shot timer, I don't rearm the timer after
expire, therefore traces are only logged once after rtnl_lock was held
for more than 2 seconds, rather than every 2 seconds. In this way, even
if this rtnl_lock debug mechanism is enabled, the overhead is also
affordable in the user load.

As far as I understand it, there won't be the problem you mentioned.

In my experience, it's more appropriate to print trace when
call rtnl_print_btrace(), we can't expect that rtnl_lock will be
released before kernel reboot. And after holding the rtnl_lock for more
than 2s, we will print these logs, which is more helpful for us.

quoted

+	show_stack(rtnl_instance.task, NULL, KERN_DEBUG);

Why KERN_DEBUG ?

If you retrieve the output via dmesg, KERN_DEBUG would be fine.
But for syzkaller (which counts on printk() messages being printed to
consoles), KERN_INFO (or default) is expected.

KERN_INFO is expected, I will fix it in patch v2.

quoted

+	pr_info("------------ %s end -----------\n", __func__);
+}
+
+static void rtnl_relase_btrace(void)
+{
+	rtnl_instance.end_time = sched_clock();
+

You should del_timer_sync() here than

quoted

+	if (rtnl_instance.end_time - rtnl_instance.start_time > 2000000000ULL) {
+		pr_info("rtnl_lock is held by [%d] from [%llu] to [%llu]\n",
+			rtnl_instance.pid,
+			rtnl_instance.start_time,
+			rtnl_instance.end_time);
+	}
+
+	del_timer(&rtnl_chk_timer);

here in order to make sure that end message is printed only after
rtnl_print_btrace() messages are printed.

Will have a separate patch for this.

quoted

+}
+#endif
+

Best Regards
Rocco

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