On 8/6/21 1:59 AM, Dave Chinner wrote:

On Thu, Aug 05, 2021 at 11:12:42PM +0300, Pavel Skripkin wrote:

quoted

On 8/5/21 10:45 PM, Theodore Ts'o wrote:

quoted

On Thu, Aug 05, 2021 at 06:14:18PM +0300, Pavel Skripkin wrote:

quoted

Syzbot reported task hung bug in ext4_fill_super(). The problem was in
too huge mmp update interval.

Syzkaller reproducer setted s_mmp_update_interval to 39785 seconds. This
update interaval is unreasonable huge and it can cause tasks to hung on
kthread_stop() call, since it will wait until timeout timer expires.

I must be missing something.  kthread_stop() should wake up the
kmmpd() thread, which should see kthread_should_stop(), and then it
should exit.  What is causing it to wait until the timeout timer
expires?

					- Ted

Hi, Ted!

I guess, I've explained my idea badly, sorry :)

I mean, that there is a chance to hit this situation:

CPU0				CPU1
				kthread_should_stop()  <-- false
kthread_stop()
set_bit(KTHREAD_SHOULD_STOP)				
wake_up_process()
wait_for_completion()
				schedule_timeout_interruptible()

*waits until timer expires*

Yeah, so the bug here is checking kthread_should_stop() while
the task state is TASK_RUNNING.

What you need to do here is:

while (run) {

	....
	set_current_state(TASK_INTERRUPTIBLE);
	if (kthread_should_stop()) {
		__set_current_state(TASK_RUNNING);
		break;
	}
	schedule_timeout(tout);

	.....
}


That means in the case above where schedule() occurs after the
kthread_should_stop() check has raced with kthread_stop(), then
wake_up_process() will handle any races with schedule() correctly.

i.e.  wake_up_process() will set the task state to TASK_RUNNING and
schedule() will not sleep if it is called after wake_up_process().
Or if schedule() runs first then wake_up_process() will wake it
correctly after setting the state to TASK_RUNNING.

Either way, the loop then runs around again straight away to the next
kthread_should_stop() call, at which point it breaks out.

I note that the "wait_to_exit:" code in the same function does this
properly....

Hi, Dave!

I've tested your suggestion with syzbot and it works, thank you!


Anyway, @Ted, does it make sense to add boundaries for 
s_mmp_update_interval? I think, too big update interval for mmp isn't 
reasonable. I can send patch series with Dave's suggestion and previous 
patch. What do you think?




With regards,
Pavel Skripkin