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Date:   Fri, 6 Aug 2021 13:20:29 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu>
Cc:     adilger.kernel@...ger.ca, johann@...mcloud.com,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+c9ff4822a62eee994ea3@...kaller.appspotmail.com
Subject: Re: [PATCH] ext4: avoid huge mmp update interval value

On 8/6/21 1:59 AM, Dave Chinner wrote:
> On Thu, Aug 05, 2021 at 11:12:42PM +0300, Pavel Skripkin wrote:
>> On 8/5/21 10:45 PM, Theodore Ts'o wrote:
>> > On Thu, Aug 05, 2021 at 06:14:18PM +0300, Pavel Skripkin wrote:
>> > > 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

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