linux-kernel - Re: [rfcomm_run] WARNING: CPU: 1 PID: 79 at kernel/sched/core.c:7156 __might

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Date:	Thu, 02 Oct 2014 09:49:04 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Fengguang Wu <fengguang.wu@...el.com>
CC:	Jet Chen <jet.chen@...el.com>, Su Tao <tao.su@...el.com>,
	Yuanhan Liu <yuanhan.liu@...el.com>, LKP <lkp@...org>,
	linux-kernel@...r.kernel.org,
	Marcel Holtmann <marcel@...tmann.org>, oleg@...hat.com
Subject: Re: [rfcomm_run] WARNING: CPU: 1 PID: 79 at kernel/sched/core.c:7156
 __might_sleep()

On 10/02/2014 08:42 AM, Peter Zijlstra wrote:
> On Thu, Oct 02, 2014 at 02:31:50PM +0200, Peter Zijlstra wrote:
>> @@ -2086,24 +2086,22 @@ static void rfcomm_kill_listener(void)
>>  
>>  static int rfcomm_run(void *unused)
>>  {
>> +	DEFINE_WAIT_FUNC(wait, woken_wake_function);
>>  	BT_DBG("");
>>  
>>  	set_user_nice(current, -10);
>>  
>>  	rfcomm_add_listener(BDADDR_ANY);
>>  
>> -	while (1) {
>> -		set_current_state(TASK_INTERRUPTIBLE);
>> -
>> -		if (kthread_should_stop())
>> -			break;
>> +	add_wait_queue(&rfcomm_wq, &wait);
>> +	while (!kthread_should_stop()) {
>>  
>>  		/* Process stuff */
>>  		rfcomm_process_sessions();
>>  
>> -		schedule();
>> +		wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
>>  	}
>> -	__set_current_state(TASK_RUNNING);
>> +	remove_wait_queue(&rfcomm_wq, &wait);
>>  
>>  	rfcomm_kill_listener();
>>  
> 
> Hmm, I think there's a problem there. If someone were to do
> kthread_stop() before wait_woken() we'd not actually stop, because
> wait_woken() doesn't test KTHREAD_SHOULD_STOP before calling schedule().

Do you mean this situation?

CPU 0                                    | CPU 1
                                         |
rfcomm_run()                             | kthread_stop()
  ...                                    |
  if (!test_bit(KTHREAD_SHOULD_STOP))    |
                                         |   set_bit(KTHREAD_SHOULD_STOP)
                                         |   wake_up_process()
    wait_woken()                         |   wait_for_completion()
      set_current_state(INTERRUPTIBLE)   |
      if (!WQ_FLAG_WOKEN)                |
        schedule_timeout()               |
                                         |

Now both tasks are sleeping forever.

If yes, then wakeups from signals don't work either, right?

Regards,
Peter Hurley
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