Re: [PATCH 1/1] timer: Warn about schedule_timeout() called for tasks in TASK_RUNNING state

[Alexander Popov]

On 16.01.2020 17:52, Steven Rostedt wrote:
> On Thu, 16 Jan 2020 17:02:18 +0300
> Alexander Popov <alex.popov@xxxxxxxxx> wrote:
> 
>> When we were preparing the patch 6dcd5d7a7a29c1e, we made a mistake noticed
>> by Linus: schedule_timeout() was called without setting the task state to
>> anything particular. It calls the scheduler, but doesn't delay anything,
>> because the task stays runnable. That happens because sched_submit_work()
>> does nothing for tasks in TASK_RUNNING state.
>>
>> Let's add a WARN_ONCE() under CONFIG_SCHED_DEBUG to detect such kernel
>> API misuse.
>>
>> Signed-off-by: Alexander Popov <alex.popov@xxxxxxxxx>
>> ---
>>  kernel/time/timer.c | 5 +++++
>>  1 file changed, 5 insertions(+)
>>
>> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
>> index 4820823515e9..52ad2d6ce352 100644
>> --- a/kernel/time/timer.c
>> +++ b/kernel/time/timer.c
>> @@ -1887,6 +1887,11 @@ signed long __sched schedule_timeout(signed long timeout)
>>  		}
>>  	}
>>  
>> +#ifdef CONFIG_SCHED_DEBUG
>> +	WARN_ONCE(current->state == TASK_RUNNING,
>> +			"schedule_timeout for TASK_RUNNING\n");
>> +#endif
>> +
> 
> But this can trigger false warnings. For example, if we are waiting on
> an event with a timeout:
> 
> 
> 	DEFINE_WAIT(wait);
> 
> 	for (;;) {
> 		prepare_to_wait(&waitq, &wait, TASK_UNINTERRUPTIBLE);
> 		if (event)
> 			break;
> 		timeout = schedule_timeout(timeout);
> 		if (!timeout)
> 			break;
> 	}
> 	finish_wait(&waitq, &wait);
> 
> 
> If the event happens between "prepare_to_wait" and just before
> schedule_timeout(), the wait queue will set this task's state to
> TASK_RUNNING, which in turn triggers your warning.

Steven, thanks for the explanation.

If I understand you right, it is the intended behavior of schedule_timeout() in
some sense.

So the best thing I can do here is adding an explanatory comment to the
schedule_timeout() description.

Maybe that would help against such situations:
https://lore.kernel.org/lkml/CAHk-=wgE-veRb7+mw9oMmsD97BLnL+q8Gxu0QRrK65S2yQfMdQ@xxxxxxxxxxxxxx/#t

I'll come with the patch soon.

Best regards,
Alexander