Messages in this thread | | | Subject | Re: [PATCH 1/1] timer: Warn about schedule_timeout() called for tasks in TASK_RUNNING state | From | Alexander Popov <> | Date | Fri, 17 Jan 2020 18:33:34 +0300 |
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On 16.01.2020 17:52, Steven Rostedt wrote: > On Thu, 16 Jan 2020 17:02:18 +0300 > Alexander Popov <alex.popov@linux.com> 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@linux.com> >> --- >> 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@mail.gmail.com/#t
I'll come with the patch soon.
Best regards, Alexander
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