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Date:   Thu, 16 Jan 2020 09:52:20 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Alexander Popov <alex.popov@linux.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        John Stultz <john.stultz@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephen Boyd <sboyd@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org, notify@kernel.org
Subject: Re: [PATCH 1/1] timer: Warn about schedule_timeout() called for
 tasks in TASK_RUNNING state
Message-ID: <20200116095220.7368a604@gandalf.local.home>
In-Reply-To: <20200116140218.1328022-1-alex.popov@linux.com>
References: <20200116140218.1328022-1-alex.popov@linux.com>
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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.

-- Steve


>  	expire = timeout + jiffies;
>  
>  	timer.task = current;