Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Mon, 30 Jan 2023 14:43:01 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Pietro Borrello <borrello@diag.uniroma1.it>
Cc:     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>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Dmitry Adamushko <dmitry.adamushko@gmail.com>,
        Cristiano Giuffrida <c.giuffrida@vu.nl>,
        "Bos, H.J." <h.j.bos@vu.nl>, Jakob Koschel <jkl820.git@gmail.com>,
        Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched: pick_next_rt_entity(): checked list_entry
Message-ID: <Y9fJZdMYbc4gRVJ+@gmail.com>
References: <20230128-list-entry-null-check-sched-v1-1-c93085ee0055@diag.uniroma1.it>
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* Pietro Borrello <borrello@diag.uniroma1.it> wrote:

> Commit 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
> removed any path which could make pick_next_rt_entity() return NULL.
> However, BUG_ON(!rt_se) in _pick_next_task_rt() (the only caller of
> pick_next_rt_entity()) still checks the error condition, which can
> never happen, since list_entry() never returns NULL. Return
> list_first_entry_or_null(queue, ...) to allow BUG to check the only
> possible error condition here: the queue being empty which should
> never happen.
> 
> Fixes: 326587b84078 ("sched: fix goto retry in pick_next_task_rt()")
> Signed-off-by: Pietro Borrello <borrello@diag.uniroma1.it>
> ---
>  kernel/sched/rt.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ed2a47e4ddae..6088d5d83b75 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1777,7 +1777,7 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rt_rq *rt_rq)
>  	BUG_ON(idx >= MAX_RT_PRIO);
>  
>  	queue = array->queue + idx;
> -	next = list_entry(queue->next, struct sched_rt_entity, run_list);
> +	next = list_first_entry_or_null(queue, struct sched_rt_entity, run_list);
>  
>  	return next;

I'd much rather have something like this:

	SCHED_WARN_ON(list_empty(next));

So the debug check is part of CONFIG_SCHED_DEBUG=y, and not if it's turned 
off. Also, this makes sure we don't crash (BUG_ON()) on the error 
condition, in the hope of getting debug info out of the system.

Thanks,

	Ingo