Date:   Tue, 7 Feb 2023 07:29:43 -0500
From:   Phil Auld <pauld@redhat.com>
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 v3] sched: pick_next_rt_entity(): check list_entry
Message-ID: <Y+JEN3/iEPEn6JZq@lorien.usersys.redhat.com>
References: <20230128-list-entry-null-check-sched-v3-1-b1a71bd1ac6b@diag.uniroma1.it>
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In-Reply-To: <20230128-list-entry-null-check-sched-v3-1-b1a71bd1ac6b@diag.uniroma1.it>
Precedence: bulk

Hi Pietro,

On Mon, Feb 06, 2023 at 10:33:54PM +0000 Pietro Borrello 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.
> Remove the BUG_ON check, and instead emit a warning in 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>
> ---
> Changes in v3:
> - return NULL if the list is empty
> - Link to v2: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v2-1-d8e010cce91b@diag.uniroma1.it
> 
> Changes in v2:
> - pick_next_rt_entity(): emit warning instead of crashing
> - Link to v1: https://lore.kernel.org/r/20230128-list-entry-null-check-sched-v1-1-c93085ee0055@diag.uniroma1.it
> ---
>  kernel/sched/rt.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index ed2a47e4ddae..0a11f44adee5 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1777,6 +1777,8 @@ static struct sched_rt_entity *pick_next_rt_entity(struct rt_rq *rt_rq)
>  	BUG_ON(idx >= MAX_RT_PRIO);
>  
>  	queue = array->queue + idx;
> +	if (SCHED_WARN_ON(list_empty(queue)))
> +		return NULL;
>  	next = list_entry(queue->next, struct sched_rt_entity, run_list);
>  
>  	return next;
> @@ -1789,7 +1791,8 @@ static struct task_struct *_pick_next_task_rt(struct rq *rq)
>  
>  	do {
>  		rt_se = pick_next_rt_entity(rt_rq);
> -		BUG_ON(!rt_se);
> +		if (unlikely(!rt_se))
> +			return NULL;
>  		rt_rq = group_rt_rq(rt_se);
>  	} while (rt_rq);
>  
> 
> ---
> base-commit: 2241ab53cbb5cdb08a6b2d4688feb13971058f65
> change-id: 20230128-list-entry-null-check-sched-a3f3dfd6d468
> 
> Best regards,
> -- 
> Pietro Borrello <borrello@diag.uniroma1.it>
> 


Reviewed-by: Phil Auld <pauld@redhat.com>

Thanks. I think this will be useful.


Cheers,
Phil


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