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Subject: Re: [PATCH] sched/rt: Plug rt_mutex_setprio() vs push_rt_task() race
To:     Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org
Cc:     John Keeping <john@metanate.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>
References: <20220120194037.650433-1-valentin.schneider@arm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <aa5bf2d0-bef7-485a-9a51-0da7df67d8c5@arm.com>
Date:   Mon, 24 Jan 2022 10:37:43 +0100
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On 20/01/2022 20:40, Valentin Schneider wrote:

[...]

> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 7b4f4fbbb404..48fc8c04b038 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -2026,6 +2026,16 @@ static int push_rt_task(struct rq *rq, bool pull)
>  		return 0;
>  
>  retry:
> +	/*
> +	 * It's possible that the next_task slipped in of
> +	 * higher priority than current. If that's the case
> +	 * just reschedule current.
> +	 */
> +	if (unlikely(next_task->prio < rq->curr->prio)) {
> +		resched_curr(rq);
> +		return 0;
> +	}

If we do this before `is_migration_disabled(next_task), shouldn't then
the related condition in push_dl_task() also be moved up?

  if (dl_task(rq->curr) &&
    dl_time_before(next_task->dl.deadline, rq->curr->dl.deadline) &&
    rq->curr->nr_cpus_allowed > 1)

To enforce resched_curr(rq) in the `is_migration_disabled(next_task)`
case there as well?

> +
>  	if (is_migration_disabled(next_task)) {
>  		struct task_struct *push_task = NULL;
>  		int cpu;
> @@ -2033,6 +2043,17 @@ static int push_rt_task(struct rq *rq, bool pull)
>  		if (!pull || rq->push_busy)
>  			return 0;
>  
> +		/*
> +		 * Per the above priority check, curr is at least RT. If it's
> +		 * of a higher class than RT, invoking find_lowest_rq() on it
> +		 * doesn't make sense.
> +		 *
> +		 * Note that the stoppers are masqueraded as SCHED_FIFO
> +		 * (cf. sched_set_stop_task()), so we can't rely on rt_task().
> +		 */
> +		if (rq->curr->sched_class != &rt_sched_class)

s/ != / > / ... since the `unlikely(next_task->prio < rq->curr->prio)`
already filters tasks from lower sched classes (CFS)?

> +			return 0;
> +

[...]