Date:   Mon, 23 Jan 2023 17:30:03 +0100
From:   Oleg Nesterov <oleg@redhat.com>
To:     Wander Lairson Costa <wander@redhat.com>
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>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Stafford Horne <shorne@gmail.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        "Liam R. Howlett" <Liam.Howlett@Oracle.com>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Andrei Vagin <avagin@gmail.com>,
        open list <linux-kernel@vger.kernel.org>,
        Paul McKenney <paulmck@kernel.org>
Subject: Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe
 function
Message-ID: <20230123163002.GB6268@redhat.com>
References: <20230120150246.20797-1-wander@redhat.com>
 <20230120150246.20797-2-wander@redhat.com>
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On 01/20, Wander Lairson Costa wrote:
>
> +static inline void put_task_struct_atomic_safe(struct task_struct *task)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> +		/*
> +		 * Decrement the refcount explicitly to avoid unnecessarily
> +		 * calling call_rcu.
> +		 */
> +		if (refcount_dec_and_test(&task->usage))
> +			/*
> +			 * under PREEMPT_RT, we can't call put_task_struct
> +			 * in atomic context because it will indirectly
> +			 * acquire sleeping locks.
> +			 */
> +			call_rcu(&task->rcu, __delayed_put_task_struct);
                                  ^^^^^^^^^
I am not sure the usage of task->rcu is safe...

Suppose that, before __delayed_put_task_struct() is called by RCU, this task
does the last schedule and calls put_task_struct_rcu_user().

And, can't we simply turn put_task_struct() into something like

	put_task_struct(struct task_struct *t)
	{
		if (refcount_dec_and_test(&t->usage)) {
			if (IS_ENABLED(CONFIG_PREEMPT_RT)
			    && (in_atomic() || irqs_disabled()))
				call_rcu(...);
			else
				__put_task_struct(t);
		}
	}

?

Oleg.