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Message-ID: <20230123163002.GB6268@redhat.com>
Date: Mon, 23 Jan 2023 17:30:03 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Wander Lairson Costa <wander@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Stafford Horne <shorne@...il.com>,
Kefeng Wang <wangkefeng.wang@...wei.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
Andy Lutomirski <luto@...nel.org>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Fenghua Yu <fenghua.yu@...el.com>,
Andrei Vagin <avagin@...il.com>,
open list <linux-kernel@...r.kernel.org>,
Paul McKenney <paulmck@...nel.org>
Subject: Re: [PATCH v2 1/4] sched/task: Add the put_task_struct_atomic_safe
function
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.
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