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Date:   Wed, 25 Oct 2023 10:40:08 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, rcu <rcu@...r.kernel.org>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH 2/4] rcu/tasks: Handle new PF_IDLE semantics

On Tue, Oct 24, 2023 at 11:46:23PM +0200, Frederic Weisbecker wrote:

> +/* Check for quiescent states since the pregp's synchronize_rcu() */
> +static bool rcu_tasks_is_holdout(struct task_struct *t)
> +{
> +	int cpu;
> +
> +	/* Has the task been seen voluntarily sleeping? */
> +	if (!READ_ONCE(t->on_rq))
> +		return false;
> +
> +	cpu = task_cpu(t);
> +
> +	/*
> +	 * Idle tasks within the idle loop or offline CPUs are RCU-tasks
> +	 * quiescent states. But CPU boot code performed by the idle task
> +	 * isn't a quiescent state.
> +	 */
> +	if (t == idle_task(cpu)) {
> +		if (is_idle_task(t))
> +			return false;
> +
> +		if (!rcu_cpu_online(cpu))
> +			return false;
> +	}

Hmm, why is this guarded by t == idle_task() ?

Notably, there is the idle-injection thing that uses FIFO tasks to run
'idle', see play_idle_precise(). This will (temporarily) get PF_IDLE on
tasks that are not idle_task().

> +
> +	return true;
> +}
> +
>  /* Per-task initial processing. */
>  static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
>  {
> -	if (t != current && READ_ONCE(t->on_rq) && !is_idle_task(t)) {
> +	if (t != current && rcu_tasks_is_holdout(t)) {


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