Re: [PATCH v2 6/6] rcu-tasks: Maintain real-time response in rcu_tasks_postscan()

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Feb 23, 2024 at 01:17:14PM +0100, Frederic Weisbecker wrote:
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 866743e0796f..0ff2b554f5b5 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -973,12 +973,13 @@ static void rcu_tasks_postscan(struct list_head *hop)
>  	for_each_possible_cpu(cpu) {
>  		unsigned long j = jiffies + 1;
>  		struct rcu_tasks_percpu *rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, cpu);
> -		struct task_struct *t;
> -		struct task_struct *t1;
> -		struct list_head tmp;
>  
>  		raw_spin_lock_irq_rcu_node(rtpcp);
> -		list_for_each_entry_safe(t, t1, &rtpcp->rtp_exit_list, rcu_tasks_exit_list) {
> +		while (!list_empty(&rtpcp->rtp_exit_list)) {
> +			struct task_struct *t;
> +			t = list_first_entry(&rtpcp->rtp_exit_list, typeof(*t), rcu_tasks_exit_list);
> +			list_del_init(&t->rcu_tasks_exit_list);

Oh no! The task has to stay in the list for subsequent grace periods! Please
forget that suggestion... Yours looks good!

Thanks.

> +
>  			if (list_empty(&t->rcu_tasks_holdout_list))
>  				rcu_tasks_pertask(t, hop);
>  
> @@ -987,14 +988,9 @@ static void rcu_tasks_postscan(struct list_head *hop)
>  			if (!IS_ENABLED(CONFIG_PREEMPT_RT) && time_before(jiffies, j))
>  				continue;
>  
> -			// Keep our place in the list while pausing.
> -			// Nothing else traverses this list, so adding a
> -			// bare list_head is OK.
> -			list_add(&tmp, &t->rcu_tasks_exit_list);
>  			raw_spin_unlock_irq_rcu_node(rtpcp);
>  			cond_resched(); // For CONFIG_PREEMPT=n kernels
>  			raw_spin_lock_irq_rcu_node(rtpcp);
> -			list_del(&tmp);
>  			j = jiffies + 1;
>  		}
>  		raw_spin_unlock_irq_rcu_node(rtpcp);
> @@ -1219,7 +1215,6 @@ void exit_tasks_rcu_stop(void)
>  	struct rcu_tasks_percpu *rtpcp;
>  	struct task_struct *t = current;
>  
> -	WARN_ON_ONCE(list_empty(&t->rcu_tasks_exit_list));
>  	rtpcp = per_cpu_ptr(rcu_tasks.rtpcpu, t->rcu_tasks_exit_cpu);
>  	raw_spin_lock_irqsave_rcu_node(rtpcp, flags);
>  	list_del_init(&t->rcu_tasks_exit_list);
>