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

Frederic Weisbecker
2024-02-23T15:14:52Z

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);
>