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

[Paul E. McKenney]

On Fri, Feb 23, 2024 at 04:14:49PM +0100, Frederic Weisbecker wrote:
> 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!

You had me going for a bit, and I do know that feeling!  ;-)

							Thanx, Paul

> 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);
> >