linux-kernel - Re: [PATCH rcu 1/6] rcu/tasks: Check processor-ID assumptions

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Message-ID: <20240814121654.GA1412801@neeraj.linux>
Date: Wed, 14 Aug 2024 17:46:54 +0530
From: Neeraj Upadhyay <Neeraj.Upadhyay@...nel.org>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: rcu@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com,
	rostedt@...dmis.org, Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH rcu 1/6] rcu/tasks: Check processor-ID assumptions

Hi Paul,

On Thu, Aug 01, 2024 at 05:34:21PM -0700, Paul E. McKenney wrote:
> The current mapping of smp_processor_id() to a CPU processing Tasks-RCU
> callbacks makes some assumptions about layout.  This commit therefore
> adds a WARN_ON() to check these assumptions.
> 
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  kernel/rcu/tasks.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index b6fcf744af75d..d473846a572aa 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -355,6 +355,7 @@ static void call_rcu_tasks_generic(struct rcu_head *rhp, rcu_callback_t func,
>  	rcu_read_lock();
>  	ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
>  	chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_possible_mask);
> +	WARN_ON_ONCE(chosen_cpu >= nr_cpu_ids);

I have changed this s/nr_cpu_ids/rcu_task_cpu_ids/ , as there is a
another commit 	fd70e9f1d85f "rcu-tasks: Fix access non-existent percpu
rtpcp variable in rcu_tasks_need_gpcb()" which is included in the tree
here [1]. Please let me know if something looks incorrect.


[1] https://git.kernel.org/pub/scm/linux/kernel/git/neeraj.upadhyay/linux-rcu.git/log/?h=next.14.08.24a


- Neeraj

>  	rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
>  	if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
>  		raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
> -- 
> 2.40.1
>