linux-kernel - Re: [PATCH] rcu-tasks: Remove unnecessary smp_store operations in the cblist_init

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Message-ID: <b8cae37a-65c9-4380-9290-8494c854373e@paulmck-laptop>
Date: Thu, 25 Dec 2025 10:57:55 -0800
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zqiang <qiang.zhang@...ux.dev>
Cc: frederic@...nel.org, neeraj.upadhyay@...nel.org, joelagnelf@...dia.com,
	rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu-tasks: Remove unnecessary smp_store operations in
 the cblist_init_generic()

On Sun, Dec 14, 2025 at 10:49:04AM +0800, Zqiang wrote:
> The cblist_init_generic() is executed during the CPU early boot
> phase due to commit:30ef09635b9e ("rcu-tasks: Initialize callback
> lists at rcu_init() time"), at this time, only one boot CPU is
> online. this commit therefore use WRITE_ONCE() to replace the
> smp_store_release() to assigning rtp->percpu_enqueue_lim in the
> cblist_init_generic().
> 
> Signed-off-by: Zqiang <qiang.zhang@...ux.dev>

Good eyes, thank you!

But in that case, we are running on a single CPU with interrupts disables,
so do we even need WRITE_ONCE()?

							Thanx, Paul

> ---
>  kernel/rcu/tasks.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
> index 76f952196a29..f763f60d5e50 100644
> --- a/kernel/rcu/tasks.h
> +++ b/kernel/rcu/tasks.h
> @@ -292,7 +292,7 @@ static void cblist_init_generic(struct rcu_tasks *rtp)
>  		shift++;
>  	WRITE_ONCE(rtp->percpu_enqueue_shift, shift);
>  	WRITE_ONCE(rtp->percpu_dequeue_lim, lim);
> -	smp_store_release(&rtp->percpu_enqueue_lim, lim);
> +	WRITE_ONCE(rtp->percpu_enqueue_lim, lim);
>  
>  	pr_info("%s: Setting shift to %d and lim to %d rcu_task_cb_adjust=%d rcu_task_cpu_ids=%d.\n",
>  			rtp->name, data_race(rtp->percpu_enqueue_shift), data_race(rtp->percpu_enqueue_lim),
> -- 
> 2.48.1
>