linux-kernel - Re: [PATCH 2/4] rcu: Remove useless WRITE_ONCE() on rcu_data.exp_deferred

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Date:   Thu, 16 Sep 2021 09:43:40 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, rcu@...r.kernel.org,
        urezki@...il.com, boqun.feng@...il.com,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        joel@...lfernandes.org
Subject: Re: [PATCH 2/4] rcu: Remove useless WRITE_ONCE() on
 rcu_data.exp_deferred_qs

On Thu, Sep 16, 2021 at 02:10:46PM +0200, Frederic Weisbecker wrote:
> This variable is never written nor read remotely. Remove this confusion.
> 
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> ---
>  kernel/rcu/tree_exp.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> index f3947c49eee7..4266610b4587 100644
> --- a/kernel/rcu/tree_exp.h
> +++ b/kernel/rcu/tree_exp.h
> @@ -255,7 +255,7 @@ static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
>   */
>  static void rcu_report_exp_rdp(struct rcu_data *rdp)
>  {
> -	WRITE_ONCE(rdp->exp_deferred_qs, false);
> +	rdp->exp_deferred_qs = false;

Are you sure that this can never be invoked from an interrupt handler?
And that rdp->exp_deferred_qs is never read from an interrupt handler?
If either can happen, then the WRITE_ONCE() does play a role, right?

							Thanx, Paul

>  	rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
>  }
>  
> -- 
> 2.25.1
>