linux-kernel - Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on rcu_report_qs

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Date:   Fri, 21 Jul 2023 14:43:47 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Levi Yun <ppbuk5246@...il.com>
Cc:     frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, osh@...htriplett.org, boqun.feng@...il.com,
        rostedt@...dmis.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, qiang.zhang1211@...il.com,
        rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on
 rcu_report_qs_rdp

On Fri, Jul 21, 2023 at 01:15:33PM +0100, Levi Yun wrote:
> Wherever rcu_report_qs_rdp is called, cpu_no_qs.norm value is false.
> Therefore, Remove unnecessary check in rcu_report_qs_rdp.
> 
> Signed-off-by: Levi Yun <ppbuk5246@...il.com>

Why not start with something like this?

	if (!WARN_ON_ONCE(!rdp->cpu_no_qs.b.norm) ||
	    rdp->gp_seq != rnp->gp_seq || rdp->gpwrap) {

Except that rcu_report_qs_rdp() is invoked with interrupts enabled,
which means that there is some possibility of state changes up to the
raw_spin_lock_irqsave_rcu_node(rnp, flags) statement.

So, did you check whether RCU's interrupt paths change this state?

						Thanx, Paul

> ---
>  kernel/rcu/tree.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 1449cb69a0e0..d840596e9903 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1962,8 +1962,7 @@ rcu_report_qs_rdp(struct rcu_data *rdp)
>  	WARN_ON_ONCE(rdp->cpu != smp_processor_id());
>  	rnp = rdp->mynode;
>  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> -	if (rdp->cpu_no_qs.b.norm || rdp->gp_seq != rnp->gp_seq ||
> -	    rdp->gpwrap) {
> +	if (rdp->gp_seq != rnp->gp_seq || rdp->gpwrap) {
> 
>  		/*
>  		 * The grace period in which this quiescent state was
> --
> 2.37.2