linux-kernel - Re: [RFC PATCH 6/8] rcuscale: Allow rcu_scale_ops::get_gp

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Message-ID: <6dbbdfa0-ca6b-425a-85a0-7c80041573fb@paulmck-laptop>
Date: Thu, 10 Jul 2025 18:00:21 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Breno Leitao <leitao@...ian.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>, aeh@...a.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	edumazet@...gle.com, jhs@...atatu.com, kernel-team@...a.com,
	Erik Lundgren <elundgren@...a.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org
Subject: Re: [RFC PATCH 6/8] rcuscale: Allow rcu_scale_ops::get_gp_seq to be
 NULL

On Sun, Apr 13, 2025 at 11:00:53PM -0700, Boqun Feng wrote:
> For synchronization mechanisms similar to RCU, there could be no "grace
> period" concept (e.g. hazard pointers), therefore allow
> rcu_scale_ops::get_gp_seq to be a NULL pointer for these cases, and
> simply treat started and finished grace period as 0.
> 
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>

Reviewed-by: Paul E. McKenney <paulmck@...nel.org>

> ---
>  kernel/rcu/rcuscale.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
> index 0f3059b1b80d..d9bff4b1928b 100644
> --- a/kernel/rcu/rcuscale.c
> +++ b/kernel/rcu/rcuscale.c
> @@ -568,8 +568,10 @@ rcu_scale_writer(void *arg)
>  		if (gp_exp) {
>  			b_rcu_gp_test_started =
>  				cur_ops->exp_completed() / 2;
> -		} else {
> +		} else if (cur_ops->get_gp_seq) {
>  			b_rcu_gp_test_started = cur_ops->get_gp_seq();
> +		} else {
> +			b_rcu_gp_test_started = 0;
>  		}
>  	}
>  
> @@ -625,9 +627,11 @@ rcu_scale_writer(void *arg)
>  				if (gp_exp) {
>  					b_rcu_gp_test_finished =
>  						cur_ops->exp_completed() / 2;
> -				} else {
> +				} else if (cur_ops->get_gp_seq) {
>  					b_rcu_gp_test_finished =
>  						cur_ops->get_gp_seq();
> +				} else {
> +					b_rcu_gp_test_finished = 0;
>  				}
>  				if (shutdown) {
>  					smp_mb(); /* Assign before wake. */
> -- 
> 2.47.1
>