Re: [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency

From: Frederic Weisbecker <frederic@kernel.org>
To: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>,
	RCU <rcu@vger.kernel.org>,
	Neeraj upadhyay <Neeraj.Upadhyay@amd.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Hillf Danton <hdanton@sina.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency
Date: Tue, 27 Feb 2024 17:15:07 +0100	[thread overview]
Message-ID: <Zd4KizFuP4aGuNLE@localhost.localdomain> (raw)
In-Reply-To: <20240220183115.74124-3-urezki@gmail.com>

Le Tue, Feb 20, 2024 at 07:31:13PM +0100, Uladzislau Rezki (Sony) a écrit :
> +static struct llist_node *rcu_sr_get_wait_head(void)
> +{
> +	struct sr_wait_node *sr_wn;
> +	int i;
> +
> +	for (i = 0; i < SR_NORMAL_GP_WAIT_HEAD_MAX; i++) {
> +		sr_wn = &(rcu_state.srs_wait_nodes)[i];
> +
> +		if (!atomic_cmpxchg_acquire(&sr_wn->inuse, 0, 1))
> +			return &sr_wn->node;
> +	}
> +
> +	return NULL;
> +}
> +
> +static void rcu_sr_put_wait_head(struct llist_node *node)
> +{
> +	struct sr_wait_node *sr_wn = container_of(node, struct sr_wait_node, node);
> +	atomic_set_release(&sr_wn->inuse, 0);

Can we comment this release/acquire pair?

IIUC, the point is that we must make sure that the wait_head->next fetch
happens before it gets reused and rewritten to something else.

Thanks.