linux-kernel - [RFC PATCHv2 net-next 3/3] ipv4/udp: Add 4-tuple hash for connected socket

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Message-ID: <20240924123113.1688315-1-gur.stavi@huawei.com>
Date: Tue, 24 Sep 2024 15:31:13 +0300
From: Gur Stavi <gur.stavi@...wei.com>
To: <lulie@...ux.alibaba.com>
CC: <antony.antony@...unet.com>, <davem@...emloft.net>, <dsahern@...nel.org>,
	<dust.li@...ux.alibaba.com>, <edumazet@...gle.com>,
	<fred.cc@...baba-inc.com>, <jakub@...udflare.com>, <kuba@...nel.org>,
	<linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>,
	<pabeni@...hat.com>, <steffen.klassert@...unet.com>,
	<willemdebruijn.kernel@...il.com>, <yubing.qiuyubing@...baba-inc.com>
Subject: [RFC PATCHv2 net-next 3/3] ipv4/udp: Add 4-tuple hash for connected socket

> +/* In hash4, rehash can also happen in connect(), where hash4_cnt keeps unchanged. */
> +static void udp4_rehash4(struct udp_table *udptable, struct sock *sk, u16 newhash4)
> +{
> +	struct udp_hslot *hslot4, *nhslot4;
> +
> +	hslot4 = udp_hashslot4(udptable, udp_sk(sk)->udp_lrpa_hash);
> +	nhslot4 = udp_hashslot4(udptable, newhash4);
> +	udp_sk(sk)->udp_lrpa_hash = newhash4;
> +
> +	if (hslot4 != nhslot4) {
> +		spin_lock_bh(&hslot4->lock);
> +		hlist_del_init_rcu(&udp_sk(sk)->udp_lrpa_node);
> +		hslot4->count--;
> +		spin_unlock_bh(&hslot4->lock);

I realize this is copied from udp_lib_rehash, but isn't it an RCU bug?
Once a node is removed from a list, shouldn't synchronize_rcu be called
before it is reused for a new list? A reader that was traversing the
old list may find itself on the new list.

> +
> +		spin_lock_bh(&nhslot4->lock);
> +		hlist_add_head_rcu(&udp_sk(sk)->udp_lrpa_node, &nhslot4->head);
> +		nhslot4->count++;
> +		spin_unlock_bh(&nhslot4->lock);
> +	}
> +}
> +