netdev - [RFC PATCH v3 net-next 4/4] tcp: change SYN ACK retransmit behaviour to account for rehash

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Date:   Mon,  6 Dec 2021 22:11:11 +0300
From:   Akhmat Karakotov <hmukos@...dex-team.ru>
To:     netdev@...r.kernel.org
Cc:     hmukos@...dex-team.ru, edumazet@...gle.com, eric.dumazet@...il.com,
        mitradir@...dex-team.ru, tom@...bertland.com, zeil@...dex-team.ru
Subject: [RFC PATCH v3 net-next 4/4] tcp: change SYN ACK retransmit behaviour to account for rehash

Disabling rehash behavior did not affect SYN ACK retransmits because hash
was forcefully changed bypassing the sk_rethink_hash function. This patch
adds a condition which checks for rehash mode before resetting hash.

Signed-off-by: Akhmat Karakotov <hmukos@...dex-team.ru>
---
 net/core/sock.c       | 3 ++-
 net/ipv4/tcp_output.c | 4 +++-
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index daace0d10156..f2515f657974 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -1372,7 +1372,8 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
 			ret = -EINVAL;
 			break;
 		}
-		sk->sk_txrehash = (u8)val;
+		/* Paired with READ_ONCE() in tcp_rtx_synack() */
+		WRITE_ONCE(sk->sk_txrehash, (u8)val);
 		break;
 
 	default:
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 6d72f3ea48c4..bbb5f68b947a 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -4108,7 +4108,9 @@ int tcp_rtx_synack(const struct sock *sk, struct request_sock *req)
 	struct flowi fl;
 	int res;
 
-	tcp_rsk(req)->txhash = net_tx_rndhash();
+	/* Paired with WRITE_ONCE() in sock_setsockopt() */
+	if (READ_ONCE(sk->sk_txrehash) == SOCK_TXREHASH_ENABLED)
+		tcp_rsk(req)->txhash = net_tx_rndhash();
 	res = af_ops->send_synack(sk, NULL, &fl, req, NULL, TCP_SYNACK_NORMAL,
 				  NULL);
 	if (!res) {
-- 
2.17.1