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

On Dec 6, 2021, at 22:11, Akhmat Karakotov [off-list ref] wrote:

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 <redacted>
---
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