Re: [PATCH v2 bpf-next 5/8] tcp: Split out __tcp_set_rcvlowat().

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Wed Sep 23, 2026 at 9:35 PM UTC, Kuniyuki Iwashima wrote:
> We will add a kfunc for bpf_tcp_ops.{enqueue,dequeue}_rcvq()
> to adjust sk->sk_rcvlowat.
>
> These hooks are triggered
>
>   * when the TCP stack enqueues an skb to sk->sk_receive_queue
>   * after data is dequeued from sk->sk_receive_queue
>
> In the enqueue path, tcp_data_ready() is always called after
> the hooks in tcp_queue_rcv() and tcp_ofo_queue().
>
> If tcp_set_rcvlowat() were used as is, tcp_data_ready() could
> be called twice for the same skb, which is redundant and also
> confusing.
>
> Let's split out __tcp_set_rcvlowat() and add a flag to control
> wakeup behaviour.
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
> Acked-by: Stanislav Fomichev <sdf@fomichev.me>

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

> ---
>  include/net/tcp.h |  1 +
>  net/ipv4/tcp.c    | 12 +++++++++---
>  2 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index f2d838bcb0a7..07426e8641b7 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -512,6 +512,7 @@ void tcp_set_keepalive(struct sock *sk, int val);
>  void tcp_syn_ack_timeout(const struct request_sock *req);
>  int tcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
>  		int flags);
> +int __tcp_set_rcvlowat(struct sock *sk, int val, bool wakeup);
>  int tcp_set_rcvlowat(struct sock *sk, int val);
>  void tcp_set_rcvbuf(struct sock *sk, int val);
>  int tcp_set_window_clamp(struct sock *sk, int val);
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index a714b36a7494..aa7593fc8334 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1828,8 +1828,7 @@ int tcp_peek_len(struct socket *sock)
>  	return tcp_inq(sock->sk);
>  }
>  
> -/* Make sure sk_rcvbuf is big enough to satisfy SO_RCVLOWAT hint */
> -int tcp_set_rcvlowat(struct sock *sk, int val)
> +int __tcp_set_rcvlowat(struct sock *sk, int val, bool wakeup)
>  {
>  	struct tcp_sock *tp = tcp_sk(sk);
>  	int space, cap;
> @@ -1842,7 +1841,8 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
>  	WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
>  
>  	/* Check if we need to signal EPOLLIN right now */
> -	tcp_data_ready(sk);
> +	if (wakeup)
> +		tcp_data_ready(sk);
>  
>  	if (sk->sk_userlocks & SOCK_RCVBUF_LOCK)
>  		return 0;
> @@ -1857,6 +1857,12 @@ int tcp_set_rcvlowat(struct sock *sk, int val)
>  	return 0;
>  }
>  
> +/* Make sure sk_rcvbuf is big enough to satisfy SO_RCVLOWAT hint */
> +int tcp_set_rcvlowat(struct sock *sk, int val)
> +{
> +	return __tcp_set_rcvlowat(sk, val, true);
> +}
> +
>  void tcp_set_rcvbuf(struct sock *sk, int val)
>  {
>  	tcp_set_window_clamp(sk, tcp_win_from_space(sk, val));