netdev - Re: [PATCH net-next v2] net/tcp: optimise locking for blocking splice

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Date: Fri, 23 Jun 2023 16:17:53 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, dsahern@...nel.org, 
	pabeni@...hat.com, kuba@...nel.org
Subject: Re: [PATCH net-next v2] net/tcp: optimise locking for blocking splice

On Fri, Jun 23, 2023 at 2:40 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> Even when tcp_splice_read() reads all it was asked for, for blocking
> sockets it'll release and immediately regrab the socket lock, loop
> around and break on the while check.
>
> Check tss.len right after we adjust it, and return if we're done.
> That saves us one release_sock(); lock_sock(); pair per successful
> blocking splice read.
>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>
> v2: go with Paolo's suggestion
>     aggressively shrink the patch
>
>  net/ipv4/tcp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index 71b42eef9dbf..d56edc2c885f 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -839,7 +839,7 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,
>                 tss.len -= ret;
>                 spliced += ret;
>
> -               if (!timeo)
> +               if (!tss.len || !timeo)
>                         break;
>                 release_sock(sk);
>                 lock_sock(sk);

SGTM, thanks.

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

I wonder if the "release_sock();sock_lock();"  could be replaced by
sk_flush_backlog() anyway ?
Or is there any other reason for this dance ?

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 71b42eef9dbf527098963bc03deecf55042e2021..d03d38060944d63d2728a7bf90a5c117b7852d8b
100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -841,8 +841,7 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos,

                if (!timeo)
                        break;
-               release_sock(sk);
-               lock_sock(sk);
+               sk_flush_backlog();

                if (sk->sk_err || sk->sk_state == TCP_CLOSE ||
                    (sk->sk_shutdown & RCV_SHUTDOWN) ||