linux-kernel - Re: [PATCH net-next v2] net/smc: align the connect behaviour with TCP

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Date:   Mon, 23 May 2022 14:24:14 +0200
From:   Karsten Graul <kgraul@...ux.ibm.com>
To:     Guangguan Wang <guangguan.wang@...ux.alibaba.com>,
        davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2] net/smc: align the connect behaviour with TCP

On 13/05/2022 04:24, Guangguan Wang wrote:
> Connect with O_NONBLOCK will not be completed immediately
> and returns -EINPROGRESS. It is possible to use selector/poll
> for completion by selecting the socket for writing. After select
> indicates writability, a second connect function call will return
> 0 to indicate connected successfully as TCP does, but smc returns
> -EISCONN. Use socket state for smc to indicate connect state, which
> can help smc aligning the connect behaviour with TCP.
> 
> Signed-off-by: Guangguan Wang <guangguan.wang@...ux.alibaba.com>
> Acked-by: Karsten Graul <kgraul@...ux.ibm.com>
> ---
>  net/smc/af_smc.c | 50 ++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 46 insertions(+), 4 deletions(-)
> 
> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> index fce16b9d6e1a..5f70642a8044 100644
> --- a/net/smc/af_smc.c
> +++ b/net/smc/af_smc.c
> @@ -1544,9 +1544,29 @@ static int smc_connect(struct socket *sock, struct sockaddr *addr,
>  		goto out_err;
>  
>  	lock_sock(sk);
> +	switch (sock->state) {
> +	default:
> +		rc = -EINVAL;
> +		goto out;
> +	case SS_CONNECTED:
> +		rc = sk->sk_state == SMC_ACTIVE ? -EISCONN : -EINVAL;
> +		goto out;
> +	case SS_CONNECTING:
> +		if (sk->sk_state == SMC_ACTIVE)
> +			goto connected;

I stumbled over this when thinking about the fallback processing. If for whatever reason
fallback==true during smc_connect(), the "if (smc->use_fallback)" below would set sock->state
to e.g. SS_CONNECTED. But in the fallback case sk_state keeps SMC_INIT. So during the next call
the SS_CONNECTING case above would break because sk_state in NOT SMC_ACTIVE, and we would end
up calling kernel_connect() again. Which seems to be no problem when kernel_connect() returns 
-EISCONN and we return this to the caller. But is this how it should work, or does it work by chance?

> +		break;
> +	case SS_UNCONNECTED:
> +		sock->state = SS_CONNECTING;
> +		break;
> +	}
> +
>  	switch (sk->sk_state) {
>  	default:
>  		goto out;
> +	case SMC_CLOSED:
> +		rc = sock_error(sk) ? : -ECONNABORTED;
> +		sock->state = SS_UNCONNECTED;
> +		goto out;
>  	case SMC_ACTIVE:
>  		rc = -EISCONN;
>  		goto out;
> @@ -1565,20 +1585,24 @@ static int smc_connect(struct socket *sock, struct sockaddr *addr,
>  		goto out;
>  
>  	sock_hold(&smc->sk); /* sock put in passive closing */
> -	if (smc->use_fallback)
> +	if (smc->use_fallback) {
> +		sock->state = rc ? SS_CONNECTING : SS_CONNECTED;
>  		goto out;
> +	}
>  	if (flags & O_NONBLOCK) {
>  		if (queue_work(smc_hs_wq, &smc->connect_work))
>  			smc->connect_nonblock = 1;
>  		rc = -EINPROGRESS;
> +		goto out;
>  	} else {
>  		rc = __smc_connect(smc);
>  		if (rc < 0)
>  			goto out;
> -		else
> -			rc = 0; /* success cases including fallback */
>  	}
>  
> +connected:
> +	rc = 0;
> +	sock->state = SS_CONNECTED;
>  out:
>  	release_sock(sk);
>  out_err:
> @@ -1693,6 +1717,7 @@ struct sock *smc_accept_dequeue(struct sock *parent,
>  		}
>  		if (new_sock) {
>  			sock_graft(new_sk, new_sock);
> +			new_sock->state = SS_CONNECTED;
>  			if (isk->use_fallback) {
>  				smc_sk(new_sk)->clcsock->file = new_sock->file;
>  				isk->clcsock->file->private_data = isk->clcsock;
> @@ -2424,7 +2449,7 @@ static int smc_listen(struct socket *sock, int backlog)
>  
>  	rc = -EINVAL;
>  	if ((sk->sk_state != SMC_INIT && sk->sk_state != SMC_LISTEN) ||
> -	    smc->connect_nonblock)
> +	    smc->connect_nonblock || sock->state != SS_UNCONNECTED)
>  		goto out;
>  
>  	rc = 0;
> @@ -2716,6 +2741,17 @@ static int smc_shutdown(struct socket *sock, int how)
>  
>  	lock_sock(sk);
>  
> +	if (sock->state == SS_CONNECTING) {
> +		if (sk->sk_state == SMC_ACTIVE)
> +			sock->state = SS_CONNECTED;
> +		else if (sk->sk_state == SMC_PEERCLOSEWAIT1 ||
> +			 sk->sk_state == SMC_PEERCLOSEWAIT2 ||
> +			 sk->sk_state == SMC_APPCLOSEWAIT1 ||
> +			 sk->sk_state == SMC_APPCLOSEWAIT2 ||
> +			 sk->sk_state == SMC_APPFINCLOSEWAIT)
> +			sock->state = SS_DISCONNECTING;
> +	}
> +
>  	rc = -ENOTCONN;
>  	if ((sk->sk_state != SMC_ACTIVE) &&
>  	    (sk->sk_state != SMC_PEERCLOSEWAIT1) &&
> @@ -2729,6 +2765,7 @@ static int smc_shutdown(struct socket *sock, int how)
>  		sk->sk_shutdown = smc->clcsock->sk->sk_shutdown;
>  		if (sk->sk_shutdown == SHUTDOWN_MASK) {
>  			sk->sk_state = SMC_CLOSED;
> +			sk->sk_socket->state = SS_UNCONNECTED;
>  			sock_put(sk);
>  		}
>  		goto out;
> @@ -2754,6 +2791,10 @@ static int smc_shutdown(struct socket *sock, int how)
>  	/* map sock_shutdown_cmd constants to sk_shutdown value range */
>  	sk->sk_shutdown |= how + 1;
>  
> +	if (sk->sk_state == SMC_CLOSED)
> +		sock->state = SS_UNCONNECTED;
> +	else
> +		sock->state = SS_DISCONNECTING;
>  out:
>  	release_sock(sk);
>  	return rc ? rc : rc1;
> @@ -3139,6 +3180,7 @@ static int __smc_create(struct net *net, struct socket *sock, int protocol,
>  
>  	rc = -ENOBUFS;
>  	sock->ops = &smc_sock_ops;
> +	sock->state = SS_UNCONNECTED;
>  	sk = smc_sock_alloc(net, sock, protocol);
>  	if (!sk)
>  		goto out;

-- 
Karsten