netdev - Re: [PATCH v2 net] net/smc: postpone sk

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Date:   Mon, 23 May 2022 14:45:24 +0200
From:   Karsten Graul <kgraul@...ux.ibm.com>
To:     liuyacan@...p.netease.com, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com
Cc:     linux-s390@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, ubraun@...ux.ibm.com
Subject: Re: [PATCH v2 net] net/smc: postpone sk_refcnt increment in connect()

On 23/05/2022 06:57, liuyacan@...p.netease.com wrote:
> From: liuyacan <liuyacan@...p.netease.com>
> 
> Same trigger condition as commit 86434744. When setsockopt runs
> in parallel to a connect(), and switch the socket into fallback
> mode. Then the sk_refcnt is incremented in smc_connect(), but
> its state stay in SMC_INIT (NOT SMC_ACTIVE). This cause the
> corresponding sk_refcnt decrement in __smc_release() will not be
> performed.
> 
> Fixes: 86434744fedf ("net/smc: add fallback check to connect()")
> Signed-off-by: liuyacan <liuyacan@...p.netease.com>
> ---
>  net/smc/af_smc.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> index fce16b9d6..45a24d242 100644
> --- a/net/smc/af_smc.c
> +++ b/net/smc/af_smc.c
> @@ -1564,9 +1564,9 @@ static int smc_connect(struct socket *sock, struct sockaddr *addr,
>  	if (rc && rc != -EINPROGRESS)
>  		goto out;
>  
> -	sock_hold(&smc->sk); /* sock put in passive closing */
>  	if (smc->use_fallback)
>  		goto out;
> +	sock_hold(&smc->sk); /* sock put in passive closing */
>  	if (flags & O_NONBLOCK) {
>  		if (queue_work(smc_hs_wq, &smc->connect_work))
>  			smc->connect_nonblock = 1;

This is a rather unusual problem that can come up when fallback=true BEFORE smc_connect()
is called. But nevertheless, it is a problem.

Right now I am not sure if it is okay when we NOT hold a ref to smc->sk during all fallback
processing. This change also conflicts with a patch that is already on net-next (3aba1030).

With the new patch on net-next it would also be possible to detect in __smc_release() that
the socket is in state sk->sk_state == SMC_INIT but the sock->state is SS_CONNECTING or 
SS_CONNECTED and call sock_put() in this case.
What do you think?