netdev - Re: [PATCH net v3] net/smc: avoid data corruption caused by decline

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Date: Mon, 20 Nov 2023 11:37:20 +0800
From: Wen Gu <guwen@...ux.alibaba.com>
To: "D. Wythe" <alibuda@...ux.alibaba.com>, kgraul@...ux.ibm.com,
 wenjia@...ux.ibm.com, jaka@...ux.ibm.com, wintera@...ux.ibm.com
Cc: kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
 linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org,
 tonylu@...ux.alibaba.com, pabeni@...hat.com, edumazet@...gle.com
Subject: Re: [PATCH net v3] net/smc: avoid data corruption caused by decline



On 2023/11/19 23:28, D. Wythe wrote:
> From: "D. Wythe" <alibuda@...ux.alibaba.com>
> 
> We found a data corruption issue during testing of SMC-R on Redis
> applications.
> 
> The benchmark has a low probability of reporting a strange error as
> shown below.
> 
> "Error: Protocol error, got "\xe2" as reply type byte"
> 
> Finally, we found that the retrieved error data was as follows:
> 
> 0xE2 0xD4 0xC3 0xD9 0x04 0x00 0x2C 0x20 0xA6 0x56 0x00 0x16 0x3E 0x0C
> 0xCB 0x04 0x02 0x01 0x00 0x00 0x20 0x00 0x00 0x00 0x00 0x00 0x00 0x00
> 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0xE2
> 
> It is quite obvious that this is a SMC DECLINE message, which means that
> the applications received SMC protocol message.
> We found that this was caused by the following situations:
> 
> client                  server
>          ¦  proposal
>          ------------->
>          ¦  accept
>          <-------------
>          ¦  confirm
>          ------------->
> wait confirm

I think there may be an ambiguity here, better for 'wait for llc confirm link'.
Could you please add 'clc' and 'llc' prefix to distinguish flows on the diagram?

Thanks.

> 
>          ¦failed llc confirm
>          ¦   x------
> (after 2s)timeout
>                          wait rsp
> 
> wait decline
> 
> (after 1s) timeout
>                          (after 2s) timeout
>          ¦   decline
>          -------------->
>          ¦   decline
>          <--------------
> 
> As a result, a decline message was sent in the implementation, and this
> message was read from TCP by the already-fallback connection.
> 
> This patch double the client timeout as 2x of the server value,
> With this simple change, the Decline messages should never cross or
> collide (during Confirm link timeout).
> 
> This issue requires an immediate solution, since the protocol updates
> involve a more long-term solution.
> 
> Fixes: 0fb0b02bd6fd ("net/smc: adapt SMC client code to use the LLC flow")
> Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com>


> ---
>   net/smc/af_smc.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> index abd2667..8615cc0 100644
> --- a/net/smc/af_smc.c
> +++ b/net/smc/af_smc.c
> @@ -598,8 +598,12 @@ static int smcr_clnt_conf_first_link(struct smc_sock *smc)
>   	struct smc_llc_qentry *qentry;
>   	int rc;
>   
> -	/* receive CONFIRM LINK request from server over RoCE fabric */
> -	qentry = smc_llc_wait(link->lgr, NULL, SMC_LLC_WAIT_TIME,
> +	/* Receive CONFIRM LINK request from server over RoCE fabric.
> +	 * Increasing the client's timeout by twice as much as the server's
> +	 * timeout by default can temporarily avoid decline messages of
> +	 * both sides crossing or colliding
> +	 */
> +	qentry = smc_llc_wait(link->lgr, NULL, 2 * SMC_LLC_WAIT_TIME,
>   			      SMC_LLC_CONFIRM_LINK);
>   	if (!qentry) {
>   		struct smc_clc_msg_decline dclc;