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Date: Mon, 13 Nov 2023 11:44:57 +0800
From: Dust Li <>
To: "D. Wythe" <>,,,,
Subject: Re: [PATCH net v1] net/smc: avoid data corruption caused by decline

On Wed, Nov 08, 2023 at 05:48:29PM +0800, D. Wythe wrote:
>From: "D. Wythe" <>
>We found a data corruption issue during testing of SMC-R on Redis
>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
>	 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 <>
> 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 abd2667..5b91f55 100644
>--- a/net/smc/af_smc.c
>+++ b/net/smc/af_smc.c
>@@ -599,7 +599,7 @@ static int smcr_clnt_conf_first_link(struct smc_sock *smc)
> 	int rc;
> 	/* receive CONFIRM LINK request from server over RoCE fabric */
>-	qentry = smc_llc_wait(link->lgr, NULL, SMC_LLC_WAIT_TIME,
>+	qentry = smc_llc_wait(link->lgr, NULL, 2 * SMC_LLC_WAIT_TIME,

It may be difficult for people to understand why LLC_WAIT_TIME is
different, especially without any comments explaining its purpose.
People are required to use git to find the reason, which I believe is
not conducive to easy maintenance.

Best regards,

> 	if (!qentry) {
> 		struct smc_clc_msg_decline dclc;

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