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Date: Wed, 15 Nov 2023 15:06:21 +0100
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: dust.li@...ux.alibaba.com, "D. Wythe" <alibuda@...ux.alibaba.com>,
        kgraul@...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
Subject: Re: [PATCH net v1] net/smc: avoid data corruption caused by decline



On 13.11.23 04:44, Dust Li wrote:
> On Wed, Nov 08, 2023 at 05:48:29PM +0800, 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
>>
>> 	 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 | 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,
>> 			      SMC_LLC_CONFIRM_LINK);
> 
> 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,
> Dust
> 
> 
Good point! @D.Wythe, could you please try to add a simple commet to 
explain it?

Thanks,
Wenjia
> 
>> 	if (!qentry) {
>> 		struct smc_clc_msg_decline dclc;
>> -- 
>> 1.8.3.1

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