netdev - Re: [PATCH net v1] net/smc: avoid data corruption caused by decline

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Date: Thu, 16 Nov 2023 19:50:03 +0800
From: "D. Wythe" <alibuda@...ux.alibaba.com>
To: Wenjia Zhang <wenjia@...ux.ibm.com>, dust.li@...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 11/15/23 10:06 PM, Wenjia Zhang wrote:
>
>
> 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?
>

Also good to me, i will add comment to explain it.

D. Wythe

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