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Date: Wed, 15 Nov 2023 14:52:24 +0100
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: "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 14.11.23 10:52, D. Wythe wrote:
> 
> 
> On 11/13/23 6:57 PM, Wenjia Zhang wrote:
>>
>>
>> On 13.11.23 03:50, D. Wythe wrote:
>>>
>>>
>>> On 11/10/23 10:51 AM, D. Wythe wrote:
>>>>
>>>>
>>>> On 11/8/23 9:00 PM, Wenjia Zhang wrote:
>>>>>
>>>>>
>>>>> On 08.11.23 10:48, 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);
>>>>>>       if (!qentry) {
>>>>>>           struct smc_clc_msg_decline dclc;
>>>>> I'm wondering if the double time (if sufficient) of timeout could 
>>>>> be for waiting for CLC_DECLINE on the client's side. i.e.
>>>>>
>>>>
>>>> It depends. We can indeed introduce a sysctl to allow server to 
>>>> manager their Confirm Link timeout,
>>>> but if there will be protocol updates, this introduction will no 
>>>> longer be necessary, and we will
>>>> have to maintain it continuously.
>>>>
>> no, I don't think, either, that we need a sysctl for that.
> 
> I am okay about that.
> 
>>>> I believe the core of the solution is to ensure that decline 
>>>> messages never cross or collide. Increasing
>>>> the client's timeout by twice as much as the server's timeout can 
>>>> temporarily solve this problem.
>>
>> I have no objection with that, but my question is why you don't 
>> increase the timeout waiting for CLC_DECLINE instead of waiting 
>> LLC_Confirm_Link? Shouldn't they have the same effect?
>>
> 
> Logically speaking, of course, they have the same effect, but there are 
> two reasons that i choose to increase LLC timeout here:
> 
> 1. to avoid DECLINE  cross or collide, we need a bigger time gap, a 
> simple math is
> 
>      2 ( LLC_Confirm_Link) + 1 (CLC_DECLINE) = 3
>      2 (LLC_Confirm_Link)  + 1 * 2 (CLC_DECLINE) = 4
>      2 * 2(LLC_Confirm_Link) + 1 (CLC_DECLINE) = 5
> 
> Obviously, double the LLC_Confirm_Link will result in more time gaps.
> 
That's already clear to me. That's why I stressed "(if sufficient)".

> 2. increase LLC timeout to allow as many RDMA link as possible to 
> succeed, rather than fallback.
> 
ok, that sounds reasonable. And I think that's the answer which 
persuaded me. Thank you!

> D. Wythe
> 
>>>> If Jerry's proposed protocol updates are too complex or if there 
>>>> won't be any future protocol updates,
>>>> it's still not late to let server manager their Confirm Link timeout 
>>>> then.
>>>>
>>>> Best wishes,
>>>> D. Wythe
>>>>
>>>
>>> FYI:
>>>
>>> It seems that my email was not successfully delivered due to some 
>>> reasons. Sorry
>>> for that.
>>>
>>> D. Wythe
>>>
>>>
>>
>>>>> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
>>>>> index 35ddebae8894..9b1feef1013d 100644
>>>>> --- a/net/smc/af_smc.c
>>>>> +++ b/net/smc/af_smc.c
>>>>> @@ -605,7 +605,7 @@ static int smcr_clnt_conf_first_link(struct 
>>>>> smc_sock *smc)
>>>>>                 struct smc_clc_msg_decline dclc;
>>>>>
>>>>>                 rc = smc_clc_wait_msg(smc, &dclc, sizeof(dclc),
>>>>> -                                     SMC_CLC_DECLINE, 
>>>>> CLC_WAIT_TIME_SHORT);
>>>>> +                                     SMC_CLC_DECLINE, 2 * 
>>>>> CLC_WAIT_TIME_SHORT);
>>>>>                 return rc == -EAGAIN ? SMC_CLC_DECL_TIMEOUT_CL : rc;
>>>>>         }
>>>>>         smc_llc_save_peer_uid(qentry);
>>>>>
>>>>> Because the purpose is to let the server have the control to deline.
>>>>
>>>
> 

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