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Date:   Sun, 23 Oct 2022 13:05:34 -0500
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Subject: Re: [for-next PATCH v2 1/2] RDMA/rxe: Remove unnecessary mr testing
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To:     Li Zhijian <lizhijian@fujitsu.com>,
        Zhu Yanjun <zyjzyj2000@gmail.com>
Cc:     jgg@ziepe.ca, leon@kernel.org, linux-rdma@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <1666338764-2-1-git-send-email-lizhijian@fujitsu.com>
 <1666338764-2-2-git-send-email-lizhijian@fujitsu.com>
 <CAD=hENeGMKri0nYMtcu98K2j3D+ZBkxO6fFX5cvvyJsEghrrmQ@mail.gmail.com>
 <1846f2e1-ff13-5fa2-240f-fd7749921ce2@fujitsu.com>
From:   Bob Pearson <rpearsonhpe@gmail.com>
In-Reply-To: <1846f2e1-ff13-5fa2-240f-fd7749921ce2@fujitsu.com>
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On 10/21/22 20:09, Li Zhijian wrote:
> 
> 
> On 21/10/2022 22:39, Zhu Yanjun wrote:
>> On Fri, Oct 21, 2022 at 3:53 PM Li Zhijian <lizhijian@fujitsu.com> wrote:
>>> Before the testing, we already passed it to rxe_mr_copy() where mr could
>>> be dereferenced. so this checking is not exactly correct.
>>>
>>> I tried to figure out the details how/when mr could be NULL, but failed
>>> at last. Add a WARN_ON(!mr) to that path to tell us more when it
>>> happends.
>> If I get you correctly, you confronted a problem,
> Not exactly,  I removed the mr checking since i think this checking is not correct.
> the newly added WARN_ON(!mr) is the only once place where the mr can be NULL but not handled correctly.
> At least with/without this patch, once WARN_ON(!mr) is triggered, kernel will go something wrong.
> 
> so i want to place this  WARN_ON(!mr) to point to the problem.
> 
> Thanks
> Zhijian
> 
>>   but you can not figure it out.
>> So you send it upstream as a patch?
>>
>> I am not sure if it is a good idea.
>>
>> Zhu Yanjun
>>
>>> Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
>>> ---
>>>   drivers/infiniband/sw/rxe/rxe_resp.c | 4 ++--
>>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/infiniband/sw/rxe/rxe_resp.c b/drivers/infiniband/sw/rxe/rxe_resp.c
>>> index ed5a09e86417..218c14fb07c6 100644
>>> --- a/drivers/infiniband/sw/rxe/rxe_resp.c
>>> +++ b/drivers/infiniband/sw/rxe/rxe_resp.c
>>> @@ -778,6 +778,7 @@ static enum resp_states read_reply(struct rxe_qp *qp,
>>>          if (res->state == rdatm_res_state_new) {
>>>                  if (!res->replay) {
>>>                          mr = qp->resp.mr;
>>> +                       WARN_ON(!mr);
>>>                          qp->resp.mr = NULL;
>>>                  } else {
>>>                          mr = rxe_recheck_mr(qp, res->read.rkey);
>>> @@ -811,8 +812,7 @@ static enum resp_states read_reply(struct rxe_qp *qp,
>>>
>>>          rxe_mr_copy(mr, res->read.va, payload_addr(&ack_pkt),
>>>                      payload, RXE_FROM_MR_OBJ);
>>> -       if (mr)
>>> -               rxe_put(mr);
>>> +       rxe_put(mr);
>>>
>>>          if (bth_pad(&ack_pkt)) {
>>>                  u8 *pad = payload_addr(&ack_pkt) + payload
>>> -- 
>>> 2.31.1
>>>
> 

Li is correct that the only way mr could be NULL is if qp->resp.mr == NULL. So the
'if (mr)' is not needed if that is the case. The read_reply subroutine is reached
from a new rdma read operation after going through check_rkey or from a previous
rdma read operations from get_req if qp->resp.res != NULL or from a duplicate request
where the previous responder resource is found. In all these cases the mr is set.
Initially in check_rkey where if it can't find the mr it causes an RKEY_VIOLATION.
Thereafter the rkey is stored in the responder resources and looked up for each
packet to get an mr or cause an RKEY_VIOLATION. So the mr can't be NULL. I think
you can leave out the WARN and just drop the if (mr).

Bob