Re: [PATCH][next] RDMA/rxe: Avoid -Wflex-array-member-not-at-end warnings

Zhu Yanjun
2025-11-11T15:37:56Z

在 2025/11/11 6:19, Leon Romanovsky 写道:
> On Tue, Nov 11, 2025 at 09:14:05PM +0900, Gustavo A. R. Silva wrote:
>>
>> On 11/11/25 20:56, Leon Romanovsky wrote:
>>> On Tue, Nov 11, 2025 at 12:35:02PM +0900, Gustavo A. R. Silva wrote:
>>>> -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
>>>> getting ready to enable it, globally.
>>>>
>>>> Use the new TRAILING_OVERLAP() helper to fix the following warning:
>>>>
>>>> 21 drivers/infiniband/sw/rxe/rxe_verbs.h:271:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
>>>>
>>>> This helper creates a union between a flexible-array member (FAM) and a
>>>> set of MEMBERS that would otherwise follow it.
>>>>
>>>> This overlays the trailing MEMBER struct ib_sge sge[RXE_MAX_SGE]; onto
>>>> the FAM struct rxe_recv_wqe::dma.sge, while keeping the FAM and the
>>>> start of MEMBER aligned.
>>>>
>>>> The static_assert() ensures this alignment remains, and it's
>>>> intentionally placed inmediately after the related structure --no
>>>> blank line in between.
>>>>
>>>> Lastly, move the conflicting declaration struct rxe_resp_info resp;
>>>> to the end of the corresponding structure.
>>>>
>>>> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
>>>> ---
>>>> drivers/infiniband/sw/rxe/rxe_verbs.h | 18 +++++++++++-------
>>>> 1 file changed, 11 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/drivers/infiniband/sw/rxe/rxe_verbs.h b/drivers/infiniband/sw/rxe/rxe_verbs.h
>>>> index fd48075810dd..6498d61e8956 100644
>>>> --- a/drivers/infiniband/sw/rxe/rxe_verbs.h
>>>> +++ b/drivers/infiniband/sw/rxe/rxe_verbs.h
>>>> @@ -219,12 +219,6 @@ struct rxe_resp_info {
>>>> u32 rkey;
>>>> u32 length;
>>>> - /* SRQ only */
>>>> - struct {
>>>> - struct rxe_recv_wqe wqe;
>>>> - struct ib_sge sge[RXE_MAX_SGE];
>>>> - } srq_wqe;
>>>> -
>>>> /* Responder resources. It's a circular list where the oldest
>>>> * resource is dropped first.
>>>> */
>>>> @@ -232,7 +226,15 @@ struct rxe_resp_info {
>>>> unsigned int res_head;
>>>> unsigned int res_tail;
>>>> struct resp_res *res;
>>>> +
>>>> + /* SRQ only */
>>>> + /* Must be last as it ends in a flexible-array member. */
>>>> + TRAILING_OVERLAP(struct rxe_recv_wqe, wqe, dma.sge,
>>>> + struct ib_sge sge[RXE_MAX_SGE];
>>>> + ) srq_wqe;
>>> Will this change be enough?
>>>
>>> diff --git a/drivers/infiniband/sw/rxe/rxe_verbs.h b/drivers/infiniband/sw/rxe/rxe_verbs.h
>>> index fd48075810dd..9ab11421a585 100644
>>> --- a/drivers/infiniband/sw/rxe/rxe_verbs.h
>>> +++ b/drivers/infiniband/sw/rxe/rxe_verbs.h
>>> @@ -219,12 +219,6 @@ struct rxe_resp_info {
>>> u32 rkey;
>>> u32 length;
>>> - /* SRQ only */
>>> - struct {
>>> - struct rxe_recv_wqe wqe;
>>> - struct ib_sge sge[RXE_MAX_SGE];
>>> - } srq_wqe;
>>> -
>>> /* Responder resources. It's a circular list where the oldest
>>> * resource is dropped first.
>>> */
>>> @@ -232,6 +226,12 @@ struct rxe_resp_info {
>>> unsigned int res_head;
>>> unsigned int res_tail;
>>> struct resp_res *res;
>>> +
>>> + /* SRQ only */
>>> + struct {
>>> + struct ib_sge sge[RXE_MAX_SGE];
>>> + struct rxe_recv_wqe wqe;
>>> + } srq_wqe;
>>> };
>> The question is if this is really what you want?
>>
>> sge[RXE_MAX_SGE] is of the following type:
>>
>> struct ib_sge {
>> u64 addr;
>> u32 length;
>> u32 lkey;
>> };
>>
>> and struct rxe_recv_wqe::dma.sge[] is of type:
>>
>> struct rxe_sge {
>> __aligned_u64 addr;
>> __u32 length;
>> __u32 lkey;
>> };
>>
>> Both types are basically the same, and the original code looks
>> pretty much like what people do when they want to pre-allocate
>> a number of elements (of the same element type as the flex array)
>> for a flexible-array member.
>>
>> Based on the above, the change you suggest seems a bit suspicious,
>> and I'm not sure that's actually what you want?
> You wrote about this error: "warning: structure containing a flexible array
> member is not at the end of another structure".
>
> My suggestion was simply to move that flex array to be the last element
> and save us from the need to have some complex, magic macro in RXE.

Thanks, I agree with this approach. The macro is rather complicated, and
this solution fixes the problem in a straightforward way.

Yanjun.Zhu

>
> Thanks
>
>> Thanks
>> -Gustavo
>>
>>
>>
>>
>>
--
Best Regards,
Yanjun.Zhu