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

[Zhu Yanjun <yanjun.zhu@linux.dev>]

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