Re: [dpdk-dev] [PATCH v2] mbuf: replace zero-length marker with unnamed union

[Kevin Traynor <ktraynor@redhat.com>]

On 13/03/2020 09:22, Gavin Hu wrote:
> Hi Bruce,
> 
>> -----Original Message-----
>> From: Bruce Richardson <bruce.richardson@intel.com>
>> Sent: Wednesday, March 11, 2020 8:08 PM
>> To: Morten Brørup <mb@smartsharesystems.com>
>> Cc: Gavin Hu <Gavin.Hu@arm.com>; Ferruh Yigit <ferruh.yigit@intel.com>;
>> dev@dpdk.org; nd <nd@arm.com>; david.marchand@redhat.com;
>> thomas@monjalon.net; ktraynor@redhat.com; jerinj@marvell.com;
>> Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
>> <Ruifeng.Wang@arm.com>; Phil Yang <Phil.Yang@arm.com>; Joyce Kong
>> <Joyce.Kong@arm.com>; stable@dpdk.org; Olivier MATZ
>> <olivier.matz@6wind.com>; Konstantin Ananyev
>> <konstantin.ananyev@intel.com>; Andrew Rybchenko
>> <arybchenko@solarflare.com>
>> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: replace zero-length marker with
>> unnamed union
>>
>> On Wed, Mar 11, 2020 at 10:04:33AM +0100, Morten Brørup wrote:
>>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Gavin Hu
>>>> Sent: Wednesday, March 11, 2020 8:50 AM
>>>>
>>>> Hi Morten,
>>>>
>>>>> From: Morten Brørup <mb@smartsharesystems.com>
>>>>> Sent: Monday, March 9, 2020 9:31 PM
>>>>>
>>>>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit
>>>>>> Sent: Monday, March 9, 2020 12:30 PM
>>>>>>
>>>>>> On 3/9/2020 9:45 AM, Gavin Hu wrote:
>>>>>>> Hi Ferruh,
>>>>>>>
>>>>>>>> -----Original Message-----
>>>>>>>> From: Ferruh Yigit <ferruh.yigit@intel.com>
>>>>>>>> Sent: Monday, March 9, 2020 4:55 PM
>>>>>>>>
>>>>>>>> On 3/7/2020 3:56 PM, Gavin Hu wrote:
>>>>>>>>> Declaring zero-length arrays in other contexts, including as
>>>>>> interior
>>>>>>>>> members of structure objects or as non-member objects, is
>>>>>> discouraged.
>>>>>>>>> Accessing elements of zero-length arrays declared in such
>>>> contexts
>>>>>> is
>>>>>>>>> undefined and may be diagnosed.[1]
>>>>>>>>>
>>>>>>>>> Fix by using unnamed union and struct.
>>>>>>>>>
>>>>>>>>> https://bugs.dpdk.org/show_bug.cgi?id=396
>>>>>>>>>
>>>>>>>>> Bugzilla ID: 396
>>>>>>>>>
>>>>>>>>> [1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
>>>>>>>>>
>>>>>>>>> Fixes: 3e6181b07038 ("mbuf: use structure marker from EAL")
>>>>>>>>> Cc: stable@dpdk.org
>>>>>>>>>
>>>>>>>>> Signed-off-by: Gavin Hu <gavin.hu@arm.com>
>>>>>>>>> ---
>>>>>>>>> v2:
>>>>>>>>> * change 'uint64_t rearm_data' to 'uint_64_t rearm_data[1]' to
>>>> fix
>>>>>>>>>   the SFC PMD compiling error on x86. <Kevin Traynor>
>>>>>>>>> ---
>>>>>>>>>  lib/librte_mbuf/rte_mbuf_core.h | 54 +++++++++++++++++++----
>> --
>>>> ----
>>>>>> ----
>>>>>>>>>  1 file changed, 32 insertions(+), 22 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/lib/librte_mbuf/rte_mbuf_core.h
>>>>>>>> b/lib/librte_mbuf/rte_mbuf_core.h
>>>>>>>>> index b9a59c879..34cb152e2 100644
>>>>>>>>> --- a/lib/librte_mbuf/rte_mbuf_core.h
>>>>>>>>> +++ b/lib/librte_mbuf/rte_mbuf_core.h
>>>>>>>>> @@ -480,31 +480,41 @@ struct rte_mbuf {
>>>>>>>>>  		rte_iova_t buf_physaddr; /**< deprecated */
>>>>>>>>>  	} __rte_aligned(sizeof(rte_iova_t));
>>>>>>>>>
>>>>>>>>> -	/* next 8 bytes are initialised on RX descriptor rearm */
>>>>>>>>> -	RTE_MARKER64 rearm_data;
>>>>>>>>> -	uint16_t data_off;
>>>>>>>>> -
>>>>>>>>> -	/**
>>>>>>>>> -	 * Reference counter. Its size should at least equal to the
>>>> size
>>>>>>>>> -	 * of port field (16 bits), to support zero-copy broadcast.
>>>>>>>>> -	 * It should only be accessed using the following
>>>> functions:
>>>>>>>>> -	 * rte_mbuf_refcnt_update(), rte_mbuf_refcnt_read(), and
>>>>>>>>> -	 * rte_mbuf_refcnt_set(). The functionality of these
>>>> functions
>>>>>> (atomic,
>>>>>>>>> -	 * or non-atomic) is controlled by the
>>>>>>>> CONFIG_RTE_MBUF_REFCNT_ATOMIC
>>>>>>>>> -	 * config option.
>>>>>>>>> -	 */
>>>>>>>>>  	RTE_STD_C11
>>>>>>>>>  	union {
>>>>>>>>> -		rte_atomic16_t refcnt_atomic; /**< Atomically
>>>> accessed
>>>>>>>> refcnt */
>>>>>>>>> -		/** Non-atomically accessed refcnt */
>>>>>>>>> -		uint16_t refcnt;
>>>>>>>>> -	};
>>>>>>>>> -	uint16_t nb_segs;         /**< Number of segments. */
>>>>>>>>> +		/* next 8 bytes are initialised on RX descriptor
>>>> rearm */
>>>>>>>>> +		uint64_t rearm_data[1];
>>>>>>>> We are using zero length array as markers only and know what we
>>>> are
>>>>>> doing
>>>>>>>> with them,
>>>>>>>> what would you think disabling the warning instead of increasing
>>>> the
>>>>>>>> complexity
>>>>>>>> in mbuf struct?
>>>>>>> Okay, I will add -Wno-zero-length-bounds to the compiler
>>>> toolchain
>>>>>> flags.
>>>>>>
>>>>>> This would be my preference but I would like to get more input, can
>>>> you
>>>>>> please
>>>>>> for more comments before changing the implementation in case there
>>>> are
>>>>>> some
>>>>>> strong opinion on it?
>>>>>>
>>>>>
>>>>> I have some input to this discussion.
>>>>>
>>>>> Let me repeat what Gavin's GCC reference states: Declaring zero-
>>>> length
>>>>> arrays [...] as interior members of structure objects [...] is
>>>> discouraged.
>>>>>
>>>>> Why would we do something that the compiler documentation says is
>>>>> discouraged? I think the problem (i.e. using discouraged techniques)
>>>> should
>>>>> be fixed, not the symptom (i.e. getting warnings about using
>>>> discouraged
>>>>> techniques).
>>>>>
>>>>> Compiler warnings are here to help, and in my experience they are
>>>> actually
>>>>> very helpful, although avoiding them often requires somewhat more
>>>>> verbose source code. Disabling this warning not only affects this
>>>> file, but
>>>>> disables warnings about potential bugs in other source code too.
>>>>>
>>>>> Generally, disabling compiler warnings is a slippery slope. It would
>>>> be
>>>>> optimal if DPDK could be compiled with -Wall, and it would probably
>>>> reduce
>>>>> the number of released bugs too.
>>>>>
>>>>> With that said, sometimes the optimal solution has to give way for
>>>> the
>>>>> practical solution. And this is a core file, so we should thread
>>>> lightly.
>>>>>
>>>>>
>>>>> As for an alternative solution, perhaps we can get rid of the MARKERs
>>>> in the
>>>>> struct and #define them instead. Not as elegant as Gavin's suggested
>>>> union
>>>>> based solution, but it might bring inspiration...
>>>>>
>>>>> struct rte_mbuf {
>>>>>     ...
>>>>>     } __rte_aligned(sizeof(rte_iova_t));
>>>>>
>>>>>     uint16_t data_off;
>>>>>     ...
>>>>> }
>>>>>
>>>>> #define rte_mbuf_rearm_data(m) ((uint64_t *)m->data_off)
>>>>
>>>> This does not work out, it generates new errors:
>>>> /root/dpdk/build/include/rte_mbuf_core.h:485:33: error: dereferencing
>>>> type-punned pointer will break strict-aliasing rules [-Werror=strict-
>>>> aliasing]
>>>>   485 | #define rte_mbuf_rearm_data(m) ((uint64_t *)&m->data_off)
>>>>
>>>
>>> OK. Then Bruce's suggestion probably won't work either.
>>>
>>> I found this article about strict aliasing:
>> https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8
>>>
>>> The article basically says that the union based method (i.e. your original
>> suggestion) is valid C (but not C++) and is the common solution.
>>>
>>> Alternatives have now been discussed and tested, so we should all support
>> your original suggestion, which seems to be the only correct and viable solution.
>>>
>>> Please go ahead with that, and then someone should update the SFC PMD
>> accordingly.
>>>
>>> Furthermore, I think that Stephen's suggestion about getting rid of the
>> markers all together is good thinking, but it would require updating a lot of
>> PMDs accordingly. So please also consider removing other markers that can be
>> removed without affecting a whole bunch of other files.
>>>
>>
>> Does it still give errors if we don't have the cast in the macro?
> 
> Yes, it gives errors elsewhere that have the cast. 
> 

Hi Gavin, I lost track if v2 is still a candidate for merge. fwiw, it
compiles without giving the zero-length-bounds warning on my system.

Kevin.