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

[Ferruh Yigit]

On 3/9/2020 9:45 AM, Gavin Hu wrote:
> Hi Ferruh,
> 
>> -----Original Message-----
>> From: Ferruh Yigit <ferruh.yigit at intel.com>
>> Sent: Monday, March 9, 2020 4:55 PM
>> To: Gavin Hu <Gavin.Hu at arm.com>; dev at dpdk.org
>> Cc: nd <nd at arm.com>; david.marchand at redhat.com; thomas at monjalon.net;
>> ktraynor at redhat.com; jerinj at marvell.com; Honnappa Nagarahalli
>> <Honnappa.Nagarahalli at arm.com>; Ruifeng Wang
>> <Ruifeng.Wang at arm.com>; Phil Yang <Phil.Yang at arm.com>; Joyce Kong
>> <Joyce.Kong at arm.com>; stable at dpdk.org; Olivier MATZ
>> <olivier.matz at 6wind.com>; Konstantin Ananyev
>> <konstantin.ananyev at intel.com>; Andrew Rybchenko
>> <arybchenko at solarflare.com>
>> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: replace zero-length marker with
>> unnamed union
>>
>> 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 at dpdk.org
>>>
>>> Signed-off-by: Gavin Hu <gavin.hu at 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?