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

[Richardson, Bruce]

> -----Original Message-----
> From: dev <dev-bounces at dpdk.org> On Behalf Of Morten Brørup
> Sent: Monday, March 9, 2020 1:31 PM
> To: Yigit, Ferruh <ferruh.yigit at intel.com>; 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>; Ananyev,
> Konstantin <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
> 
> > From: dev [mailto:dev-bounces at 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 at 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 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?
> >
> 
> 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)
> 
> 
+1 for this, it's very similar to what I was going to suggest, which was:

uint16_t data_off;
#define _REARM_DATA data_off

but your suggestion is probably cleaner than mine.

/Bruce