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

[Gavin Hu <Gavin.Hu@arm.com>]

Hi Ferruh,

> -----Original Message-----
> From: Ferruh Yigit <ferruh.yigit@intel.com>
> Sent: Monday, March 9, 2020 4:55 PM
> To: Gavin Hu <Gavin.Hu@arm.com>; dev@dpdk.org
> Cc: 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 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. 
/Gavin