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From: Gavin Hu <Gavin.Hu@arm.com>
To: Morten Brørup <mb@smartsharesystems.com>,
	Ferruh Yigit <ferruh.yigit@intel.com>,
	"dev@dpdk.org" <dev@dpdk.org>
Cc: nd <nd@arm.com>,
	"david.marchand@redhat.com" <david.marchand@redhat.com>,
	 "thomas@monjalon.net" <thomas@monjalon.net>,
	"ktraynor@redhat.com" <ktraynor@redhat.com>,
	"jerinj@marvell.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" <stable@dpdk.org>,
	Olivier MATZ <olivier.matz@6wind.com>,
	Konstantin Ananyev <konstantin.ananyev@intel.com>,
	Andrew Rybchenko <arybchenko@solarflare.com>, nd <nd@arm.com>
Subject: Re: [dpdk-stable] [dpdk-dev] [PATCH v2] mbuf: replace zero-length marker with unnamed union
Date: Wed, 11 Mar 2020 07:50:12 +0000
Message-ID: <VI1PR08MB5376761A8FD748CA357472C78FFC0@VI1PR08MB5376.eurprd08.prod.outlook.com> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35C60EA3@smartserver.smartshare.dk>

Hi Morten,

> -----Original Message-----
> From: Morten Brørup <mb@smartsharesystems.com>
> Sent: Monday, March 9, 2020 9:31 PM
> To: Ferruh Yigit <ferruh.yigit@intel.com>; 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
> 
> > 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)

> 
> 
> Med venlig hilsen / kind regards
> - Morten Brørup


  parent reply	other threads:[~2020-03-11  7:50 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-03 16:27 [dpdk-stable] [PATCH v1] " Gavin Hu
2020-03-04 12:32 ` [dpdk-stable] [dpdk-dev] " Kevin Traynor
2020-03-07 14:52   ` Gavin Hu
2020-03-07 15:56 ` [dpdk-stable] [PATCH v2] " Gavin Hu
2020-03-09  8:55   ` [dpdk-stable] [dpdk-dev] " Ferruh Yigit
2020-03-09  9:45     ` Gavin Hu
2020-03-09 11:29       ` Ferruh Yigit
2020-03-09 13:30         ` Morten Brørup
2020-03-09 14:16           ` Richardson, Bruce
2020-03-09 14:50             ` [dpdk-stable] [dpdk-dev] [PATCH v2] mbuf: replace zero-length markerwith " Morten Brørup
2020-03-11  7:50           ` Gavin Hu [this message]
2020-03-11  9:04             ` [dpdk-stable] [dpdk-dev] [PATCH v2] mbuf: replace zero-length marker with " Morten Brørup
2020-03-11 12:07               ` Bruce Richardson
2020-03-13  7:36                 ` Gavin Hu
2020-03-13  9:22                 ` Gavin Hu
2020-04-07 17:13                   ` Kevin Traynor
2020-04-08 15:04                     ` Gavin Hu
2020-04-08 15:22                       ` David Marchand
2020-04-09  9:48                         ` Gavin Hu
2020-04-09 10:49                           ` Thomas Monjalon
2020-04-09 16:09                             ` Ray Kinsella
2020-04-11  2:50                             ` Gavin Hu
2020-05-14 13:24                         ` Kevin Traynor
2020-03-09 15:47     ` Stephen Hemminger

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