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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Richardson, Bruce
> Sent: Monday, March 9, 2020 3:16 PM
> > From: dev <dev-bounces@dpdk.org> On Behalf Of Morten Brørup
> > Sent: Monday, March 9, 2020 1:31 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)
> >
> >
> +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

I prefer Gavin's academically correct solution, i.e. using unions.

But check out the code using rearm_data! It's all drivers, and they all type cast it, except the SFC PMD.

So Bruce's suggestion is probably more workable than mine, although the #define'd name is missing an RTE prefix. (And mine is missing an &.)


And a completely alternative route is open: Type cast rearm_data in the SFC PMD, and postpone the MARKER discussion to later.

-Morten