[dpdk-dev] [PATCH 1/6] ring: change head and tail to pointer-width size

[Eads, Gage]

> -----Original Message-----
> From: Burakov, Anatoly
> Sent: Monday, January 21, 2019 8:15 AM
> To: Eads, Gage <gage.eads at intel.com>; Richardson, Bruce
> <bruce.richardson at intel.com>
> Cc: dev at dpdk.org; olivier.matz at 6wind.com; arybchenko at solarflare.com;
> Ananyev, Konstantin <konstantin.ananyev at intel.com>
> Subject: Re: [dpdk-dev] [PATCH 1/6] ring: change head and tail to pointer-width
> size
> 
> On 11-Jan-19 7:27 PM, Eads, Gage wrote:
> >
> >
> >> -----Original Message-----
> >> From: Richardson, Bruce
> >> Sent: Friday, January 11, 2019 5:59 AM
> >> To: Burakov, Anatoly <anatoly.burakov at intel.com>
> >> Cc: Eads, Gage <gage.eads at intel.com>; dev at dpdk.org;
> >> olivier.matz at 6wind.com; arybchenko at solarflare.com; Ananyev,
> >> Konstantin <konstantin.ananyev at intel.com>
> >> Subject: Re: [dpdk-dev] [PATCH 1/6] ring: change head and tail to
> >> pointer-width size
> >>
> >> On Fri, Jan 11, 2019 at 11:30:24AM +0000, Burakov, Anatoly wrote:
> >>> On 11-Jan-19 10:58 AM, Bruce Richardson wrote:
> >>>> On Fri, Jan 11, 2019 at 10:40:19AM +0000, Burakov, Anatoly wrote:
> >>>>> <...>
> >>>>>
> >>>>>> + * Copyright(c) 2016-2019 Intel Corporation
> >>>>>>      */
> >>>>>>     /**
> >>>>>> @@ -88,7 +88,7 @@ rte_event_ring_enqueue_burst(struct
> >> rte_event_ring *r,
> >>>>>>     		const struct rte_event *events,
> >>>>>>     		unsigned int n, uint16_t *free_space)
> >>>>>>     {
> >>>>>> -	uint32_t prod_head, prod_next;
> >>>>>> +	uintptr_t prod_head, prod_next;
> >>>>>
> >>>>> I would also question the use of uinptr_t. I think semnatically,
> >>>>> size_t is more appropriate.
> >>>>>
> >>>> Yes, it would, but I believe in this case they want to use the
> >>>> largest size of (unsigned)int where there exists an atomic for
> >>>> manipulating 2 of them simultaneously. [The largest size is to
> >>>> minimize any chance of an ABA issue occuring]. Therefore we need
> >>>> 32-bit values on 32-bit and 64-bit on 64, and I suspect the best
> >>>> way to guarantee this is to use pointer-sized values. If size_t is
> >>>> guaranteed across all OS's to have the same size as uintptr_t it
> >>>> could also be
> >> used, though.
> >>>>
> >>>> /Bruce
> >>>>
> >>>
> >>> Technically, size_t and uintptr_t are not guaranteed to match. In
> >>> practice, they won't match only on architectures that DPDK doesn't
> >>> intend to run on (such as 16-bit segmented archs, where size_t would
> >>> be 16-bit but uinptr_t would be 32-bit).
> >>>
> >>> In all the rest of DPDK code, we use size_t for this kind of thing.
> >>>
> >>
> >> Ok.
> >> If we do use size_t, I think we also need to add a compile-time check
> >> into the build too, to error out if sizeof(size_t) != sizeof(uintptr_t).
> >
> > Ok, I wasn't aware of the precedent of using size_t for this purpose. I'll change
> it and look into adding a static assert.
> 
> RTE_BUILD_BUG_ON?

Appreciate the pointer, but with the changes needed to preserve ABI compatibility* this is no longer necessary.

*http://mails.dpdk.org/archives/dev/2019-January/123775.html

> 
> >
> > Thanks,
> > Gage
> >
> 
> 
> --
> Thanks,
> Anatoly