Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt operations

[Phil Yang <Phil.Yang@arm.com>]

> -----Original Message-----
> From: Olivier Matz <olivier.matz@6wind.com>
> Sent: Wednesday, July 8, 2020 7:43 PM
> To: Phil Yang <Phil.Yang@arm.com>
> Cc: Stephen Hemminger <stephen@networkplumber.org>;
> david.marchand@redhat.com; dev@dpdk.org; drc@linux.vnet.ibm.com;
> Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
> <Ruifeng.Wang@arm.com>; nd <nd@arm.com>
> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt
> operations
> 
> On Wed, Jul 08, 2020 at 04:48:33AM +0000, Phil Yang wrote:
> > > -----Original Message-----
> > > From: Stephen Hemminger <stephen@networkplumber.org>
> > > Sent: Wednesday, July 8, 2020 1:13 AM
> > > To: Phil Yang <Phil.Yang@arm.com>
> > > Cc: david.marchand@redhat.com; dev@dpdk.org;
> drc@linux.vnet.ibm.com;
> > > Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> > > olivier.matz@6wind.com; Ruifeng Wang <Ruifeng.Wang@arm.com>; nd
> > > <nd@arm.com>
> > > Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use C11 atomics for refcnt
> > > operations
> > >
> > > On Tue,  7 Jul 2020 18:10:33 +0800
> > > Phil Yang <phil.yang@arm.com> wrote:
> > >
> > > > +	return (uint16_t)(__atomic_add_fetch((int16_t *)&shinfo-
> > > >refcnt_atomic,
> > > > +
> > >
> > > Why do you need so many casts here?
> > > The type of refcnt_atomic is now uint16 after your patch.
> >
> > In the existing code, the input parameter type for this API is signed integer.
> For example:
> > drivers/net/netvsc/hn_rxtx.c:531
> > lib/librte_mbuf/rte_mbuf.h:1194
> >
> > However, the output type of rte_mbuf_ext_refcnt related APIs is not
> uniform. We use these typecast to consistent with the current API definition.
> 
> Would it make sense to cast the increment instead?

Yes. It is better.
Thanks.

> 
> I mean:
> 
> 	return __atomic_add_fetch(&m->refcnt, (uint16_t)value,
> __ATOMIC_ACQ_REL);
> 
> instead of:
> 
> 	return (uint16_t)(__atomic_add_fetch((int16_t *)&m->refcnt, value,
> 				__ATOMIC_ACQ_REL));
> 
> 
> The same could apply to __rte_pktmbuf_pinned_extbuf_decref() I think:
> 
> > -	if (likely(rte_atomic16_add_return
> > -			(&shinfo->refcnt_atomic, -1)))
> > +	if (likely(__atomic_add_fetch((int *)&shinfo->refcnt_atomic, -1,
> > +				     __ATOMIC_ACQ_REL)))
> 
> By the way, why was the cast was to (int *) in this case? I think
> it can overwrite fields beside refcnt.
Fixed in the next version.

Thanks,
Phil