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

From: Olivier Matz <olivier.matz@6wind.com>
To: Phil Yang <Phil.Yang@arm.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>,
	"david.marchand@redhat.com" <david.marchand@redhat.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	"drc@linux.vnet.ibm.com" <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
Date: Wed, 8 Jul 2020 13:43:02 +0200	[thread overview]
Message-ID: <20200708114301.GN5869@platinum> (raw)
In-Reply-To: <VE1PR08MB4640935CA367E216F788DEE5E9670@VE1PR08MB4640.eurprd08.prod.outlook.com>

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?

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.