Re: [dpdk-dev] [PATCH] gso: fix free issue of mbuf gso segments attach to

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> 
> > -----Original Message-----
> > From: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > Sent: Wednesday, October 14, 2020 8:06 PM
> > To: yang_y_yi <yang_y_yi@163.com>; Hu, Jiayu <jiayu.hu@intel.com>
> > Cc: dev@dpdk.org; olivier.matz@6wind.com; thomas@monjalon.net;
> > yangyi01@inspur.com
> > Subject: RE: Re:RE: [PATCH] gso: fix free issue of mbuf gso segments attach to
> >
> >
> > > From: yang_y_yi <yang_y_yi@163.com>
> > > Sent: Wednesday, October 14, 2020 3:56 AM
> > > To: Hu, Jiayu <jiayu.hu@intel.com>
> > > Cc: Ananyev, Konstantin <konstantin.ananyev@intel.com>; dev@dpdk.org;
> > olivier.matz@6wind.com; thomas@monjalon.net;
> > > yangyi01@inspur.com
> > > Subject: Re:RE: [PATCH] gso: fix free issue of mbuf gso segments attach to
> > >
> > > I think it isn't a good idea to free it in rte_gso_segment, maybe application
> > will continue to use this pkt for other purpose, rte_gso_segment
> > > can't make decision for application without any notice, it is better to return
> > this decision right backt to application.
> > >
> >
> > I think, if user wants to keep original packet, he can always call
> > rte_pktmbuf_refcnt_update(pkt, 1)
> > just before calling gso function.
> >
> > Also, as I remember in some cases it is not safe to do free() for input packet
> > (as pkt_out[] can contain input pkt itself). Would it also be user responsibility
> > to determine
> > such situations?
> 
> In what case will pkt_out[] contain the input pkt? Can you give an example?

As I can read the code, whenever gso code decides that
no segmentation is not really needed, or it is not capable
of doing it properly.
Let say:

gso_tcp4_segment(struct rte_mbuf *pkt,
                uint16_t gso_size,
                uint8_t ipid_delta,
                struct rte_mempool *direct_pool,
                struct rte_mempool *indirect_pool,
                struct rte_mbuf **pkts_out,
                uint16_t nb_pkts_out)
{
	...
	/* Don't process the fragmented packet */
        ipv4_hdr = (struct rte_ipv4_hdr *)(rte_pktmbuf_mtod(pkt, char *) +
                        pkt->l2_len);
        frag_off = rte_be_to_cpu_16(ipv4_hdr->fragment_offset);
        if (unlikely(IS_FRAGMENTED(frag_off))) {
                pkts_out[0] = pkt;
                return 1;
        }

        /* Don't process the packet without data */
        hdr_offset = pkt->l2_len + pkt->l3_len + pkt->l4_len;
        if (unlikely(hdr_offset >= pkt->pkt_len)) {
                pkts_out[0] = pkt;
                return 1;
        }

That's why in rte_gso_segment() we update refcnt only when ret > 1.