Re: [dpdk-dev] [PATCH] mbuf: decrease refcnt when detaching

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

Hi Hiroyuki,

> 
> Hi Konstantin,
> 
> Now, the attach operation increases refcnt, but the detach does not decrease it.
> I think both operations should affect refcnt or not.
> Which design is intended?
> 
> In "6.7. Direct and Indirect Buffers" of Programmer's Guide,
> it is mentioned that "...whenever an indirect buffer is attached to
> the direct buffer,
> the reference counter on the direct buffer is incremented.
> Similarly, whenever the indirect buffer is detached,
> the reference counter on the direct buffer is decremented."

Well, right now the rte_pktmbuf_detach(struct rte_mbuf *m) just restores 
the fields of indirect mbufs to the default values, nothing more.
Actual freeing of the mbuf it was attached to is done in the __rte_pktmbuf_prefree_seg().
I suppose the name rte_pktmbuf_detach() is a bit confusing here,
might be, when created, it should be named rte_pktmbuf_restore() or so.
About proposed changes - it would introduce an extra unnecessary read of refcnt (as it is a volatile field).
If we'll decide to go that way, then I think rte_pktmbuf_detach() have to deal with freeing md.
Something like that:

static inline void 
rte_pktmbuf_detach(struct rte_mbuf *m)
{
         struct rte_mbuf *md = rte_mbuf_from_indirect(m);
         
          /* former rte_pktmbuf_detach */
          rte_pktmbuf_restore(m);    
          if (rte_mbuf_refcnt_update(md, -1) == 0)
               __rte_mbuf_raw_free(md);
}

That might be a good thing in terms of API usability and clearness,
but would cause a change in public API behaviour, so I am not sure it is worth it.
Konstantin 

> 
> Regards,
> Hiroyuki
> 
> 2016-05-16 9:05 GMT+09:00 Ananyev, Konstantin <konstantin.ananyev@intel.com>:
> >
> >
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Hiroyuki Mikita
> >> Sent: Sunday, May 15, 2016 4:51 PM
> >> To: olivier.matz@6wind.com
> >> Cc: dev@dpdk.org
> >> Subject: [dpdk-dev] [PATCH] mbuf: decrease refcnt when detaching
> >>
> >> The rte_pktmbuf_detach() function should decrease refcnt on a direct
> >> buffer.
> >
> > Hmm, why is that?
> > What's wrong with the current approach?
> > Konstantin
> >
> >>
> >> Signed-off-by: Hiroyuki Mikita <h.mikita89@gmail.com>
> >> ---
> >>  lib/librte_mbuf/rte_mbuf.h | 4 +++-
> >>  1 file changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> >> index 529debb..3b117ca 100644
> >> --- a/lib/librte_mbuf/rte_mbuf.h
> >> +++ b/lib/librte_mbuf/rte_mbuf.h
> >> @@ -1468,9 +1468,11 @@ static inline void rte_pktmbuf_attach(struct rte_mbuf *mi, struct rte_mbuf *m)
> >>   */
> >>  static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
> >>  {
> >> +     struct rte_mbuf *md = rte_mbuf_from_indirect(m);
> >>       struct rte_mempool *mp = m->pool;
> >>       uint32_t mbuf_size, buf_len, priv_size;
> >>
> >> +     rte_mbuf_refcnt_update(md, -1);
> >>       priv_size = rte_pktmbuf_priv_size(mp);
> >>       mbuf_size = sizeof(struct rte_mbuf) + priv_size;
> >>       buf_len = rte_pktmbuf_data_room_size(mp);
> >> @@ -1498,7 +1500,7 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> >>               if (RTE_MBUF_INDIRECT(m)) {
> >>                       struct rte_mbuf *md = rte_mbuf_from_indirect(m);
> >>                       rte_pktmbuf_detach(m);
> >> -                     if (rte_mbuf_refcnt_update(md, -1) == 0)
> >> +                     if (rte_mbuf_refcnt_read(md) == 0)
> >>                               __rte_mbuf_raw_free(md);
> >>               }
> >>               return m;
> >> --
> >> 1.9.1
> >