[dpdk-dev] [PATCH v2] mbuf: use refcnt = 0 when debugging

Ananyev, Konstantin konstantin.ananyev at intel.com
Wed Sep 6 13:58:47 CEST 2017



> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Chas Williams
> Sent: Wednesday, September 6, 2017 11:46 AM
> To: Nicolau, Radu <radu.nicolau at intel.com>; dev at dpdk.org
> Cc: olivier.matz at 6wind.com; cw817q at att.com
> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use refcnt = 0 when debugging
> 
> [Note: My former email address is going away eventually.  I am moving the
> conversation to my other email address which is a bit more permanent.]
> 
> On Mon, 2017-09-04 at 15:27 +0100, Radu Nicolau wrote:
> >
> > On 8/7/2017 5:11 PM, Charles (Chas) Williams wrote:
> > > After commit 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool") is it
> > > much harder to detect a "double free".  If the developer makes a copy
> > > of an mbuf pointer and frees it twice, this condition is never detected
> > > and the mbuf gets returned to the pool twice.
> > >
> > > Since this requires extra work to track, make this behavior conditional
> > > on CONFIG_RTE_LIBRTE_MBUF_DEBUG.
> > >
> > > Signed-off-by: Chas Williams <ciwillia at brocade.com>
> > > ---
> > >
> > > @@ -1304,10 +1329,13 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > >   			m->next = NULL;
> > >   			m->nb_segs = 1;
> > >   		}
> > > +#ifdef RTE_LIBRTE_MBUF_DEBUG
> > > +		rte_mbuf_refcnt_set(m, RTE_MBUF_UNUSED_CNT);
> > > +#endif
> > >
> > >   		return m;
> > >
> > > -       } else if (rte_atomic16_add_return(&m->refcnt_atomic, -1) == 0) {
> > > +	} else if (rte_mbuf_refcnt_update(m, -1) == 0) {
> > Why replace the use of atomic operation?
> 
> It doesn't.  rte_mbuf_refcnt_update() is also atomic(ish) but it slightly more
> optimal.  This whole section is a little hazy actually.  It looks like
> rte_pktmbuf_prefree_seg() unwraps rte_mbuf_refcnt_update() so they can avoid
> setting the refcnt when the refcnt is already the 'correct' value.

You don't need to use refcnt_update() here - if you take that path it already means
that m->refcnt_atomic != 1.
In fact, I think using refcnt_update () here might be a bit slower - as it means extra read.
Konstantin

> 
> > >
> > >
> > >   		if (RTE_MBUF_INDIRECT(m))
> > > @@ -1317,7 +1345,7 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> > >   			m->next = NULL;
> > >   			m->nb_segs = 1;
> > >   		}
> > > -		rte_mbuf_refcnt_set(m, 1);
> > > +		rte_mbuf_refcnt_set(m, RTE_MBUF_UNUSED_CNT);
> > >
> > >   		return m;
> > >   	}
> > Reviewed-by:  Radu Nicolau <radu.nicolau at intel.com>
> 
> Thanks for the review.


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