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Message-ID: <1504706130.2192.11.camel@gmail.com>
From: Chas Williams <3chas3@gmail.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>, "Nicolau, Radu"
<radu.nicolau@intel.com>, "dev@dpdk.org" <dev@dpdk.org>
Cc: "olivier.matz@6wind.com" <olivier.matz@6wind.com>, "cw817q@att.com"
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Date: Wed, 06 Sep 2017 09:55:30 -0400
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Subject: Re: [dpdk-dev] [PATCH v2] mbuf: use refcnt = 0 when debugging
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On Wed, 2017-09-06 at 11:58 +0000, Ananyev, Konstantin wrote:
>
> > -----Original Message-----
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Chas Williams
> > Sent: Wednesday, September 6, 2017 11:46 AM
> > To: Nicolau, Radu <radu.nicolau@intel.com>; dev@dpdk.org
> > Cc: olivier.matz@6wind.com; cw817q@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@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
Yes, that is somewhat the point. If a mbuf can have a refcnt of 0,
then we want to go into rte_mbuf_refcnt_update() which detects 0 -> -1.
I could explicitly check this in prefree_seg but I was just restored the
previous call into refcnt_update. I could explicitly check for refcnt =
0 in prefree_seg() but since we do have a routine for this...
> > > >
> > > >
> > > > 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@intel.com>
> >
> > Thanks for the review.