On Wed, 2017-09-06 at 11:58 +0000, Ananyev, Konstantin wrote: > > > -----Original Message----- > > From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Chas Williams > > Sent: Wednesday, September 6, 2017 11:46 AM > > To: Nicolau, Radu <radu.nico...@intel.com>; dev@dpdk.org > > Cc: olivier.m...@6wind.com; cw8...@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 <ciwil...@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.nico...@intel.com> > > > > Thanks for the review.
