[dpdk-dev] [PATCH] mbuf: fix reset on mbuf free

[Olivier Matz]

m->nb_seg must be reset on mbuf free whatever the value of m->next,
because it can happen that m->nb_seg is != 1. For instance in this
case:

  m1 = rte_pktmbuf_alloc(mp);
  rte_pktmbuf_append(m1, 500);
  m2 = rte_pktmbuf_alloc(mp);
  rte_pktmbuf_append(m2, 500);
  rte_pktmbuf_chain(m1, m2);
  m0 = rte_pktmbuf_alloc(mp);
  rte_pktmbuf_append(m0, 500);
  rte_pktmbuf_chain(m0, m1);

As rte_pktmbuf_chain() does not reset nb_seg in the initial m1
segment (this is not required), after this code the mbuf chain
have 3 segments:
  - m0: next=m1, nb_seg=3
  - m1: next=m2, nb_seg=2
  - m2: next=NULL, nb_seg=1

Freeing this mbuf chain will not restore nb_seg=1 in the second
segment. This is expected that mbufs stored in pool have their
nb_seg field set to 1.

Fixes: 8f094a9ac5d7 ("mbuf: set mbuf fields while in pool")
Cc: stable at dpdk.org

Signed-off-by: Olivier Matz <olivier.matz at 6wind.com>
---
 lib/librte_mbuf/rte_mbuf.c |  6 ++----
 lib/librte_mbuf/rte_mbuf.h | 12 ++++--------
 2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/lib/librte_mbuf/rte_mbuf.c b/lib/librte_mbuf/rte_mbuf.c
index 8a456e5e64..e632071c23 100644
--- a/lib/librte_mbuf/rte_mbuf.c
+++ b/lib/librte_mbuf/rte_mbuf.c
@@ -129,10 +129,8 @@ rte_pktmbuf_free_pinned_extmem(void *addr, void *opaque)
 
 	rte_mbuf_ext_refcnt_set(m->shinfo, 1);
 	m->ol_flags = EXT_ATTACHED_MBUF;
-	if (m->next != NULL) {
-		m->next = NULL;
-		m->nb_segs = 1;
-	}
+	m->next = NULL;
+	m->nb_segs = 1;
 	rte_mbuf_raw_free(m);
 }
 
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index a1414ed7cd..ef5800c8ef 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -1329,10 +1329,8 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
 				return NULL;
 		}
 
-		if (m->next != NULL) {
-			m->next = NULL;
-			m->nb_segs = 1;
-		}
+		m->next = NULL;
+		m->nb_segs = 1;
 
 		return m;
 
@@ -1346,10 +1344,8 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
 				return NULL;
 		}
 
-		if (m->next != NULL) {
-			m->next = NULL;
-			m->nb_segs = 1;
-		}
+		m->next = NULL;
+		m->nb_segs = 1;
 		rte_mbuf_refcnt_set(m, 1);
 
 		return m;
-- 
2.25.1