[dpdk-dev] [PATCH v4] mbuf: decrease refcnt when detaching

[Hiroyuki Mikita <h.mikita89@gmail.com>]

The rte_pktmbuf_detach() function should decrease refcnt on a direct
buffer.

Signed-off-by: Hiroyuki Mikita <h.mikita89@gmail.com>
---
v4:
* fixed formatting.
* fixed doxygen comments of rte_pktmbuf_detach() to be clearer.

v3:
* fixed rte_pktmbuf_detach() to decrease refcnt.
* free the direct mbuf when refcnt becomes 0.
* added this issue to Resolved Issues in release notes.

v2:
* introduced a new function rte_pktmbuf_detach2() which decrease refcnt.
* marked rte_pktmbuf_detach() as deprecated.
* added comments about refcnt to rte_pktmbuf_attach() and rte_pktmbuf_detach().
* checked refcnt when detaching in unit tests.
* added this issue to release notes.

 app/test/test_mbuf.c                   |  4 ++++
 doc/guides/rel_notes/release_16_07.rst |  6 ++++++
 lib/librte_mbuf/rte_mbuf.h             | 23 ++++++++++++-----------
 3 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/app/test/test_mbuf.c b/app/test/test_mbuf.c
index 98ff93a..8460db7 100644
--- a/app/test/test_mbuf.c
+++ b/app/test/test_mbuf.c
@@ -511,10 +511,14 @@ test_attach_from_different_pool(void)
 	rte_pktmbuf_detach(clone);
 	if (c_data != rte_pktmbuf_mtod(clone, char *))
 		GOTO_FAIL("clone was not detached properly\n");
+	if (rte_mbuf_refcnt_read(m) != 2)
+		GOTO_FAIL("invalid refcnt in m\n");
 
 	rte_pktmbuf_detach(clone2);
 	if (c_data2 != rte_pktmbuf_mtod(clone2, char *))
 		GOTO_FAIL("clone2 was not detached properly\n");
+	if (rte_mbuf_refcnt_read(m) != 1)
+		GOTO_FAIL("invalid refcnt in m\n");
 
 	/* free the clones and the initial mbuf */
 	rte_pktmbuf_free(clone2);
diff --git a/doc/guides/rel_notes/release_16_07.rst b/doc/guides/rel_notes/release_16_07.rst
index 58c8ef9..a8c50c8 100644
--- a/doc/guides/rel_notes/release_16_07.rst
+++ b/doc/guides/rel_notes/release_16_07.rst
@@ -69,6 +69,12 @@ Drivers
 Libraries
 ~~~~~~~~~
 
+* **mbuf: Fixed refcnt update when detaching.**
+
+  Fix the ``rte_pktmbuf_detach()`` function to decrement the direct
+  mbuf's reference counter. The previous behavior was not to affect
+  the reference counter. It lead a memory leak of the direct mbuf.
+
 
 Examples
 ~~~~~~~~
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 7b92b88..48911a6 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -1412,6 +1412,8 @@ static inline int rte_pktmbuf_alloc_bulk(struct rte_mempool *pool,
  *
  * After attachment we refer the mbuf we attached as 'indirect',
  * while mbuf we attached to as 'direct'.
+ * The direct mbuf's reference counter is incremented.
+ *
  * Right now, not supported:
  *  - attachment for already indirect mbuf (e.g. - mi has to be direct).
  *  - mbuf we trying to attach (mi) is used by someone else
@@ -1465,13 +1467,17 @@ static inline void rte_pktmbuf_attach(struct rte_mbuf *mi, struct rte_mbuf *m)
  *
  *  - restore original mbuf address and length values.
  *  - reset pktmbuf data and data_len to their default values.
- *  All other fields of the given packet mbuf will be left intact.
+ *  - decrement the direct mbuf's reference counter. When the
+ *  reference counter becomes 0, the direct mbuf is freed.
+ *
+ * All other fields of the given packet mbuf will be left intact.
  *
  * @param m
  *   The indirect attached packet mbuf.
  */
 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;
 
@@ -1486,6 +1492,9 @@ static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
 	m->data_off = RTE_MIN(RTE_PKTMBUF_HEADROOM, (uint16_t)m->buf_len);
 	m->data_len = 0;
 	m->ol_flags = 0;
+
+	if (rte_mbuf_refcnt_update(md, -1) == 0)
+		__rte_mbuf_raw_free(md);
 }
 
 static inline struct rte_mbuf* __attribute__((always_inline))
@@ -1494,17 +1503,9 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
 	__rte_mbuf_sanity_check(m, 0);
 
 	if (likely(rte_mbuf_refcnt_update(m, -1) == 0)) {
-
-		/* if this is an indirect mbuf, then
-		 *  - detach mbuf
-		 *  - free attached mbuf segment
-		 */
-		if (RTE_MBUF_INDIRECT(m)) {
-			struct rte_mbuf *md = rte_mbuf_from_indirect(m);
+		/* if this is an indirect mbuf, it is detached. */
+		if (RTE_MBUF_INDIRECT(m))
 			rte_pktmbuf_detach(m);
-			if (rte_mbuf_refcnt_update(md, -1) == 0)
-				__rte_mbuf_raw_free(md);
-		}
 		return m;
 	}
 	return NULL;
-- 
1.9.1