[dpdk-dev,2/5] mbuf: new helper to check if a mbuf is shared

diff --git a/app/test/test_mbuf.c b/app/test/test_mbuf.c
index c0823ea..7656a4d 100644
--- a/app/test/test_mbuf.c
+++ b/app/test/test_mbuf.c
@@ -333,6 +333,7 @@  testclone_testupdate_testdetach(void)
 	struct rte_mbuf *m = NULL;
 	struct rte_mbuf *clone = NULL;
 	struct rte_mbuf *clone2 = NULL;
+	struct rte_mbuf *m2 = NULL;
 	unaligned_uint32_t *data;
 
 	/* alloc a mbuf */
@@ -384,6 +385,11 @@  testclone_testupdate_testdetach(void)
 	if (*data != MAGIC_DATA)
 		GOTO_FAIL("invalid data in clone->next\n");
 
+	if (rte_pktmbuf_seg_is_shared(m) == 0)
+		GOTO_FAIL("m should be marked as shared\n");
+	if (rte_pktmbuf_seg_is_shared(clone) == 0)
+		GOTO_FAIL("clone should be marked as shared\n");
+
 	if (rte_mbuf_refcnt_read(m) != 2)
 		GOTO_FAIL("invalid refcnt in m\n");
 
@@ -410,14 +416,32 @@  testclone_testupdate_testdetach(void)
 	if (rte_mbuf_refcnt_read(m->next) != 3)
 		GOTO_FAIL("invalid refcnt in m->next\n");
 
+	/* prepend data to one of the clone */
+	m2 = rte_pktmbuf_alloc(pktmbuf_pool);
+	if (m2 == NULL)
+		GOTO_FAIL("cannot allocate m2");
+	rte_pktmbuf_append(m2, sizeof(uint32_t));
+	rte_pktmbuf_chain(m2, clone);
+	clone = NULL;
+
+	if (rte_pktmbuf_data_is_shared(m2, 0, sizeof(uint32_t)))
+		GOTO_FAIL("m2 headers should not be marked as shared");
+	if (rte_pktmbuf_data_is_shared(m2, sizeof(uint32_t),
+			rte_pktmbuf_pkt_len(m2) - sizeof(uint32_t)) == 0)
+		GOTO_FAIL("m2 data should be marked as shared");
+
 	/* free mbuf */
 	rte_pktmbuf_free(m);
-	rte_pktmbuf_free(clone);
-	rte_pktmbuf_free(clone2);
-
 	m = NULL;
-	clone = NULL;
+	rte_pktmbuf_free(m2);
+	m2 = NULL;
+
+	if (rte_pktmbuf_seg_is_shared(clone2))
+		GOTO_FAIL("clone2 should not be marked as shared\n");
+
+	rte_pktmbuf_free(clone2);
 	clone2 = NULL;
+
 	printf("%s ok\n", __func__);
 	return 0;
 
@@ -428,6 +452,8 @@  testclone_testupdate_testdetach(void)
 		rte_pktmbuf_free(clone);
 	if (clone2)
 		rte_pktmbuf_free(clone2);
+	if (m2)
+		rte_pktmbuf_free(m2);
 	return -1;
 }
 
diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 14956f6..cd77a56 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -576,7 +576,7 @@  rte_mbuf_data_dma_addr_default(const struct rte_mbuf *mb)
  *   The address of the direct mbuf corresponding to buffer_addr.
  */
 static inline struct rte_mbuf *
-rte_mbuf_from_indirect(struct rte_mbuf *mi)
+rte_mbuf_from_indirect(const struct rte_mbuf *mi)
 {
 	return (struct rte_mbuf *)RTE_PTR_SUB(mi->buf_addr, sizeof(*mi) + mi->priv_size);
 }
@@ -1574,6 +1574,75 @@  static inline int rte_pktmbuf_is_contiguous(const struct rte_mbuf *m)
 }
 
 /**
+ * Test if a mbuf segment is shared
+ *
+ * Return true if the data embedded in this segment is shared by several
+ * mbufs. In this case, the mbuf data should be considered as read-only.
+ *
+ * @param m
+ *   The packet mbuf.
+ * @return
+ *   - (1), the mbuf segment is shared (read-only)
+ *   - (0), the mbuf segment is not shared (writable)
+ */
+static inline int rte_pktmbuf_seg_is_shared(const struct rte_mbuf *m)
+{
+	if (rte_mbuf_refcnt_read(m) > 1)
+		return 1;
+
+	if (RTE_MBUF_INDIRECT(m) &&
+			rte_mbuf_refcnt_read(rte_mbuf_from_indirect(m)) > 1)
+		return 1;
+
+	return 0;
+}
+
+/**
+ * Test if some data in an mbuf chain is shared
+ *
+ * Return true if the specified data area in the mbuf chain is shared by
+ * several mbufs. In this case, this data should be considered as
+ * read-only.
+ *
+ * If the area described by off and len exceeds the bounds of the mbuf
+ * chain (off + len <= rte_pktmbuf_pkt_len()), the exceeding part of the
+ * area is ignored.
+ *
+ * @param m
+ *   The packet mbuf.
+ * @return
+ *   - (1), the mbuf data is shared (read-only)
+ *   - (0), the mbuf data is not shared (writable)
+ */
+static inline int rte_pktmbuf_data_is_shared(const struct rte_mbuf *m,
+	uint32_t off, uint32_t len)
+{
+	const struct rte_mbuf *seg = m;
+
+	if (likely(off + len <= rte_pktmbuf_data_len(seg)))
+		return rte_pktmbuf_seg_is_shared(seg);
+
+	while (seg->next && off >= rte_pktmbuf_data_len(seg)) {
+		off -= rte_pktmbuf_data_len(seg);
+		seg = seg->next;
+	}
+
+	if (off + len <= rte_pktmbuf_data_len(seg))
+		return rte_pktmbuf_seg_is_shared(seg);
+
+	while (seg->next && len > 0) {
+		if (rte_pktmbuf_seg_is_shared(seg))
+			return 1;
+
+		len -= (rte_pktmbuf_data_len(seg) - off);
+		off = 0;
+		seg = seg->next;
+	}
+
+	return 0;
+}
+
+/**
  * @internal used by rte_pktmbuf_read().
  */
 void *__rte_pktmbuf_read(const struct rte_mbuf *m, uint32_t off,