[dpdk-dev] [PATCH] mbuf: add comment explaining confusing code

[Bruce Richardson <bruce.richardson@intel.com>]

The logic used in the condition check before freeing an mbuf is
sometimes confusing, so explain it in a proper comment.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---
 lib/librte_mbuf/rte_mbuf.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
index 17ba791..0265172 100644
--- a/lib/librte_mbuf/rte_mbuf.h
+++ b/lib/librte_mbuf/rte_mbuf.h
@@ -764,6 +764,16 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
 {
 	__rte_mbuf_sanity_check(m, 0);
 
+	/*
+	 * Check to see if this is the last reference to the mbuf.
+	 * Note: the double check here is deliberate. If the ref_cnt is "atomic"
+	 * the call to "refcnt_update" is a very expensive operation, so we
+	 * don't want to call it in the case where we know we are the holder
+	 * of the last reference to this mbuf i.e. ref_cnt == 1.
+	 * If however, ref_cnt != 1, it's still possible that we may still be
+	 * the final decrementer of the count, so we need to check that
+	 * result also, to make sure the mbuf is freed properly.
+	 */
 	if (likely (rte_mbuf_refcnt_read(m) == 1) ||
 			likely (rte_mbuf_refcnt_update(m, -1) == 0)) {
 
-- 
2.1.0