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* [dpdk-dev] [PATCH] mbuf: add comment explaining confusing code
@ 2015-03-26 21:14 Bruce Richardson
  2015-03-26 21:31 ` Olivier MATZ
  2015-03-27 10:29 ` Neil Horman
  0 siblings, 2 replies; 18+ messages in thread
From: Bruce Richardson @ 2015-03-26 21:14 UTC (permalink / raw)
  To: dev

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

^ permalink raw reply	[flat|nested] 18+ messages in thread

end of thread, other threads:[~2015-03-31 12:33 UTC | newest]

Thread overview: 18+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2015-03-26 21:14 [dpdk-dev] [PATCH] mbuf: add comment explaining confusing code Bruce Richardson
2015-03-26 21:31 ` Olivier MATZ
2015-03-27 10:29 ` Neil Horman
2015-03-27 10:49   ` Ananyev, Konstantin
2015-03-27 11:32   ` Bruce Richardson
2015-03-27 12:42     ` Neil Horman
2015-03-27 14:07     ` Neil Horman
2015-03-27 14:30       ` Bruce Richardson
2015-03-27 14:38         ` Neil Horman
2015-03-27 14:55           ` Bruce Richardson
2015-03-27 16:43             ` Neil Horman
2015-03-27 16:56               ` Richardson, Bruce
2015-03-30 17:11                 ` Thomas Monjalon
2015-03-30 17:39                 ` Don Provan
2015-03-30 18:15                   ` Stephen Hemminger
2015-03-31 12:33                   ` Zoltan Kiss
2015-03-30 19:39         ` Marc Sune
2015-03-30 20:26           ` Neil Horman

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