[dpdk-stable] patch 'hash: use ordered loads only if signature matches' has been queued to LTS release 18.11.3

[Kevin Traynor]

Hi,

FYI, your patch has been queued to LTS release 18.11.3

Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable yet.
It will be pushed if I get no objections before 09/03/19. So please
shout if anyone has objections.

Also note that after the patch there's a diff of the upstream commit vs the
patch applied to the branch. This will indicate if there was any rebasing
needed to apply to the stable branch. If there were code changes for rebasing
(ie: not only metadata diffs), please double check that the rebase was
correctly done.

Queued patches are on a temporary branch at:
https://github.com/kevintraynor/dpdk-stable-queue

This queued commit can be viewed at:
https://github.com/kevintraynor/dpdk-stable-queue/commit/2f0d98c0a96c87467ffc5b1be76ff45688eb039c

Thanks.

Kevin Traynor

---
>From 2f0d98c0a96c87467ffc5b1be76ff45688eb039c Mon Sep 17 00:00:00 2001
From: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
Date: Tue, 2 Jul 2019 16:16:33 -0500
Subject: [PATCH] hash: use ordered loads only if signature matches

[ upstream commit 4d9ca3ed213320eaef0d8a7e95b3b00c2d70800b ]

Relaxed signature comparison is done first. Further ordered loads
are done only if the signature matches. Any false positives are
caught by the full key comparison. This provides performance
benefits as load-acquire is executed only when required.

Fixes: e605a1d36 ("hash: add lock-free r/w concurrency")

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
Reviewed-by: Gavin Hu <gavin.hu at arm.com>
Tested-by: Ruifeng Wang <ruifeng.wang at arm.com>
Acked-by: Yipeng Wang <yipeng1.wang at intel.com>
---
 lib/librte_hash/rte_cuckoo_hash.c | 37 +++++++++++++++++++------------
 1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/lib/librte_hash/rte_cuckoo_hash.c b/lib/librte_hash/rte_cuckoo_hash.c
index d7a5f4c21..e76277167 100644
--- a/lib/librte_hash/rte_cuckoo_hash.c
+++ b/lib/librte_hash/rte_cuckoo_hash.c
@@ -1169,20 +1169,29 @@ search_one_bucket_lf(const struct rte_hash *h, const void *key, uint16_t sig,
 
 	for (i = 0; i < RTE_HASH_BUCKET_ENTRIES; i++) {
-		key_idx = __atomic_load_n(&bkt->key_idx[i],
+		/* Signature comparison is done before the acquire-load
+		 * of the key index to achieve better performance.
+		 * This can result in the reader loading old signature
+		 * (which matches), while the key_idx is updated to a
+		 * value that belongs to a new key. However, the full
+		 * key comparison will ensure that the lookup fails.
+		 */
+		if (bkt->sig_current[i] == sig) {
+			key_idx = __atomic_load_n(&bkt->key_idx[i],
 					  __ATOMIC_ACQUIRE);
-		if (bkt->sig_current[i] == sig && key_idx != EMPTY_SLOT) {
-			k = (struct rte_hash_key *) ((char *)keys +
-					key_idx * h->key_entry_size);
-			pdata = __atomic_load_n(&k->pdata,
-					__ATOMIC_ACQUIRE);
+			if (key_idx != EMPTY_SLOT) {
+				k = (struct rte_hash_key *) ((char *)keys +
+						key_idx * h->key_entry_size);
+				pdata = __atomic_load_n(&k->pdata,
+						__ATOMIC_ACQUIRE);
 
-			if (rte_hash_cmp_eq(key, k->key, h) == 0) {
-				if (data != NULL)
-					*data = pdata;
-				/*
-				 * Return index where key is stored,
-				 * subtracting the first dummy index
-				 */
-				return key_idx - 1;
+				if (rte_hash_cmp_eq(key, k->key, h) == 0) {
+					if (data != NULL)
+						*data = pdata;
+					/*
+					 * Return index where key is stored,
+					 * subtracting the first dummy index
+					 */
+					return key_idx - 1;
+				}
 			}
 		}
-- 
2.20.1

---
  Diff of the applied patch vs upstream commit (please double-check if non-empty:
---
--- -	2019-08-27 09:40:12.828307491 +0100
+++ 0035-hash-use-ordered-loads-only-if-signature-matches.patch	2019-08-27 09:40:10.925144267 +0100
@@ -1 +1 @@
-From 4d9ca3ed213320eaef0d8a7e95b3b00c2d70800b Mon Sep 17 00:00:00 2001
+From 2f0d98c0a96c87467ffc5b1be76ff45688eb039c Mon Sep 17 00:00:00 2001
@@ -5,0 +6,2 @@
+[ upstream commit 4d9ca3ed213320eaef0d8a7e95b3b00c2d70800b ]
+
@@ -12 +13,0 @@
-Cc: stable at dpdk.org
@@ -23 +24 @@
-index 74d5bbd99..a3ac2fead 100644
+index d7a5f4c21..e76277167 100644
@@ -26 +27 @@
-@@ -1190,20 +1190,29 @@ search_one_bucket_lf(const struct rte_hash *h, const void *key, uint16_t sig,
+@@ -1169,20 +1169,29 @@ search_one_bucket_lf(const struct rte_hash *h, const void *key, uint16_t sig,