[dpdk-stable] [PATCH v2 1/2] lib/hash: use ordered loads only if signature matches

[Honnappa Nagarahalli]

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")
Cc: stable at dpdk.org

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 953928f27..0e042d924 100644
--- a/lib/librte_hash/rte_cuckoo_hash.c
+++ b/lib/librte_hash/rte_cuckoo_hash.c
@@ -1188,22 +1188,31 @@ search_one_bucket_lf(const struct rte_hash *h, const void *key, uint16_t sig,
 	struct rte_hash_key *k, *keys = h->key_store;
 
 	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;
+				}
 			}
 		}
 	}
-- 
2.17.1