[dpdk-dev] [PATCH v3 2/3] lib/lpm: memory orderings to avoid race conditions for v1604

[Ruifeng Wang <ruifeng.wang@arm.com>]

When a tbl8 group is getting attached to a tbl24 entry, lookup
might fail even though the entry is configured in the table.

For ex: consider a LPM table configured with 10.10.10.1/24.
When a new entry 10.10.10.32/28 is being added, a new tbl8
group is allocated and tbl24 entry is changed to point to
the tbl8 group. If the tbl24 entry is written without the tbl8
group entries updated, a lookup on 10.10.10.9 will return
failure.

Correct memory orderings are required to ensure that the
store to tbl24 does not happen before the stores to tbl8 group
entries complete.

The orderings have impact on LPM performance test.
On Arm A72 platform, delete operation has 2.7% degradation, while
add / lookup has no notable performance change.
On x86 E5 platform, add operation has 8% boost, delete operation
has 1% degradation, lookup has no performance change.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Signed-off-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Gavin Hu <gavin.hu@arm.com>
---
v3: updated test data
v2: no changes
v1: initial version

 lib/librte_lpm/rte_lpm.c | 32 +++++++++++++++++++++++++-------
 lib/librte_lpm/rte_lpm.h |  4 ++++
 2 files changed, 29 insertions(+), 7 deletions(-)

diff --git a/lib/librte_lpm/rte_lpm.c b/lib/librte_lpm/rte_lpm.c
index eb835f052..fabd13fb0 100644
--- a/lib/librte_lpm/rte_lpm.c
+++ b/lib/librte_lpm/rte_lpm.c
@@ -806,7 +806,8 @@ add_depth_small_v1604(struct rte_lpm *lpm, uint32_t ip, uint8_t depth,
 			/* Setting tbl24 entry in one go to avoid race
 			 * conditions
 			 */
-			lpm->tbl24[i] = new_tbl24_entry;
+			__atomic_store(&lpm->tbl24[i], &new_tbl24_entry,
+					__ATOMIC_RELEASE);
 
 			continue;
 		}
@@ -1017,7 +1018,11 @@ add_depth_big_v1604(struct rte_lpm *lpm, uint32_t ip_masked, uint8_t depth,
 			.depth = 0,
 		};
 
-		lpm->tbl24[tbl24_index] = new_tbl24_entry;
+		/* The tbl24 entry must be written only after the
+		 * tbl8 entries are written.
+		 */
+		__atomic_store(&lpm->tbl24[tbl24_index], &new_tbl24_entry,
+				__ATOMIC_RELEASE);
 
 	} /* If valid entry but not extended calculate the index into Table8. */
 	else if (lpm->tbl24[tbl24_index].valid_group == 0) {
@@ -1063,7 +1068,11 @@ add_depth_big_v1604(struct rte_lpm *lpm, uint32_t ip_masked, uint8_t depth,
 				.depth = 0,
 		};
 
-		lpm->tbl24[tbl24_index] = new_tbl24_entry;
+		/* The tbl24 entry must be written only after the
+		 * tbl8 entries are written.
+		 */
+		__atomic_store(&lpm->tbl24[tbl24_index], &new_tbl24_entry,
+				__ATOMIC_RELEASE);
 
 	} else { /*
 		* If it is valid, extended entry calculate the index into tbl8.
@@ -1391,6 +1400,7 @@ delete_depth_small_v1604(struct rte_lpm *lpm, uint32_t ip_masked,
 	/* Calculate the range and index into Table24. */
 	tbl24_range = depth_to_range(depth);
 	tbl24_index = (ip_masked >> 8);
+	struct rte_lpm_tbl_entry zero_tbl24_entry = {0};
 
 	/*
 	 * Firstly check the sub_rule_index. A -1 indicates no replacement rule
@@ -1405,7 +1415,8 @@ delete_depth_small_v1604(struct rte_lpm *lpm, uint32_t ip_masked,
 
 			if (lpm->tbl24[i].valid_group == 0 &&
 					lpm->tbl24[i].depth <= depth) {
-				lpm->tbl24[i].valid = INVALID;
+				__atomic_store(&lpm->tbl24[i],
+					&zero_tbl24_entry, __ATOMIC_RELEASE);
 			} else if (lpm->tbl24[i].valid_group == 1) {
 				/*
 				 * If TBL24 entry is extended, then there has
@@ -1450,7 +1461,8 @@ delete_depth_small_v1604(struct rte_lpm *lpm, uint32_t ip_masked,
 
 			if (lpm->tbl24[i].valid_group == 0 &&
 					lpm->tbl24[i].depth <= depth) {
-				lpm->tbl24[i] = new_tbl24_entry;
+				__atomic_store(&lpm->tbl24[i], &new_tbl24_entry,
+						__ATOMIC_RELEASE);
 			} else  if (lpm->tbl24[i].valid_group == 1) {
 				/*
 				 * If TBL24 entry is extended, then there has
@@ -1713,8 +1725,11 @@ delete_depth_big_v1604(struct rte_lpm *lpm, uint32_t ip_masked,
 	tbl8_recycle_index = tbl8_recycle_check_v1604(lpm->tbl8, tbl8_group_start);
 
 	if (tbl8_recycle_index == -EINVAL) {
-		/* Set tbl24 before freeing tbl8 to avoid race condition. */
+		/* Set tbl24 before freeing tbl8 to avoid race condition.
+		 * Prevent the free of the tbl8 group from hoisting.
+		 */
 		lpm->tbl24[tbl24_index].valid = 0;
+		__atomic_thread_fence(__ATOMIC_RELEASE);
 		tbl8_free_v1604(lpm->tbl8, tbl8_group_start);
 	} else if (tbl8_recycle_index > -1) {
 		/* Update tbl24 entry. */
@@ -1725,8 +1740,11 @@ delete_depth_big_v1604(struct rte_lpm *lpm, uint32_t ip_masked,
 			.depth = lpm->tbl8[tbl8_recycle_index].depth,
 		};
 
-		/* Set tbl24 before freeing tbl8 to avoid race condition. */
+		/* Set tbl24 before freeing tbl8 to avoid race condition.
+		 * Prevent the free of the tbl8 group from hoisting.
+		 */
 		lpm->tbl24[tbl24_index] = new_tbl24_entry;
+		__atomic_thread_fence(__ATOMIC_RELEASE);
 		tbl8_free_v1604(lpm->tbl8, tbl8_group_start);
 	}
 #undef group_idx
diff --git a/lib/librte_lpm/rte_lpm.h b/lib/librte_lpm/rte_lpm.h
index b886f54b4..6f5704c5c 100644
--- a/lib/librte_lpm/rte_lpm.h
+++ b/lib/librte_lpm/rte_lpm.h
@@ -354,6 +354,10 @@ rte_lpm_lookup(struct rte_lpm *lpm, uint32_t ip, uint32_t *next_hop)
 	ptbl = (const uint32_t *)(&lpm->tbl24[tbl24_index]);
 	tbl_entry = *ptbl;
 
+	/* Memory ordering is not required in lookup. Because dataflow
+	 * dependency exists, compiler or HW won't be able to re-order
+	 * the operations.
+	 */
 	/* Copy tbl8 entry (only if needed) */
 	if (unlikely((tbl_entry & RTE_LPM_VALID_EXT_ENTRY_BITMASK) ==
 			RTE_LPM_VALID_EXT_ENTRY_BITMASK)) {
-- 
2.17.1