[dpdk-dev] [PATCH v2] ring: enforce reading the tails before ring operations

[gavin hu]

In weak memory models, like arm64, reading the {prod,cons}.tail may get
reordered after reading or writing the ring slots, which corrupts the ring
and stale data is observed.

This issue was reported by NXP on 8-A72 DPAA2 board. The problem is most
likely caused by missing the acquire semantics when reading cons.tail (in
SP enqueue) or prod.tail (in SC dequeue) which makes it possible to read a
stale value from the ring slots.

For MP (and MC) case, rte_atomic32_cmpset() already provides the required
ordering. This patch is to prevent reading and writing the ring slots get
reordered before reading {prod,cons}.tail for SP (and SC) case.

Signed-off-by: gavin hu <gavin.hu at arm.com>
Reviewed-by: Ola Liljedahl <Ola.Liljedahl at arm.com>
Tested-by: Nipun Gupta <nipun.gupta at nxp.com>
---
 lib/librte_ring/rte_ring_generic.h | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/lib/librte_ring/rte_ring_generic.h b/lib/librte_ring/rte_ring_generic.h
index ea7dbe5..1bd3dfd 100644
--- a/lib/librte_ring/rte_ring_generic.h
+++ b/lib/librte_ring/rte_ring_generic.h
@@ -90,9 +90,11 @@ __rte_ring_move_prod_head(struct rte_ring *r, unsigned int is_sp,
 			return 0;
 
 		*new_head = *old_head + n;
-		if (is_sp)
-			r->prod.head = *new_head, success = 1;
-		else
+		if (is_sp) {
+			r->prod.head = *new_head;
+			rte_smp_rmb();
+			success = 1;
+		} else
 			success = rte_atomic32_cmpset(&r->prod.head,
 					*old_head, *new_head);
 	} while (unlikely(success == 0));
@@ -158,9 +160,11 @@ __rte_ring_move_cons_head(struct rte_ring *r, unsigned int is_sc,
 			return 0;
 
 		*new_head = *old_head + n;
-		if (is_sc)
-			r->cons.head = *new_head, success = 1;
-		else
+		if (is_sc) {
+			r->cons.head = *new_head;
+			rte_smp_rmb();
+			success = 1;
+		} else
 			success = rte_atomic32_cmpset(&r->cons.head, *old_head,
 					*new_head);
 	} while (unlikely(success == 0));
-- 
2.7.4