[dpdk-dev] [PATCH v1] ring: enforce reading the tails before ring operations

[gavin hu <gavin.hu@arm.com>]

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.  There must be a read fence before writing
or reading the ring slots, rte_atomic32_cmpset() provides the same ordering
for MP (and MC) case. This patch is to enforce this ordering for SP (and
SC) case.

Signed-off-by: gavin hu <gavin.hu@arm.com>
Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
Tested-by: Nipun Gupta <nipun.gupta@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