[dpdk-stable] patch 'ring: enforce reading tail before slots' has been queued to LTS release 18.11.2

[Kevin Traynor]

Hi,

FYI, your patch has been queued to LTS release 18.11.2

Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable yet.
It will be pushed if I get no objections before 04/24/19. 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.

Thanks.

Kevin Traynor

---
>From 76b5e7cde680dc9cacaaec01566d577422c32273 Mon Sep 17 00:00:00 2001
From: Gavin Hu <gavin.hu at arm.com>
Date: Wed, 13 Mar 2019 00:58:53 +0800
Subject: [PATCH] ring: enforce reading tail before slots

[ upstream commit 85cffb2eccd9edf89a9e28539206a79258a6e9d9 ]

In weak memory models, like arm64, reading the prod.tail may get
reordered after reading 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
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. For SP case, the control depependency between if-statement (which
depends on the read of r->cons.tail) and the later stores to the ring slots
make RMB unnecessary. About the control dependency, read more at:
https://www.cl.cam.ac.uk/~pes20/ppc-supplemental/test7.pdf

This patch is adding the required read barrier to prevent reading the ring
slots get reordered before reading prod.tail for SC case.

Fixes: c9fb3c62896f ("ring: move code in a new header file")

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>
Acked-by: Nipun Gupta <nipun.gupta at nxp.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev at intel.com>
---
 lib/librte_ring/rte_ring_generic.h | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/lib/librte_ring/rte_ring_generic.h b/lib/librte_ring/rte_ring_generic.h
index ea7dbe5b9..953cdbbd5 100644
--- a/lib/librte_ring/rte_ring_generic.h
+++ b/lib/librte_ring/rte_ring_generic.h
@@ -159,9 +159,12 @@ __rte_ring_move_cons_head(struct rte_ring *r, unsigned int is_sc,
 
 		*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));
 	return n;
-- 
2.20.1

---
  Diff of the applied patch vs upstream commit (please double-check if non-empty:
---
--- -	2019-04-16 15:34:27.649879139 +0100
+++ 0055-ring-enforce-reading-tail-before-slots.patch	2019-04-16 15:34:25.234178752 +0100
@@ -1,8 +1,10 @@
-From 85cffb2eccd9edf89a9e28539206a79258a6e9d9 Mon Sep 17 00:00:00 2001
+From 76b5e7cde680dc9cacaaec01566d577422c32273 Mon Sep 17 00:00:00 2001
 From: Gavin Hu <gavin.hu at arm.com>
 Date: Wed, 13 Mar 2019 00:58:53 +0800
 Subject: [PATCH] ring: enforce reading tail before slots
 
+[ upstream commit 85cffb2eccd9edf89a9e28539206a79258a6e9d9 ]
+
 In weak memory models, like arm64, reading the prod.tail may get
 reordered after reading the ring slots, which corrupts the ring and
 stale data is observed.
@@ -22,7 +24,6 @@
 slots get reordered before reading prod.tail for SC case.
 
 Fixes: c9fb3c62896f ("ring: move code in a new header file")
-Cc: stable at dpdk.org
 
 Signed-off-by: Gavin Hu <gavin.hu at arm.com>
 Reviewed-by: Ola Liljedahl <ola.liljedahl at arm.com>