[dpdk-stable] patch 'mcslock: fix hang in weak memory model' has been queued to stable release 19.11.6

[luca.boccassi at gmail.com]

Hi,

FYI, your patch has been queued to stable release 19.11.6

Note it hasn't been pushed to http://dpdk.org/browse/dpdk-stable yet.
It will be pushed if I get no objections before 12/02/20. 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.

Queued patches are on a temporary branch at:
https://github.com/bluca/dpdk-stable

This queued commit can be viewed at:
https://github.com/bluca/dpdk-stable/commit/22023fd68b0f0fff0c4a2fefcd3c33b5a941a5d7

Thanks.

Luca Boccassi

---
>From 22023fd68b0f0fff0c4a2fefcd3c33b5a941a5d7 Mon Sep 17 00:00:00 2001
From: Diogo Behrens <diogo.behrens at huawei.com>
Date: Wed, 26 Aug 2020 11:20:02 +0200
Subject: [PATCH] mcslock: fix hang in weak memory model

[ upstream commit 021b698eb56e48009d74d818bea5a9bdad4f54fa ]

The initialization me->locked=1 in lock() must happen before
next->locked=0 in unlock(), otherwise a thread may hang forever,
waiting me->locked become 0. On weak memory systems (such as ARMv8),
the current implementation allows me->locked=1 to be reordered with
announcing the node (pred->next=me) and, consequently, to be
reordered with next->locked=0 in unlock().

This fix adds a release barrier to pred->next=me, forcing
me->locked=1 to happen before this operation.

Fixes: 2173f3333b61 ("mcslock: add MCS queued lock implementation")

Signed-off-by: Diogo Behrens <diogo.behrens at huawei.com>
Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
---
 lib/librte_eal/common/include/generic/rte_mcslock.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/librte_eal/common/include/generic/rte_mcslock.h b/lib/librte_eal/common/include/generic/rte_mcslock.h
index 2bef28351c..ce553f5475 100644
--- a/lib/librte_eal/common/include/generic/rte_mcslock.h
+++ b/lib/librte_eal/common/include/generic/rte_mcslock.h
@@ -68,7 +68,14 @@ rte_mcslock_lock(rte_mcslock_t **msl, rte_mcslock_t *me)
 		 */
 		return;
 	}
-	__atomic_store_n(&prev->next, me, __ATOMIC_RELAXED);
+	/* The store to me->next above should also complete before the node is
+	 * visible to predecessor thread releasing the lock. Hence, the store
+	 * prev->next also requires release semantics. Note that, for example,
+	 * on ARM, the release semantics in the exchange operation is not
+	 * strong as a release fence and is not sufficient to enforce the
+	 * desired order here.
+	 */
+	__atomic_store_n(&prev->next, me, __ATOMIC_RELEASE);
 
 	/* The while-load of me->locked should not move above the previous
 	 * store to prev->next. Otherwise it will cause a deadlock. Need a
-- 
2.27.0

---
  Diff of the applied patch vs upstream commit (please double-check if non-empty:
---
--- -	2020-11-30 11:49:30.431485317 +0000
+++ 0001-mcslock-fix-hang-in-weak-memory-model.patch	2020-11-30 11:49:30.382271364 +0000
@@ -1 +1 @@
-From 021b698eb56e48009d74d818bea5a9bdad4f54fa Mon Sep 17 00:00:00 2001
+From 22023fd68b0f0fff0c4a2fefcd3c33b5a941a5d7 Mon Sep 17 00:00:00 2001
@@ -5,0 +6,2 @@
+[ upstream commit 021b698eb56e48009d74d818bea5a9bdad4f54fa ]
+
@@ -17 +18,0 @@
-Cc: stable at dpdk.org
@@ -22 +23 @@
- lib/librte_eal/include/generic/rte_mcslock.h | 9 ++++++++-
+ lib/librte_eal/common/include/generic/rte_mcslock.h | 9 ++++++++-
@@ -25,5 +26,5 @@
-diff --git a/lib/librte_eal/include/generic/rte_mcslock.h b/lib/librte_eal/include/generic/rte_mcslock.h
-index 78b0df295e..d370bef17a 100644
---- a/lib/librte_eal/include/generic/rte_mcslock.h
-+++ b/lib/librte_eal/include/generic/rte_mcslock.h
-@@ -64,7 +64,14 @@ rte_mcslock_lock(rte_mcslock_t **msl, rte_mcslock_t *me)
+diff --git a/lib/librte_eal/common/include/generic/rte_mcslock.h b/lib/librte_eal/common/include/generic/rte_mcslock.h
+index 2bef28351c..ce553f5475 100644
+--- a/lib/librte_eal/common/include/generic/rte_mcslock.h
++++ b/lib/librte_eal/common/include/generic/rte_mcslock.h
+@@ -68,7 +68,14 @@ rte_mcslock_lock(rte_mcslock_t **msl, rte_mcslock_t *me)