mcslock: fix hang in weak memory model

[ upstream commit 021b698 ]

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: 2173f33 ("mcslock: add MCS queued lock implementation")

Signed-off-by: Diogo Behrens <diogo.behrens@huawei.com>
Acked-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>

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