maple_tree: add smp_rmb() to dead node detection

Add an smp_rmb() before reading the parent pointer to ensure that anything
read from the node prior to the parent pointer hasn't been reordered ahead
of this check.

The is necessary for RCU mode.

Link: https://lkml.kernel.org/r/20230227173632.3292573-7-surenb@google.com
Fixes: 54a611b6 ("Maple Tree: add new data structure")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/maple_tree.c

@@ -539,9 +539,11 @@ static inline struct maple_node *mte_parent(const struct maple_enode *enode)
  */
 static inline bool ma_dead_node(const struct maple_node *node)
 {
-	struct maple_node *parent = (void *)((unsigned long)
-					     node->parent & ~MAPLE_NODE_MASK);
+	struct maple_node *parent;
 
+	/* Do not reorder reads from the node prior to the parent check */
+	smp_rmb();
+	parent = (void *)((unsigned long) node->parent & ~MAPLE_NODE_MASK);
 	return (parent == node);
 }
 
@@ -556,6 +558,8 @@ static inline bool mte_dead_node(const struct maple_enode *enode)
 	struct maple_node *parent, *node;
 
 	node = mte_to_node(enode);
+	/* Do not reorder reads from the node prior to the parent check */
+	smp_rmb();
 	parent = mte_parent(enode);
 	return (parent == node);
 }