XArray: Do not return sibling entries from xa_load()

It is possible for xa_load() to observe a sibling entry pointing to
another sibling entry.  An example:

Thread A:		Thread B:
			xa_store_range(xa, entry, 188, 191, gfp);
xa_load(xa, 191);
entry = xa_entry(xa, node, 63);
[entry is a sibling of 188]
			xa_store_range(xa, entry, 184, 191, gfp);
if (xa_is_sibling(entry))
offset = xa_to_sibling(entry);
entry = xa_entry(xas->xa, node, offset);
[entry is now a sibling of 184]

It is sufficient to go around this loop until we hit a non-sibling entry.
Sibling entries always point earlier in the node, so we are guaranteed
to terminate this search.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Fixes: 6b24ca4 ("mm: Use multi-index entries in the page cache")
Cc: stable@vger.kernel.org

Author: Matthew Wilcox (Oracle)

lib/xarray.c

@@ -206,7 +206,7 @@ static void *xas_descend(struct xa_state *xas, struct xa_node *node)
 	void *entry = xa_entry(xas->xa, node, offset);
 
 	xas->xa_node = node;
-	if (xa_is_sibling(entry)) {
+	while (xa_is_sibling(entry)) {
 		offset = xa_to_sibling(entry);
 		entry = xa_entry(xas->xa, node, offset);
 		if (node->shift && xa_is_node(entry))

tools/testing/radix-tree/multiorder.c

@@ -159,7 +159,7 @@ void multiorder_tagged_iteration(struct xarray *xa)
 	item_kill_tree(xa);
 }
 
-bool stop_iteration = false;
+bool stop_iteration;
 
 static void *creator_func(void *ptr)
 {
@@ -201,6 +201,7 @@ static void multiorder_iteration_race(struct xarray *xa)
 	pthread_t worker_thread[num_threads];
 	int i;
 
+	stop_iteration = false;
 	pthread_create(&worker_thread[0], NULL, &creator_func, xa);
 	for (i = 1; i < num_threads; i++)
 		pthread_create(&worker_thread[i], NULL, &iterator_func, xa);
@@ -211,19 +212,82 @@ static void multiorder_iteration_race(struct xarray *xa)
 	item_kill_tree(xa);
 }
 
+static void *load_creator(void *ptr)
+{
+	/* 'order' is set up to ensure we have sibling entries */
+	unsigned int order;
+	struct radix_tree_root *tree = ptr;
+	int i;
+
+	rcu_register_thread();
+	item_insert_order(tree, 3 << RADIX_TREE_MAP_SHIFT, 0);
+	item_insert_order(tree, 2 << RADIX_TREE_MAP_SHIFT, 0);
+	for (i = 0; i < 10000; i++) {
+		for (order = 1; order < RADIX_TREE_MAP_SHIFT; order++) {
+			unsigned long index = (3 << RADIX_TREE_MAP_SHIFT) -
+						(1 << order);
+			item_insert_order(tree, index, order);
+			item_delete_rcu(tree, index);
+		}
+	}
+	rcu_unregister_thread();
+
+	stop_iteration = true;
+	return NULL;
+}
+
+static void *load_worker(void *ptr)
+{
+	unsigned long index = (3 << RADIX_TREE_MAP_SHIFT) - 1;
+
+	rcu_register_thread();
+	while (!stop_iteration) {
+		struct item *item = xa_load(ptr, index);
+		assert(!xa_is_internal(item));
+	}
+	rcu_unregister_thread();
+
+	return NULL;
+}
+
+static void load_race(struct xarray *xa)
+{
+	const int num_threads = sysconf(_SC_NPROCESSORS_ONLN) * 4;
+	pthread_t worker_thread[num_threads];
+	int i;
+
+	stop_iteration = false;
+	pthread_create(&worker_thread[0], NULL, &load_creator, xa);
+	for (i = 1; i < num_threads; i++)
+		pthread_create(&worker_thread[i], NULL, &load_worker, xa);
+
+	for (i = 0; i < num_threads; i++)
+		pthread_join(worker_thread[i], NULL);
+
+	item_kill_tree(xa);
+}
+
 static DEFINE_XARRAY(array);
 
 void multiorder_checks(void)
 {
 	multiorder_iteration(&array);
 	multiorder_tagged_iteration(&array);
 	multiorder_iteration_race(&array);
+	load_race(&array);
 
 	radix_tree_cpu_dead(0);
 }
 
-int __weak main(void)
+int __weak main(int argc, char **argv)
 {
+	int opt;
+
+	while ((opt = getopt(argc, argv, "ls:v")) != -1) {
+		if (opt == 'v')
+			test_verbose++;
+	}
+
 	rcu_register_thread();
 	radix_tree_init();
 	multiorder_checks();