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: 6b24ca4a1a8d ("mm: Use multi-index entries in the page cache")
Cc: stable@vger.kernel.org

diff --git a/lib/xarray.c b/lib/xarray.c
index 2071a37..142e36f 100644 (file)
--- a/lib/xarray.c
+++ b/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))

diff --git a/tools/testing/radix-tree/multiorder.c b/tools/testing/radix-tree/multiorder.c
index e00520c..cffaf22 100644 (file)
--- a/tools/testing/radix-tree/multiorder.c
+++ b/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);
@@ -211,6 +212,61 @@ 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)
@@ -218,12 +274,20 @@ 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();