file-ids: Fix subnode linkage

The subtree node should be linked with common rbtree procedure. Otherwise
the very first subtree insertion results in an infinite loop in the rb
linking and balancing code.

Plus, mark the root as static and add the subroot node initialization check.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

file-ids.c

@@ -77,7 +77,7 @@ struct fd_id_entry {
 } __aligned(sizeof(long));
 
 
-struct rb_root fd_id_root = RB_ROOT;
+static struct rb_root fd_id_root = RB_ROOT;
 static unsigned long fd_id_entries_subid = 1;
 
 static struct fd_id_entry *alloc_fd_id_entry(u32 genid, pid_t pid, int fd)
@@ -98,7 +98,9 @@ static struct fd_id_entry *alloc_fd_id_entry(u32 genid, pid_t pid, int fd)
 
 	rb_init_node(&e->node);
 	rb_init_node(&e->subtree_node);
-	rb_attach_node(&e->subtree_root, &e->subtree_node);
+	e->subtree_root = RB_ROOT;
+	rb_link_and_balance(&e->subtree_root, &e->subtree_node,
+			NULL, &e->subtree_root.rb_node);
 err:
 	return e;
 }
@@ -112,6 +114,8 @@ lookup_alloc_subtree(struct fd_id_entry *e, u32 genid, pid_t pid, int fd)
 	struct rb_node **new = &e->subtree_root.rb_node;
 	struct rb_node *parent = NULL;
 
+	BUG_ON(!node);
+
 	while (node) {
 		struct fd_id_entry *this = rb_entry(node, struct fd_id_entry, subtree_node);
 		int ret = sys_kcmp(this->pid, pid, KCMP_FILE, this->fd, fd);