new helper: destroy_unused_super()

Used for disposal of super_block instances that had never been reachable
via any shared data structures.  No need for RCU delay in there, everything
can be called directly.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/super.c

@@ -171,6 +171,21 @@ static void destroy_super(struct super_block *s)
 	call_rcu(&s->rcu, destroy_super_rcu);
 }
 
+/* Free a superblock that has never been seen by anyone */
+static void destroy_unused_super(struct super_block *s)
+{
+	if (!s)
+		return;
+	up_write(&s->s_umount);
+	list_lru_destroy(&s->s_dentry_lru);
+	list_lru_destroy(&s->s_inode_lru);
+	security_sb_free(s);
+	put_user_ns(s->s_user_ns);
+	kfree(s->s_subtype);
+	/* no delays needed */
+	destroy_super_work(&s->destroy_work);
+}
+
 /**
  *	alloc_super	-	create new superblock
  *	@type:	filesystem type superblock should belong to
@@ -256,7 +271,7 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags,
 	return s;
 
 fail:
-	destroy_super(s);
+	destroy_unused_super(s);
 	return NULL;
 }
 
@@ -484,19 +499,12 @@ struct super_block *sget_userns(struct file_system_type *type,
 				continue;
 			if (user_ns != old->s_user_ns) {
 				spin_unlock(&sb_lock);
-				if (s) {
-					up_write(&s->s_umount);
-					destroy_super(s);
-				}
+				destroy_unused_super(s);
 				return ERR_PTR(-EBUSY);
 			}
 			if (!grab_super(old))
 				goto retry;
-			if (s) {
-				up_write(&s->s_umount);
-				destroy_super(s);
-				s = NULL;
-			}
+			destroy_unused_super(s);
 			return old;
 		}
 	}
@@ -511,8 +519,7 @@ struct super_block *sget_userns(struct file_system_type *type,
 	err = set(s, data);
 	if (err) {
 		spin_unlock(&sb_lock);
-		up_write(&s->s_umount);
-		destroy_super(s);
+		destroy_unused_super(s);
 		return ERR_PTR(err);
 	}
 	s->s_type = type;