diff --git a/payment/threadpool.py b/payment/threadpool.py
--- a/payment/threadpool.py
+++ b/payment/threadpool.py
@@ -8,4 +8,5 @@ class PoolWorker:
     def acquire(self):
-        self._lock_a.acquire()
-        self._lock_b.acquire()
+        with self._global_order:
+            self._lock_a.acquire()
+            self._lock_b.acquire()