diff --git a/jinja2/utils.py b/jinja2/utils.py
index 568c63f9f..35f51692a 100644
--- a/jinja2/utils.py
+++ b/jinja2/utils.py
@@ -464,17 +464,21 @@ def __getitem__(self, key):
 
         Raise a `KeyError` if it does not exist.
         """
-        rv = self._mapping[key]
-        if self._queue[-1] != key:
-            try:
-                self._remove(key)
-            except ValueError:
-                # if something removed the key from the container
-                # when we read, ignore the ValueError that we would
-                # get otherwise.
-                pass
-            self._append(key)
-        return rv
+        self._wlock.acquire()
+        try:
+            rv = self._mapping[key]
+            if self._queue[-1] != key:
+                try:
+                    self._remove(key)
+                except ValueError:
+                    # if something removed the key from the container
+                    # when we read, ignore the ValueError that we would
+                    # get otherwise.
+                    pass
+                self._append(key)
+            return rv
+        finally:
+            self._wlock.release()
 
     def __setitem__(self, key, value):
         """Sets the value for an item. Moves the item up so that it