diff --git a/django/template/base.py b/django/template/base.py
index 9a7f55f8de..c94eeb51f9 100644
--- a/django/template/base.py
+++ b/django/template/base.py
@@ -1045,7 +1045,9 @@ def get_templatetags_modules():
     global templatetags_modules
     if not templatetags_modules:
         _templatetags_modules = []
-        # Populate list once per thread.
+        # Populate list once per process. Mutate the local list first, and then
+        # assign it to the global name to ensure there are no cases where two
+        # threads try to populate it simultaneously.
         for app_module in ['django'] + list(settings.INSTALLED_APPS):
             try:
                 templatetag_module = '%s.templatetags' % app_module