diff -r 345ec7455b75 Modules/posixmodule.c
--- a/Modules/posixmodule.c	Thu Jul 14 22:00:03 2016 -0700
+++ b/Modules/posixmodule.c	Sat Jul 16 15:41:01 2016 -0700
@@ -4431,7 +4431,9 @@
     DWORD attributes;
 
     if (PyArg_ParseTuple(args, "u|:_isdir", &wpath)) {
+        Py_BEGIN_ALLOW_THREADS
         attributes = GetFileAttributesW(wpath);
+        Py_END_ALLOW_THREADS
         if (attributes == INVALID_FILE_ATTRIBUTES)
             Py_RETURN_FALSE;
         goto check;
@@ -4444,7 +4446,9 @@
                           Py_FileSystemDefaultEncoding, &path))
         return NULL;
 
+    Py_BEGIN_ALLOW_THREADS
     attributes = GetFileAttributesA(path);
+    Py_END_ALLOW_THREADS
     PyMem_Free(path);
     if (attributes == INVALID_FILE_ATTRIBUTES)
         Py_RETURN_FALSE;