diff -r 45bb27eae8d3 Lib/subprocess.py
--- a/Lib/subprocess.py	Fri Aug 21 09:40:38 2015 -0400
+++ b/Lib/subprocess.py	Fri Aug 21 12:39:05 2015 -0400
@@ -427,6 +427,7 @@
     import threading
     import msvcrt
     import _winapi
+    _global_win32_popen_lock = threading.Lock()
     class STARTUPINFO:
         dwFlags = 0
         hStdInput = None
@@ -909,6 +910,13 @@
         # are -1 when not using PIPEs. The child objects are -1
         # when not redirecting.
 
+        if _mswindows:
+            # If we are on Windows, acquire the global Popen lock. The lock
+            # ensures that only one thread at a time is creating inheritable
+            # handles. That way, if two threads call Popen at the same time,
+            # inheritable handles from thread 1 won't be leaked into thread 2.
+            _global_win32_popen_lock.acquire()
+
         (p2cread, p2cwrite,
          c2pread, c2pwrite,
          errread, errwrite) = self._get_handles(stdin, stdout, stderr)
@@ -973,6 +981,10 @@
                         pass
 
             raise
+        finally:
+            if _mswindows:
+                # Release the Popen lock.
+                _global_win32_popen_lock.release()
 
 
     def _translate_newlines(self, data, encoding):