diff -r 635817da596d Lib/subprocess.py
--- a/Lib/subprocess.py	Thu Apr 17 20:13:44 2014 +0200
+++ b/Lib/subprocess.py	Thu Apr 17 18:33:20 2014 -0700
@@ -405,6 +405,7 @@
     import _posixsubprocess
     import select
     import selectors
+    import threading
 
     # When select or poll has indicated that the file is writable,
     # we can write up to _PIPE_BUF bytes without risk of blocking.
@@ -748,6 +749,10 @@
                  pass_fds=()):
         """Create new Popen instance."""
         _cleanup()
+        # Held while anything is calling waitpid before returncode has been
+        # updated to prevent clobbering returncode is wait() is called from
+        # multiple threads at once.
+        self._waitpid_lock = threading.Lock()
 
         self._input = None
         self._communication_started = False
@@ -1450,6 +1455,7 @@
         def _handle_exitstatus(self, sts, _WIFSIGNALED=os.WIFSIGNALED,
                 _WTERMSIG=os.WTERMSIG, _WIFEXITED=os.WIFEXITED,
                 _WEXITSTATUS=os.WEXITSTATUS):
+            """All callers to this function MUST hold self._waitpid_lock."""
             # This method is called (indirectly) by __del__, so it cannot
             # refer to anything outside of its local scope.
             if _WIFSIGNALED(sts):
@@ -1471,7 +1477,13 @@
 
             """
             if self.returncode is None:
+                if not self._waitpid_lock.acquire(False):
+                    # Something else is busy calling waitpid.  Don't allow two
+                    # at once.  We know nothing yet.
+                    return None
                 try:
+                    if self.returncode is not None:
+                        return self.returncode
                     pid, sts = _waitpid(self.pid, _WNOHANG)
                     if pid == self.pid:
                         self._handle_exitstatus(sts)
@@ -1485,10 +1497,13 @@
                         # can't get the status.
                         # http://bugs.python.org/issue15756
                         self.returncode = 0
+                finally:
+                    self._waitpid_lock.release()
             return self.returncode
 
 
         def _try_wait(self, wait_flags):
+            """All callers to this function MUST hold self._waitpid_lock."""
             try:
                 (pid, sts) = _eintr_retry_call(os.waitpid, self.pid, wait_flags)
             except OSError as e:
@@ -1521,11 +1536,17 @@
                 # cribbed from Lib/threading.py in Thread.wait() at r71065.
                 delay = 0.0005 # 500 us -> initial delay of 1 ms
                 while True:
-                    (pid, sts) = self._try_wait(os.WNOHANG)
-                    assert pid == self.pid or pid == 0
-                    if pid == self.pid:
-                        self._handle_exitstatus(sts)
-                        break
+                    if self._waitpid_lock.acquire(False):
+                        try:
+                            if self.returncode is not None:
+                                break
+                            (pid, sts) = self._try_wait(os.WNOHANG)
+                            assert pid == self.pid or pid == 0
+                            if pid == self.pid:
+                                self._handle_exitstatus(sts)
+                                break
+                        finally:
+                            self._waitpid_lock.release()
                     remaining = self._remaining_time(endtime)
                     if remaining <= 0:
                         raise TimeoutExpired(self.args, timeout)
@@ -1533,11 +1554,15 @@
                     time.sleep(delay)
             else:
                 while self.returncode is None:
-                    (pid, sts) = self._try_wait(0)
-                    # Check the pid and loop as waitpid has been known to return
-                    # 0 even without WNOHANG in odd situations.  issue14396.
-                    if pid == self.pid:
-                        self._handle_exitstatus(sts)
+                    with self._waitpid_lock:
+                        if self.returncode is not None:
+                            break
+                        (pid, sts) = self._try_wait(0)
+                        # Check the pid and loop as waitpid has been known to
+                        # return 0 even without WNOHANG in odd situations.
+                        # http://bugs.python.org/issue14396.
+                        if pid == self.pid:
+                            self._handle_exitstatus(sts)
             return self.returncode