--- a/mercurial/lock.py	Mon May 01 05:52:36 2017 +0900
+++ b/mercurial/lock.py	Mon May 01 19:58:52 2017 +0900
@@ -131,6 +131,9 @@
             except (OSError, IOError) as why:
                 if why.errno == errno.EEXIST:
                     locker = self._readlock()
+                    if locker is None:
+                        continue
+
                     # special case where a parent process holds the lock -- this
                     # is different from the pid being different because we do
                     # want the unlock and postrelease functions to be called,