diff -r bae0912dd160 Lib/posixpath.py
--- a/Lib/posixpath.py	Thu Oct 01 08:56:54 2015 +0200
+++ b/Lib/posixpath.py	Sat Nov 14 15:18:07 2015 +0100
@@ -182,7 +182,8 @@
     try:
         s1 = os.lstat(path)
     except OSError:
-        # It doesn't exist -- so not a mount point. :-)
+        # the OSError should be handled with more care
+        # it could be a "permission denied" but path is still a mount
         return False
     else:
         # A symlink can never be a mount point
@@ -193,19 +194,19 @@
         parent = join(path, b'..')
     else:
         parent = join(path, '..')
+    parent = realpath(parent)
+
     try:
         s2 = os.lstat(parent)
     except OSError:
+        # one should handle the returned OSError with more care to figure
+        # out whether this is still a mount
         return False
 
-    dev1 = s1.st_dev
-    dev2 = s2.st_dev
-    if dev1 != dev2:
+    if s1.st_dev != s2.st_dev:
         return True     # path/.. on a different device as path
-    ino1 = s1.st_ino
-    ino2 = s2.st_ino
-    if ino1 == ino2:
-        return True     # path/.. is the same i-node as path
+    if s1.st_ino == s2.st_ino:
+        return True     # path/.. is the same i-node as path, i.e. path=='/'
     return False
 
 
diff -r bae0912dd160 Lib/test/test_posixpath.py
--- a/Lib/test/test_posixpath.py	Thu Oct 01 08:56:54 2015 +0200
+++ b/Lib/test/test_posixpath.py	Sat Nov 14 15:18:07 2015 +0100
@@ -1,7 +1,5 @@
-import itertools
 import os
 import posixpath
-import sys
 import unittest
 import warnings
 from posixpath import realpath, abspath, dirname, basename
@@ -213,6 +211,29 @@
         finally:
             os.lstat = save_lstat
 
+    @unittest.skipIf(posix is None, "Test requires posix module")
+    def test_ismount_directory_not_readable(self):
+        """Simulate ismount run on a directory that is not readable
+        this lead to a wrong result as stated in http://bugs.python.org/issue2466
+        """
+        save_lstat = os.lstat
+        def fake_lstat(path):
+            st_ino = 0
+            st_dev = 0
+            if path.startswith(ABSTFN) and path != ABSTFN:
+                # ismount tries to read something inside the ABSTFN directory
+                # simulate this being forbidden (no read permission)
+                raise OSError("Fake [Errno 13] Permission denied")
+            if path == ABSTFN:
+                st_dev = 1
+                st_ino = 1
+            return posix.stat_result((0, st_ino, st_dev, 0, 0, 0, 0, 0, 0, 0))
+        try:
+            os.lstat = fake_lstat
+            self.assertIs(posixpath.ismount(ABSTFN), True)
+        finally:
+            os.lstat = save_lstat
+
     def test_expanduser(self):
         self.assertEqual(posixpath.expanduser("foo"), "foo")
         self.assertEqual(posixpath.expanduser(b"foo"), b"foo")