diff -r 90b56ec318b6 Modules/_tracemalloc.c
--- a/Modules/_tracemalloc.c	Thu Nov 28 06:53:05 2013 -0800
+++ b/Modules/_tracemalloc.c	Thu Nov 28 22:34:02 2013 +0100
@@ -172,14 +172,11 @@ set_reentrant(int reentrant)
     assert(reentrant == 0 || reentrant == 1);
     if (reentrant) {
         assert(PyThread_get_key_value(tracemalloc_reentrant_key) == NULL);
-        PyThread_set_key_value(tracemalloc_reentrant_key,
-                               REENTRANT);
+        PyThread_set_key_value(tracemalloc_reentrant_key, REENTRANT);
     }
     else {
-        /* FIXME: PyThread_set_key_value() cannot be used to set the flag
-           to zero, because it does nothing if the variable has already
-           a value set. */
-        PyThread_delete_key_value(tracemalloc_reentrant_key);
+        assert(PyThread_get_key_value(tracemalloc_reentrant_key) == REENTRANT);
+        PyThread_set_key_value(tracemalloc_reentrant_key, NULL);
     }
 }
 
diff -r 90b56ec318b6 Python/thread.c
--- a/Python/thread.c	Thu Nov 28 06:53:05 2013 -0800
+++ b/Python/thread.c	Thu Nov 28 22:34:02 2013 +0100
@@ -205,7 +205,7 @@ static int nkeys = 0;  /* PyThread_creat
  * segfaults.  Now we lock the whole routine.
  */
 static struct key *
-find_key(int key, void *value)
+find_key(int key, int update, void *value)
 {
     struct key *p, *prev_p;
     long id = PyThread_get_thread_ident();
@@ -215,8 +215,11 @@ find_key(int key, void *value)
     PyThread_acquire_lock(keymutex, 1);
     prev_p = NULL;
     for (p = keyhead; p != NULL; p = p->next) {
-        if (p->id == id && p->key == key)
+        if (p->id == id && p->key == key) {
+            if (update)
+                p->value = value;
             goto Done;
+        }
         /* Sanity check.  These states should never happen but if
          * they do we must abort.  Otherwise we'll end up spinning in
          * in a tight loop with the lock held.  A similar check is done
@@ -227,7 +230,7 @@ find_key(int key, void *value)
         if (p->next == keyhead)
             Py_FatalError("tls find_key: circular list(!)");
     }
-    if (value == NULL) {
+    if (!update && value == NULL) {
         assert(p == NULL);
         goto Done;
     }
@@ -279,19 +282,12 @@ PyThread_delete_key(int key)
     PyThread_release_lock(keymutex);
 }
 
-/* Confusing:  If the current thread has an association for key,
- * value is ignored, and 0 is returned.  Else an attempt is made to create
- * an association of key to value for the current thread.  0 is returned
- * if that succeeds, but -1 is returned if there's not enough memory
- * to create the association.  value must not be NULL.
- */
 int
 PyThread_set_key_value(int key, void *value)
 {
     struct key *p;
 
-    assert(value != NULL);
-    p = find_key(key, value);
+    p = find_key(key, 1, value);
     if (p == NULL)
         return -1;
     else
@@ -304,7 +300,7 @@ PyThread_set_key_value(int key, void *va
 void *
 PyThread_get_key_value(int key)
 {
-    struct key *p = find_key(key, NULL);
+    struct key *p = find_key(key, 0, NULL);
 
     if (p == NULL)
         return NULL;
diff -r 90b56ec318b6 Python/thread_nt.h
--- a/Python/thread_nt.h	Thu Nov 28 06:53:05 2013 -0800
+++ b/Python/thread_nt.h	Thu Nov 28 22:34:02 2013 +0100
@@ -389,20 +389,11 @@ PyThread_delete_key(int key)
     TlsFree(key);
 }
 
-/* We must be careful to emulate the strange semantics implemented in thread.c,
- * where the value is only set if it hasn't been set before.
- */
 int
 PyThread_set_key_value(int key, void *value)
 {
     BOOL ok;
-    void *oldvalue;
 
-    assert(value != NULL);
-    oldvalue = TlsGetValue(key);
-    if (oldvalue != NULL)
-        /* ignore value if already set */
-        return 0;
     ok = TlsSetValue(key, value);
     if (!ok)
         return -1;
diff -r 90b56ec318b6 Python/thread_pthread.h
--- a/Python/thread_pthread.h	Thu Nov 28 06:53:05 2013 -0800
+++ b/Python/thread_pthread.h	Thu Nov 28 22:34:02 2013 +0100
@@ -627,9 +627,6 @@ int
 PyThread_set_key_value(int key, void *value)
 {
     int fail;
-    void *oldValue = pthread_getspecific(key);
-    if (oldValue != NULL)
-        return 0;
     fail = pthread_setspecific(key, value);
     return fail ? -1 : 0;
 }