diff -r 5619bc2d8207 Objects/obmalloc.c
--- a/Objects/obmalloc.c	Fri Jun 14 19:02:34 2013 -0400
+++ b/Objects/obmalloc.c	Sat Jun 15 02:06:22 2013 +0200
@@ -1679,6 +1679,22 @@ pool_is_in_list(const poolp target, pool
 
 #endif  /* Py_DEBUG */
 
+static void
+_PyMem_DebugCheckGIL(void)
+{
+    PyThreadState *tstate;
+    static int check = 1;
+
+    if (!check)
+        return;
+
+    if (PyGILState_Check())
+        return;
+
+    check = 0;
+    Py_FatalError("GIL must be held to call PyMem_Malloc and PyObject_Malloc");
+}
+
 /* Let S = sizeof(size_t).  The debug malloc asks for 4*S extra bytes and
    fills them with useful stuff, here calling the underlying malloc's result p:
 
@@ -1717,6 +1733,9 @@ static void *
         /* overflow:  can't represent total as a size_t */
         return NULL;
 
+    if (api->api_id != 'r')
+        _PyMem_DebugCheckGIL();
+
     p = (uchar *)api->alloc.malloc(api->alloc.ctx, total);
     if (p == NULL)
         return NULL;
@@ -1756,6 +1775,10 @@ static void
     nbytes += 4*SST;
     if (nbytes > 0)
         memset(q, DEADBYTE, nbytes);
+
+    if (api->api_id != 'r')
+        _PyMem_DebugCheckGIL();
+
     api->alloc.free(api->alloc.ctx, q);
 }
 
@@ -1785,6 +1808,9 @@ static void *
         memset(q + nbytes, DEADBYTE, original_nbytes - nbytes + 2*SST);
     }
 
+    if (api->api_id != 'r')
+        _PyMem_DebugCheckGIL();
+
     /* Resize and add decorations. We may get a new pointer here, in which
      * case we didn't get the chance to mark the old memory with DEADBYTE,
      * but we live with that.