Index: src/org/python/core/PythonTraceFunction.java
===================================================================
--- src/org/python/core/PythonTraceFunction.java	(revision 2868)
+++ src/org/python/core/PythonTraceFunction.java	(working copy)
@@ -1,41 +1,42 @@
 // Copyright (c) Corporation for National Research Initiatives
 package org.python.core;
 
-class PythonTraceFunction extends TraceFunction
-{
+
+class PythonTraceFunction extends TraceFunction {
+
     PyObject tracefunc;
 
     PythonTraceFunction(PyObject tracefunc) {
         this.tracefunc = tracefunc;
     }
 
-    private synchronized TraceFunction safeCall(PyFrame frame, String label,
-                                                PyObject arg)
-    {
-        ThreadState ts = Py.getThreadState();
-        if (ts.tracing)
-            return null;
-
-        if (tracefunc == null)
-            return null;
-
-        PyObject ret = null;
-        try {
-            ts.tracing = true;
-            ret = tracefunc.__call__(frame, new PyString(label), arg);
-        } catch (PyException exc) {
-            frame.tracefunc = null;
-            ts.systemState.tracefunc = null;
-            ts.systemState.profilefunc = null;
-            throw exc;
-        } finally {
-            ts.tracing = false;
+    private TraceFunction safeCall(PyFrame frame, String label, PyObject arg) {
+        synchronized(imp.class) {
+            synchronized(this) {
+                ThreadState ts = Py.getThreadState();
+                if(ts.tracing)
+                    return null;
+                if(tracefunc == null)
+                    return null;
+                PyObject ret = null;
+                try {
+                    ts.tracing = true;
+                    ret = tracefunc.__call__(frame, new PyString(label), arg);
+                } catch(PyException exc) {
+                    frame.tracefunc = null;
+                    ts.systemState.tracefunc = null;
+                    ts.systemState.profilefunc = null;
+                    throw exc;
+                } finally {
+                    ts.tracing = false;
+                }
+                if(ret == tracefunc)
+                    return this;
+                if(ret == Py.None)
+                    return null;
+                return new PythonTraceFunction(ret);
+            }
         }
-        if (ret == tracefunc)
-            return this;
-        if (ret == Py.None)
-            return null;
-        return new PythonTraceFunction(ret);
     }
 
     public TraceFunction traceCall(PyFrame frame) {
@@ -51,9 +52,10 @@
     }
 
     public TraceFunction traceException(PyFrame frame, PyException exc) {
-        return safeCall(frame, "exception",
-                        new PyTuple(new PyObject[] {exc.type, exc.value,
-                                                    exc.traceback
-                        }));
+        return safeCall(frame,
+                        "exception",
+                        new PyTuple(new PyObject[] {exc.type,
+                                                    exc.value,
+                                                    exc.traceback}));
     }
 }