Index: lucene/src/java/org/apache/lucene/index/IndexReader.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/IndexReader.java	(revision 1220751)
+++ lucene/src/java/org/apache/lucene/index/IndexReader.java	(working copy)
@@ -22,10 +22,11 @@
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.LinkedHashSet;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
-import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.lucene.document.Document;
@@ -36,7 +37,6 @@
 import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.CommandLineUtil;
-import org.apache.lucene.util.MapBackedSet;
 import org.apache.lucene.util.ReaderUtil;         // for javadocs
 
 /** IndexReader is an abstract class, providing an interface for accessing an
@@ -84,7 +84,7 @@
   }
 
   private final Set<ReaderClosedListener> readerClosedListeners = 
-      new MapBackedSet<ReaderClosedListener>(new ConcurrentHashMap<ReaderClosedListener, Boolean>());
+      Collections.synchronizedSet(new LinkedHashSet<ReaderClosedListener>());
 
   /** Expert: adds a {@link ReaderClosedListener}.  The
    * provided listener will be invoked when this reader is closed.
@@ -104,8 +104,10 @@
   }
 
   private final void notifyReaderClosedListeners() {
-    for(ReaderClosedListener listener : readerClosedListeners) {
-      listener.onClose(this);
+    synchronized(readerClosedListeners) {
+      for(ReaderClosedListener listener : readerClosedListeners) {
+        listener.onClose(this);
+      }
     }
   }
 
Index: lucene/src/java/org/apache/lucene/index/SegmentCoreReaders.java
===================================================================
--- lucene/src/java/org/apache/lucene/index/SegmentCoreReaders.java	(revision 1220751)
+++ lucene/src/java/org/apache/lucene/index/SegmentCoreReaders.java	(working copy)
@@ -18,8 +18,9 @@
  */
 
 import java.io.IOException;
+import java.util.Collections;
+import java.util.LinkedHashSet;
 import java.util.Set;
-import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.atomic.AtomicInteger;
 
 import org.apache.lucene.codecs.Codec;
@@ -34,7 +35,6 @@
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.store.IOContext;
 import org.apache.lucene.util.IOUtils;
-import org.apache.lucene.util.MapBackedSet;
 
 /** Holds core readers that are shared (unchanged) when
  * SegmentReader is cloned or reopened */
@@ -68,7 +68,7 @@
   CompoundFileDirectory storeCFSReader;
 
   final Set<CoreClosedListener> coreClosedListeners = 
-      new MapBackedSet<CoreClosedListener>(new ConcurrentHashMap<CoreClosedListener, Boolean>());
+      Collections.synchronizedSet(new LinkedHashSet<CoreClosedListener>());
   
   SegmentCoreReaders(SegmentReader owner, Directory dir, SegmentInfo si, IOContext context, int termsIndexDivisor) throws IOException {
     
@@ -138,8 +138,10 @@
     if (ref.decrementAndGet() == 0) {
       IOUtils.close(fields, perDocProducer, termVectorsReaderOrig,
           fieldsReaderOrig, cfsReader, storeCFSReader, norms);
-      for (CoreClosedListener listener : coreClosedListeners) {
-        listener.onClose(owner);
+      synchronized(coreClosedListeners) {
+        for (CoreClosedListener listener : coreClosedListeners) {
+          listener.onClose(owner);
+        }
       }
     }
   }