Hi,
the patch is as below:
Index: src/main/java/java/io/PushbackInputStream.java
===================================================================
--- src/main/java/java/io/PushbackInputStream.java      £®Version 743234£©
+++ src/main/java/java/io/PushbackInputStream.java      £®Working copy£©
@@ -282,9 +282,11 @@
             throw new ArrayIndexOutOfBoundsException();
         }

-        for (int i = offset + length - 1; i >= offset; i--) {
-            unread(buffer[i]);
+        if (buf == null) {
+            throw new IOException();
         }
+        System.arraycopy(buffer, offset, buf, pos - length, length);
+        pos = pos - length;
     }

I have write a test as below:

import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.IOException;
import java.io.PushbackInputStream;

public class PushbackInputStream_Unread_$BII {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // setUp
        String fileString = "Test_All_Tests\nTest_java_io_BufferedInputStream\nTest_java_io_BufferedOutputStream\nTest_java_io_ByteArrayInputStream\nTest_java_io_ByteArrayOutputStream\nTest_java_io_DataInputStream\nTest_java_io_File\nTest_java_io_FileDescriptor\nTest_java_io_FileInputStream\nTest_java_io_FileNotFoundException\nTest_java_io_FileOutputStream\nTest_java_io_FilterInputStream\nTest_java_io_FilterOutputStream\nTest_java_io_InputStream\nTest_java_io_IOException\nTest_java_io_OutputStream\nTest_java_io_PrintStream\nTest_java_io_RandomAccessFile\nTest_java_io_SyncFailedException\nTest_java_lang_AbstractMethodError\nTest_java_lang_ArithmeticException\nTest_java_lang_ArrayIndexOutOfBoundsException\nTest_java_lang_ArrayStoreException\nTest_java_lang_Boolean\nTest_java_lang_Byte\nTest_java_lang_Character\nTest_java_lang_Class\nTest_java_lang_ClassCastException\nTest_java_lang_ClassCircularityError\nTest_java_lang_ClassFormatError\nTest_java_lang_ClassLoader\nTest_java_lang_ClassNotFoundException\nTest_java_lang_CloneNotSupportedException\nTest_java_lang_Double\nTest_java_lang_Error\nTest_java_lang_Exception\nTest_java_lang_ExceptionInInitializerError\nTest_java_lang_Float\nTest_java_lang_IllegalAccessError\nTest_java_lang_IllegalAccessException\nTest_java_lang_IllegalArgumentException\nTest_java_lang_IllegalMonitorStateException\nTest_java_lang_IllegalThreadStateException\nTest_java_lang_IncompatibleClassChangeError\nTest_java_lang_IndexOutOfBoundsException\nTest_java_lang_InstantiationError\nTest_java_lang_InstantiationException\nTest_java_lang_Integer\nTest_java_lang_InternalError\nTest_java_lang_InterruptedException\nTest_java_lang_LinkageError\nTest_java_lang_Long\nTest_java_lang_Math\nTest_java_lang_NegativeArraySizeException\nTest_java_lang_NoClassDefFoundError\nTest_java_lang_NoSuchFieldError\nTest_java_lang_NoSuchMethodError\nTest_java_lang_NullPointerException\nTest_java_lang_Number\nTest_java_lang_NumberFormatException\nTest_java_lang_Object\nTest_java_lang_OutOfMemoryError\nTest_java_lang_RuntimeException\nTest_java_lang_SecurityManager\nTest_java_lang_Short\nTest_java_lang_StackOverflowError\nTest_java_lang_String\nTest_java_lang_StringBuffer\nTest_java_lang_StringIndexOutOfBoundsException\nTest_java_lang_System\nTest_java_lang_Thread\nTest_java_lang_ThreadDeath\nTest_java_lang_ThreadGroup\nTest_java_lang_Throwable\nTest_java_lang_UnknownError\nTest_java_lang_UnsatisfiedLinkError\nTest_java_lang_VerifyError\nTest_java_lang_VirtualMachineError\nTest_java_lang_vm_Image\nTest_java_lang_vm_MemorySegment\nTest_java_lang_vm_ROMStoreException\nTest_java_lang_vm_VM\nTest_java_lang_Void\nTest_java_net_BindException\nTest_java_net_ConnectException\nTest_java_net_DatagramPacket\nTest_java_net_DatagramSocket\nTest_java_net_DatagramSocketImpl\nTest_java_net_InetAddress\nTest_java_net_NoRouteToHostException\nTest_java_net_PlainDatagramSocketImpl\nTest_java_net_PlainSocketImpl\nTest_java_net_Socket\nTest_java_net_SocketException\nTest_java_net_SocketImpl\nTest_java_net_SocketInputStream\nTest_java_net_SocketOutputStream\nTest_java_net_UnknownHostException\nTest_java_util_ArrayEnumerator\nTest_java_util_Date\nTest_java_util_EventObject\nTest_java_util_HashEnumerator\nTest_java_util_Hashtable\nTest_java_util_Properties\nTest_java_util_ResourceBundle\nTest_java_util_tm\nTest_java_util_Vector\n";

        int count = 10000000;
        long time = System.currentTimeMillis();
        PushbackInputStream pis = new PushbackInputStream(
                new ByteArrayInputStream(fileString.getBytes()), 65535);
        byte[] buf = new byte[100];
        for (int i = 0; i < count; i++) {
            try {
                pis.read(buf, 0, 100);
                pis.unread(buf, 0, 100);
            } catch (IOException e) {
                // ignore
            }
        }

        // output
        time = System.currentTimeMillis() - time;

        System.out.println("======================");
        System.out.println("Invoke Count: " + count);
        System.out.println("Total Time: " + time + "ms");
    }

I have test on Intel(R) Core(TM)2 Duo Cpu 2.4GHZ, 2.98GB Memory machine, the result is as below:
Harmony patch before: 3362 ms
Harmony patched: 1312 ms