diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 9a4ff349d22..81ed2915cad 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -355,6 +355,8 @@ Release 2.3.0 - UNRELEASED
     HDFS-5283. Under construction blocks only inside snapshots should not be
     counted in safemode threshhold.  (Vinay via szetszwo)
 
+    HDFS-4376. Fix race conditions in Balancer.  (Junping Du via szetszwo)
+
 Release 2.2.1 - UNRELEASED
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/Balancer.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/Balancer.java
index 1ed0e1915f4..befdd90a79e 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/Balancer.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/balancer/Balancer.java
@@ -506,7 +506,7 @@ private static class BalancerDatanode {
     final DatanodeInfo datanode;
     final double utilization;
     final long maxSize2Move;
-    protected long scheduledSize = 0L;
+    private long scheduledSize = 0L;
     //  blocks being moved but not confirmed yet
     private List<PendingBlockMove> pendingBlocks = 
       new ArrayList<PendingBlockMove>(MAX_NUM_CONCURRENT_MOVES); 
@@ -555,20 +555,35 @@ protected String getStorageID() {
     }
     
     /** Decide if still need to move more bytes */
-    protected boolean hasSpaceForScheduling() {
+    protected synchronized boolean hasSpaceForScheduling() {
       return scheduledSize<maxSize2Move;
     }
 
     /** Return the total number of bytes that need to be moved */
-    protected long availableSizeToMove() {
+    protected synchronized long availableSizeToMove() {
       return maxSize2Move-scheduledSize;
     }
     
-    /* increment scheduled size */
-    protected void incScheduledSize(long size) {
+    /** increment scheduled size */
+    protected synchronized void incScheduledSize(long size) {
       scheduledSize += size;
     }
     
+    /** decrement scheduled size */
+    protected synchronized void decScheduledSize(long size) {
+      scheduledSize -= size;
+    }
+    
+    /** get scheduled size */
+    protected synchronized long getScheduledSize(){
+      return scheduledSize;
+    }
+    
+    /** get scheduled size */
+    protected synchronized void setScheduledSize(long size){
+      scheduledSize = size;
+    }
+    
     /* Check if the node can schedule more blocks to move */
     synchronized private boolean isPendingQNotFull() {
       if ( pendingBlocks.size() < MAX_NUM_CONCURRENT_MOVES ) {
@@ -702,8 +717,8 @@ private PendingBlockMove chooseNextBlockToMove() {
           pendingBlock.source = this;
           pendingBlock.target = target;
           if ( pendingBlock.chooseBlockAndProxy() ) {
-            long blockSize = pendingBlock.block.getNumBytes(); 
-            scheduledSize -= blockSize;
+            long blockSize = pendingBlock.block.getNumBytes();
+            decScheduledSize(blockSize);
             task.size -= blockSize;
             if (task.size == 0) {
               tasks.remove();
@@ -747,10 +762,11 @@ private boolean shouldFetchMoreBlocks() {
     private static final long MAX_ITERATION_TIME = 20*60*1000L; //20 mins
     private void dispatchBlocks() {
       long startTime = Time.now();
+      long scheduledSize = getScheduledSize();
       this.blocksToReceive = 2*scheduledSize;
       boolean isTimeUp = false;
       int noPendingBlockIteration = 0;
-      while(!isTimeUp && scheduledSize>0 &&
+      while(!isTimeUp && getScheduledSize()>0 &&
           (!srcBlockList.isEmpty() || blocksToReceive>0)) {
         PendingBlockMove pendingBlock = chooseNextBlockToMove();
         if (pendingBlock != null) {
@@ -779,7 +795,7 @@ private void dispatchBlocks() {
           // in case no blocks can be moved for source node's task,
           // jump out of while-loop after 5 iterations.
           if (noPendingBlockIteration >= MAX_NO_PENDING_BLOCK_ITERATIONS) {
-            scheduledSize = 0;
+            setScheduledSize(0);
           }
         }
         
@@ -992,7 +1008,7 @@ private long chooseNodes() {
 
     long bytesToMove = 0L;
     for (Source src : sources) {
-      bytesToMove += src.scheduledSize;
+      bytesToMove += src.getScheduledSize();
     }
     return bytesToMove;
   }
@@ -1093,7 +1109,7 @@ private synchronized void inc( long bytes ) {
       bytesMoved += bytes;
     }
 
-    private long get() {
+    private synchronized long get() {
       return bytesMoved;
     }
   };