diff --git a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/test/MockitoUtil.java b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/test/MockitoUtil.java
index 82abcadf279..32305b5ee78 100644
--- a/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/test/MockitoUtil.java
+++ b/hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/test/MockitoUtil.java
@@ -20,6 +20,9 @@
 import java.io.Closeable;
 
 import org.mockito.Mockito;
+import org.mockito.invocation.InvocationOnMock;
+import org.mockito.stubbing.Answer;
+import org.mockito.stubbing.Stubber;
 
 public abstract class MockitoUtil {
 
@@ -33,4 +36,29 @@ public static <T> T mockProtocol(Class<T> clazz) {
     return Mockito.mock(clazz,
         Mockito.withSettings().extraInterfaces(Closeable.class));
   }
+
+  /**
+   * Throw an exception from the mock/spy only in the case that the
+   * call stack at the time the method has a line which matches the given
+   * pattern.
+   *
+   * @param t the Throwable to throw
+   * @param pattern the pattern against which to match the call stack trace
+   * @return the stub in progress
+   */
+  public static Stubber doThrowWhenCallStackMatches(
+      final Throwable t, final String pattern) {
+    return Mockito.doAnswer(new Answer<Object>() {
+      @Override
+      public Object answer(InvocationOnMock invocation) throws Throwable {
+        t.setStackTrace(Thread.currentThread().getStackTrace());
+        for (StackTraceElement elem : t.getStackTrace()) {
+          if (elem.toString().matches(pattern)) {
+            throw t;
+          }
+        }
+        return invocation.callRealMethod();
+      }
+    });
+  }
 }
diff --git a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
index 806c36f3a7c..24d192be7eb 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
+++ b/hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt
@@ -483,6 +483,9 @@ Release 2.0.5-beta - UNRELEASED
     HDFS-4658. Standby NN will log that it has received a block report "after
     becoming active" (atm)
 
+    HDFS-3981. Fix handling of FSN lock in getBlockLocations. (Xiaobo Peng
+    and todd via todd)
+
 Release 2.0.4-alpha - UNRELEASED
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
index 47604277918..ad528f0ad0a 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/FSNamesystem.java
@@ -1378,14 +1378,14 @@ private LocatedBlocks getBlockLocationsUpdateTimes(String src,
         long now = now();
         final INodeFile inode = INodeFile.valueOf(dir.getINode(src), src);
         if (doAccessTime && isAccessTimeSupported()) {
-          if (now <= inode.getAccessTime() + getAccessTimePrecision()) {
+          if (now > inode.getAccessTime() + getAccessTimePrecision()) {
             // if we have to set access time but we only have the readlock, then
             // restart this entire operation with the writeLock.
             if (isReadOp) {
               continue;
             }
+            dir.setTimes(src, inode, -1, now, false);
           }
-          dir.setTimes(src, inode, -1, now, false);
         }
         return blockManager.createLocatedBlocks(inode.getBlocks(),
             inode.computeFileSize(false), inode.isUnderConstruction(),
diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestSetTimes.java b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestSetTimes.java
index 18341ca9558..4e6091b8122 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestSetTimes.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestSetTimes.java
@@ -27,6 +27,7 @@
 import java.text.SimpleDateFormat;
 import java.util.Date;
 import java.util.Random;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 import org.apache.hadoop.conf.Configuration;
 import org.apache.hadoop.fs.CommonConfigurationKeys;
@@ -36,8 +37,11 @@
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.hdfs.protocol.DatanodeInfo;
 import org.apache.hadoop.hdfs.protocol.HdfsConstants.DatanodeReportType;
+import org.apache.hadoop.hdfs.server.namenode.NameNodeAdapter;
+import org.apache.hadoop.test.MockitoUtil;
 import org.apache.hadoop.util.Time;
 import org.junit.Test;
+import org.mockito.Mockito;
 
 /**
  * This class tests the access time on files.
@@ -273,6 +277,37 @@ public void testTimesAtClose() throws IOException {
       cluster.shutdown();
     }
   }
+  
+  /**
+   * Test that when access time updates are not needed, the FSNamesystem
+   * write lock is not taken by getBlockLocations.
+   * Regression test for HDFS-3981.
+   */
+  @Test(timeout=60000)
+  public void testGetBlockLocationsOnlyUsesReadLock() throws IOException {
+    Configuration conf = new HdfsConfiguration();
+    conf.setInt(DFSConfigKeys.DFS_NAMENODE_ACCESSTIME_PRECISION_KEY, 100*1000);
+    MiniDFSCluster cluster = new MiniDFSCluster.Builder(conf)
+      .numDataNodes(0)
+      .build();
+    ReentrantReadWriteLock spyLock = NameNodeAdapter.spyOnFsLock(cluster.getNamesystem());
+    try {
+      // Create empty file in the FSN.
+      Path p = new Path("/empty-file");
+      DFSTestUtil.createFile(cluster.getFileSystem(), p, 0, (short)1, 0L);
+      
+      // getBlockLocations() should not need the write lock, since we just created
+      // the file (and thus its access time is already within the 100-second
+      // accesstime precision configured above). 
+      MockitoUtil.doThrowWhenCallStackMatches(
+          new AssertionError("Should not need write lock"),
+          ".*getBlockLocations.*")
+          .when(spyLock).writeLock();
+      cluster.getFileSystem().getFileBlockLocations(p, 0, 100);
+    } finally {
+      cluster.shutdown();
+    }
+  }
 
   public static void main(String[] args) throws Exception {
     new TestSetTimes().testTimes();