diff --git a/hadoop-yarn-project/CHANGES.txt b/hadoop-yarn-project/CHANGES.txt
index 00b20401eb4..8018dc1063b 100644
--- a/hadoop-yarn-project/CHANGES.txt
+++ b/hadoop-yarn-project/CHANGES.txt
@@ -1231,6 +1231,9 @@ Release 2.8.0 - UNRELEASED
     YARN-4748. ApplicationHistoryManagerOnTimelineStore should not
     swallow exceptions on generateApplicationReport. (Li Lu via jianhe)
 
+    YARN-4671. There is no need to acquire CS lock when completing a container.
+    (Meng Ding via jianhe)
+
 Release 2.7.3 - UNRELEASED
 
   INCOMPATIBLE CHANGES
diff --git a/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/CapacityScheduler.java b/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/CapacityScheduler.java
index 62ecca0909d..1235946b297 100644
--- a/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/CapacityScheduler.java
+++ b/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/CapacityScheduler.java
@@ -227,7 +227,7 @@ public class CapacityScheduler extends
   private AsyncScheduleThread asyncSchedulerThread;
   private RMNodeLabelsManager labelManager;
   private SchedulerHealth schedulerHealth = new SchedulerHealth();
-  long lastNodeUpdateTime;
+  volatile long lastNodeUpdateTime;
 
   /**
    * EXPERT
@@ -925,7 +925,6 @@ public class CapacityScheduler extends
   }
 
   @Override
-  // Note: when AM asks to release container, we will acquire scheduler lock
   @Lock(Lock.NoLock.class)
   public Allocation allocate(ApplicationAttemptId applicationAttemptId,
       List<ResourceRequest> ask, List<ContainerId> release,
@@ -1543,9 +1542,8 @@ public class CapacityScheduler extends
     }
   }
 
-  @Lock(CapacityScheduler.class)
   @Override
-  protected synchronized void completedContainerInternal(
+  protected void completedContainerInternal(
       RMContainer rmContainer, ContainerStatus containerStatus,
       RMContainerEventType event) {
     
@@ -1954,7 +1952,7 @@ public class CapacityScheduler extends
     return this.schedulerHealth;
   }
 
-  private synchronized void setLastNodeUpdateTime(long time) {
+  private void setLastNodeUpdateTime(long time) {
     this.lastNodeUpdateTime = time;
   }
 
diff --git a/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/TestCapacityScheduler.java b/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/TestCapacityScheduler.java
index bd2c4fe4b19..025bad0b540 100644
--- a/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/TestCapacityScheduler.java
+++ b/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/test/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/TestCapacityScheduler.java
@@ -1078,9 +1078,33 @@ public class TestCapacityScheduler {
         RegisterApplicationMasterRequest.newInstance("localhost", 12345, "");
     client.registerApplicationMaster(request);
 
+    // Allocate a container
+    List<ResourceRequest> asks = Collections.singletonList(
+        ResourceRequest.newInstance(
+            Priority.newInstance(1), "*", Resources.createResource(2 * GB), 1));
+    AllocateRequest allocateRequest =
+        AllocateRequest.newInstance(0, 0.0f, asks, null, null);
+    client.allocate(allocateRequest);
+
+    // Make sure the container is allocated in RM
+    nm1.nodeHeartbeat(true);
+    ContainerId containerId2 =
+        ContainerId.newContainerId(applicationAttemptId, 2);
+    Assert.assertTrue(rm.waitForState(nm1, containerId2,
+        RMContainerState.ALLOCATED, 10 * 1000));
+
+    // Acquire the container
+    allocateRequest = AllocateRequest.newInstance(1, 0.0f, null, null, null);
+    client.allocate(allocateRequest);
+
+    // Launch the container
+    final CapacityScheduler cs = (CapacityScheduler) rm.getResourceScheduler();
+    RMContainer rmContainer = cs.getRMContainer(containerId2);
+    rmContainer.handle(
+        new RMContainerEvent(containerId2, RMContainerEventType.LAUNCHED));
+
     // grab the scheduler lock from another thread
     // and verify an allocate call in this thread doesn't block on it
-    final CapacityScheduler cs = (CapacityScheduler) rm.getResourceScheduler();
     final CyclicBarrier barrier = new CyclicBarrier(2);
     Thread otherThread = new Thread(new Runnable() {
       @Override
@@ -1089,9 +1113,7 @@ public class TestCapacityScheduler {
           try {
             barrier.await();
             barrier.await();
-          } catch (InterruptedException e) {
-            e.printStackTrace();
-          } catch (BrokenBarrierException e) {
+          } catch (InterruptedException | BrokenBarrierException e) {
             e.printStackTrace();
           }
         }
@@ -1099,8 +1121,9 @@ public class TestCapacityScheduler {
     });
     otherThread.start();
     barrier.await();
-    AllocateRequest allocateRequest =
-        AllocateRequest.newInstance(0, 0.0f, null, null, null);
+    List<ContainerId> release = Collections.singletonList(containerId2);
+    allocateRequest =
+        AllocateRequest.newInstance(2, 0.0f, null, release, null);
     client.allocate(allocateRequest);
     barrier.await();
     otherThread.join();