diff --git a/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java b/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java
index 6f51566f75..340cd45a1e 100644
--- a/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java
+++ b/tez-dag/src/main/java/org/apache/tez/dag/app/rm/YarnTaskSchedulerService.java
@@ -1476,44 +1476,46 @@ private CookieContainerRequest getMatchingRequestWithoutPriority(
       return null;
     }
     CookieContainerRequest firstMatch = null;
-    for (Collection<CookieContainerRequest> requests : pRequestsList) {
-      for (CookieContainerRequest cookieContainerRequest : requests) {
-        if (firstMatch == null || // we dont have a match. So look for one 
-            // we have a match but are looking for a better container level match.
-            // skip the expensive canAssignTaskToContainer() if the request is 
-            // not affinitized to the container
-            container.getId().equals(cookieContainerRequest.getAffinitizedContainer())
-            ) {
-          if (canAssignTaskToContainer(cookieContainerRequest, container)) {
-            // request matched to container
-            if (!considerContainerAffinity) {
-              return cookieContainerRequest;
-            }
-            ContainerId affCId = cookieContainerRequest.getAffinitizedContainer();
-            boolean canMatchTaskWithAffinity = true;
-            if (affCId == null || 
-                !heldContainers.containsKey(affCId) ||
-                inUseContainers.contains(affCId)) {
-              // affinity not specified
-              // affinitized container is no longer held
-              // affinitized container is in use
-              canMatchTaskWithAffinity = false;
-            }
-            if (canMatchTaskWithAffinity) {
-              if (container.getId().equals(
-                  cookieContainerRequest.getAffinitizedContainer())) {
-                // container level match
-                  LOG.debug("Matching with affinity for request: {} container: {}",
-                    cookieContainerRequest, affCId);
+    synchronized (this) {
+      for (Collection<CookieContainerRequest> requests : pRequestsList) {
+        for (CookieContainerRequest cookieContainerRequest : requests) {
+          if (firstMatch == null || // we dont have a match. So look for one
+                  // we have a match but are looking for a better container level match.
+                  // skip the expensive canAssignTaskToContainer() if the request is
+                  // not affinitized to the container
+                  container.getId().equals(cookieContainerRequest.getAffinitizedContainer())
+          ) {
+            if (canAssignTaskToContainer(cookieContainerRequest, container)) {
+              // request matched to container
+              if (!considerContainerAffinity) {
                 return cookieContainerRequest;
               }
-              if (LOG.isDebugEnabled()) {
-                LOG.debug("Skipping request for container " + container.getId()
-                    + " due to affinity. Request: " + cookieContainerRequest
-                    + " affContainer: " + affCId);
+              ContainerId affCId = cookieContainerRequest.getAffinitizedContainer();
+              boolean canMatchTaskWithAffinity = true;
+              if (affCId == null ||
+                      !heldContainers.containsKey(affCId) ||
+                      inUseContainers.contains(affCId)) {
+                // affinity not specified
+                // affinitized container is no longer held
+                // affinitized container is in use
+                canMatchTaskWithAffinity = false;
+              }
+              if (canMatchTaskWithAffinity) {
+                if (container.getId().equals(
+                        cookieContainerRequest.getAffinitizedContainer())) {
+                  // container level match
+                  LOG.debug("Matching with affinity for request: {} container: {}",
+                          cookieContainerRequest, affCId);
+                  return cookieContainerRequest;
+                }
+                if (LOG.isDebugEnabled()) {
+                  LOG.debug("Skipping request for container " + container.getId()
+                          + " due to affinity. Request: " + cookieContainerRequest
+                          + " affContainer: " + affCId);
+                }
+              } else {
+                firstMatch = cookieContainerRequest;
               }
-            } else {
-              firstMatch = cookieContainerRequest;
             }
           }
         }
@@ -1633,7 +1635,7 @@ private void pushNewContainerToDelayed(List<Container> containers){
     delayedContainerManager.triggerScheduling(false);      
   }
 
-  private CookieContainerRequest removeTaskRequest(Object task) {
+  private synchronized CookieContainerRequest removeTaskRequest(Object task) {
     CookieContainerRequest request = taskRequests.remove(task);
     if(request != null) {
       // remove all references of the request from AMRMClient
@@ -1642,7 +1644,7 @@ private CookieContainerRequest removeTaskRequest(Object task) {
     return request;
   }
 
-  private void addTaskRequest(Object task,
+  private synchronized void addTaskRequest(Object task,
                                 CookieContainerRequest request) {
     CookieContainerRequest oldRequest = taskRequests.put(task, request);
     if (oldRequest != null) {