From f2d150ea1205b77a75c347ace667b4cd060aaf40 Mon Sep 17 00:00:00 2001
From: Karthik Kambatla <kasha@apache.org>
Date: Wed, 17 Dec 2014 12:03:16 -0800
Subject: [PATCH] YARN-2964. FSLeafQueue#assignContainer - document the reason
 for using both write and read locks. (Tsuyoshi Ozawa via kasha)

---
 hadoop-yarn-project/CHANGES.txt                                | 3 +++
 .../server/resourcemanager/scheduler/fair/FSLeafQueue.java     | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/hadoop-yarn-project/CHANGES.txt b/hadoop-yarn-project/CHANGES.txt
index 37147d7b24f..42ffa9b4922 100644
--- a/hadoop-yarn-project/CHANGES.txt
+++ b/hadoop-yarn-project/CHANGES.txt
@@ -231,6 +231,9 @@ Release 2.7.0 - UNRELEASED
     YARN-2914. [YARN-1492] Potential race condition in Singleton implementation of 
     SharedCacheUploaderMetrics, CleanerMetrics, ClientSCMMetrics. (Varun Saxena via kasha)
 
+    YARN-2964. FSLeafQueue#assignContainer - document the reason for using both write and
+    read locks. (Tsuyoshi Ozawa via kasha)
+
 Release 2.6.0 - 2014-11-18
 
   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/fair/FSLeafQueue.java b/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSLeafQueue.java
index bbf1be71755..63450da05c3 100644
--- a/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSLeafQueue.java
+++ b/hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSLeafQueue.java
@@ -252,6 +252,9 @@ public class FSLeafQueue extends FSQueue {
     } finally {
       writeLock.unlock();
     }
+    // Release write lock here for better performance and avoiding deadlocks.
+    // runnableApps can be in unsorted state because of this section,
+    // but we can accept it in practice since the probability is low.
     readLock.lock();
     try {
       for (FSAppAttempt sched : runnableApps) {