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From: jlowe@apache.org
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Subject: hadoop git commit: HADOOP-14912. FairCallQueue may defer servicing
 calls. Contributed by Daryn Sharp
Date: Tue, 10 Oct 2017 19:49:59 +0000 (UTC)
archived-at: Tue, 10 Oct 2017 19:50:01 -0000

Repository: hadoop
Updated Branches:
  refs/heads/branch-3.0 61ac7c82f -> c0d56cad5


HADOOP-14912. FairCallQueue may defer servicing calls. Contributed by Daryn Sharp

(cherry picked from commit 1123f8f0b62292197f5433cd40e66d8620044608)


Project: http://git-wip-us.apache.org/repos/asf/hadoop/repo
Commit: http://git-wip-us.apache.org/repos/asf/hadoop/commit/c0d56cad
Tree: http://git-wip-us.apache.org/repos/asf/hadoop/tree/c0d56cad
Diff: http://git-wip-us.apache.org/repos/asf/hadoop/diff/c0d56cad

Branch: refs/heads/branch-3.0
Commit: c0d56cad5c7c7345492628b4e24717bea8208ac6
Parents: 61ac7c8
Author: Jason Lowe <jlowe@apache.org>
Authored: Tue Oct 10 14:47:25 2017 -0500
Committer: Jason Lowe <jlowe@apache.org>
Committed: Tue Oct 10 14:49:15 2017 -0500

----------------------------------------------------------------------
 .../src/main/java/org/apache/hadoop/ipc/FairCallQueue.java   | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/hadoop/blob/c0d56cad/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java
----------------------------------------------------------------------
diff --git a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java
index 20161b8..6d9ea3e 100644
--- a/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java
+++ b/hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ipc/FairCallQueue.java
@@ -122,13 +122,15 @@ public class FairCallQueue<E extends Schedulable> extends AbstractQueue<E>
   private E removeNextElement() {
     int priority = multiplexer.getAndAdvanceCurrentIndex();
     E e = queues.get(priority).poll();
-    if (e == null) {
+    // a semaphore permit has been acquired, so an element MUST be extracted
+    // or the semaphore and queued elements will go out of sync.  loop to
+    // avoid race condition if elements are added behind the current position,
+    // awakening other threads that poll the elements ahead of our position.
+    while (e == null) {
       for (int idx = 0; e == null && idx < queues.size(); idx++) {
         e = queues.get(idx).poll();
       }
     }
-    // guaranteed to find an element if caller acquired permit.
-    assert e != null : "consumer didn't acquire semaphore!";
     return e;
   }
 


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