From 0636013d65d73c682510ec04ea980d3f76d78e65 Mon Sep 17 00:00:00 2001
From: Simone Bordet <simone.bordet@gmail.com>
Date: Mon, 30 Mar 2015 18:47:07 +0200
Subject: [PATCH] Using test-and-test-and-set.

See http://en.wikipedia.org/wiki/Spinlock, the section about
optimizations.
See also http://en.wikipedia.org/wiki/Test_and_Test-and-set.
---
 .../eclipse/jetty/util/thread/SpinLock.java   | 31 ++++++++-----------
 .../jetty/util/thread/SpinLockTest.java       | 13 +++-----
 2 files changed, 17 insertions(+), 27 deletions(-)

diff --git a/jetty-util/src/main/java/org/eclipse/jetty/util/thread/SpinLock.java b/jetty-util/src/main/java/org/eclipse/jetty/util/thread/SpinLock.java
index d3048feba47..fc76529b64b 100644
--- a/jetty-util/src/main/java/org/eclipse/jetty/util/thread/SpinLock.java
+++ b/jetty-util/src/main/java/org/eclipse/jetty/util/thread/SpinLock.java
@@ -20,11 +20,9 @@ package org.eclipse.jetty.util.thread;
 
 import java.util.concurrent.atomic.AtomicReference;
 
-
-/* ------------------------------------------------------------ */
-/** Spin Lock
- * <p>This is a lock designed to protect VERY short sections of 
- * critical code.  Threads attempting to take the lock will spin 
+/**
+ * <p>This is a lock designed to protect VERY short sections of
+ * critical code.  Threads attempting to take the lock will spin
  * forever until the lock is available, thus it is important that
  * the code protected by this lock is extremely simple and non
  * blocking. The reason for this lock is that it prevents a thread
@@ -40,32 +38,29 @@ public class SpinLock
 {
     private final AtomicReference<Thread> _lock = new AtomicReference<>(null);
     private final Lock _unlock = new Lock();
-    
+
     public Lock lock()
     {
-        Thread thread = Thread.currentThread();
-        while(true)
+        Thread current = Thread.currentThread();
+        while (true)
         {
-            if (!_lock.compareAndSet(null,thread))
+            // Using test-and-test-and-set for better performance.
+            Thread locker = _lock.get();
+            if (locker != null || !_lock.compareAndSet(null, current))
             {
-                if (_lock.get()==thread)
+                if (locker == current)
                     throw new IllegalStateException("SpinLock is not reentrant");
                 continue;
             }
             return _unlock;
         }
     }
-    
+
     public boolean isLocked()
     {
-        return _lock.get()!=null;
+        return _lock.get() != null;
     }
-    
-    public boolean isLockedThread()
-    {
-        return _lock.get()==Thread.currentThread();
-    }
-    
+
     public class Lock implements AutoCloseable
     {
         @Override
diff --git a/jetty-util/src/test/java/org/eclipse/jetty/util/thread/SpinLockTest.java b/jetty-util/src/test/java/org/eclipse/jetty/util/thread/SpinLockTest.java
index e4160f38a5a..49021091675 100644
--- a/jetty-util/src/test/java/org/eclipse/jetty/util/thread/SpinLockTest.java
+++ b/jetty-util/src/test/java/org/eclipse/jetty/util/thread/SpinLockTest.java
@@ -18,17 +18,16 @@
 
 package org.eclipse.jetty.util.thread;
 
-import static org.junit.Assert.assertFalse;
-import static org.junit.Assert.assertTrue;
-
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 
 import org.junit.Test;
 
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
 public class SpinLockTest
 {
-    
     @Test
     public void testLocked()
     {
@@ -70,7 +69,6 @@ public class SpinLockTest
         assertFalse(lock.isLocked());
     }
 
-
     @Test
     public void testContend() throws Exception
     {
@@ -121,7 +119,7 @@ public class SpinLockTest
         };
         thread1.start();
         // thread1 will be spinning here
-        assertFalse(held1.await(100,TimeUnit.MILLISECONDS));
+        assertFalse(held1.await(100, TimeUnit.MILLISECONDS));
         
         // Let thread0 complete
         hold0.countDown();
@@ -135,8 +133,5 @@ public class SpinLockTest
         thread1.join();
 
         assertFalse(lock.isLocked());
-        
     }
-    
-    
 }