From 5183e98d9a4e75b6df7e04efa6bf7a1a37c737b0 Mon Sep 17 00:00:00 2001
From: Garvit Dewan <d.garvit@gmail.com>
Date: Sat, 8 Jun 2019 03:32:41 +0530
Subject: [PATCH 1/2] Create LockFreeQueue.chpl

---
 example/LockFreeQueue.chpl | 98 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 98 insertions(+)
 create mode 100644 example/LockFreeQueue.chpl

diff --git a/example/LockFreeQueue.chpl b/example/LockFreeQueue.chpl
new file mode 100644
index 0000000..f7d1de2
--- /dev/null
+++ b/example/LockFreeQueue.chpl
@@ -0,0 +1,98 @@
+/**
+ * Created by Garvit Dewan - 
+ * https://github.com/dgarvit/epoch-based-manager/blob/master/src/LockFreeQueue.chpl
+ *   
+ * Lock-Free Queue that uses ABA feature of Distributed Data Structures
+ * Based on Michael Scott Queue
+ */
+module LockFreeQueue {
+
+  use LocalAtomics;
+
+  class node {
+    type eltType;
+    var val : eltType;
+    var next : LocalAtomicObject(unmanaged node(eltType));
+
+    proc init(val : ?eltType) {
+      this.eltType = eltType;
+      this.val = val;
+    }
+
+    proc init(type eltType) {
+      this.eltType = eltType;
+      val = nil;
+    }
+  }
+
+  class LockFreeQueue {
+    type objType;
+    var _head : LocalAtomicObject(unmanaged node(objType));
+    var _tail : LocalAtomicObject(unmanaged node(objType));
+
+    proc init(type objType) {
+      this.objType = objType;
+      this.complete();
+      var _node = new unmanaged node(objType);
+      _head.write(_node);
+      _tail.write(_node);
+    }
+
+    proc enqueue(newObj : objType) {
+      var n = new unmanaged node(newObj);
+      while (true) {
+        var curr_tail = _tail.readABA();
+        var next = curr_tail.next.readABA();
+        if (next.getObject() == nil) {
+          if (curr_tail.next.compareExchangeABA(next, n)) {
+            _tail.compareExchangeABA(curr_tail, n);
+            break;
+          }
+        }
+        else {
+          _tail.compareExchangeABA(curr_tail, next.getObject());
+        }
+      }
+    }
+
+    proc dequeue() : objType {
+      while (true) {
+        var curr_head = _head.readABA();
+        var curr_tail = _tail.readABA();
+        var next = curr_head.next.readABA();
+        if (_head.read() == _tail.read()) {
+          if (next.getObject() == nil) then
+            return nil;
+          _tail.compareExchangeABA(curr_tail, next.getObject());
+        }
+        else {
+          if (_head.compareExchangeABA(curr_head, next.getObject())) then
+            return next.getObject().val;
+        }
+      }
+      return nil;
+    }
+
+    iter these() : objType {
+      var ptr = _head.read().next.read();
+      while (ptr != nil) {
+        yield ptr.val;
+        ptr = ptr.next.read();
+      }
+    }
+
+    proc peek() : objType {
+      return _head.read().next.read().val;
+    }
+
+    proc deinit() {
+      var ptr = _head.read();
+      while (ptr != nil) {
+        _head = ptr.next;
+        delete ptr.val;
+        delete ptr;
+        ptr = _head.read();
+      }
+    }
+  }
+}

From a909c5d7cd58f9e16b73b9d4b99bcff71b6609e3 Mon Sep 17 00:00:00 2001
From: Garvit Dewan <d.garvit@gmail.com>
Date: Sat, 8 Jun 2019 20:03:44 +0530
Subject: [PATCH 2/2] Bug fixes

---
 example/LockFreeQueue.chpl | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/example/LockFreeQueue.chpl b/example/LockFreeQueue.chpl
index f7d1de2..cec45c9 100644
--- a/example/LockFreeQueue.chpl
+++ b/example/LockFreeQueue.chpl
@@ -2,7 +2,7 @@
  * Created by Garvit Dewan - 
  * https://github.com/dgarvit/epoch-based-manager/blob/master/src/LockFreeQueue.chpl
  *   
- * Lock-Free Queue that uses ABA feature of Distributed Data Structures
+ * Lock-Free Queue that uses ABA feature of LocalAtomicObject
  * Based on Michael Scott Queue
  */
 module LockFreeQueue {
@@ -21,7 +21,6 @@ module LockFreeQueue {
 
     proc init(type eltType) {
       this.eltType = eltType;
-      val = nil;
     }
   }
 
@@ -60,7 +59,7 @@ module LockFreeQueue {
         var curr_head = _head.readABA();
         var curr_tail = _tail.readABA();
         var next = curr_head.next.readABA();
-        if (_head.read() == _tail.read()) {
+        if (curr_head.getObject() == curr_tail.getObject()) {
           if (next.getObject() == nil) then
             return nil;
           _tail.compareExchangeABA(curr_tail, next.getObject());
@@ -82,7 +81,10 @@ module LockFreeQueue {
     }
 
     proc peek() : objType {
-      return _head.read().next.read().val;
+      var actual_head = _head.read().next.read();
+      if (actual_head != nil) then
+        return actual_head.val;
+      return nil;
     }
 
     proc deinit() {