From f7aa7730059f936e9d3b355156b63a116262cf12 Mon Sep 17 00:00:00 2001
From: Javier Serrano Polo <javier@jasp.net>
Date: Sun, 3 Jul 2016 14:09:41 +0200
Subject: [PATCH] Added LocklessList and use it for new play handles

---
 include/LocklessList.h | 86 ++++++++++++++++++++++++++++++++++++++++++
 include/Mixer.h        |  4 +-
 src/core/Mixer.cpp     | 42 ++++++++++++++-------
 3 files changed, 118 insertions(+), 14 deletions(-)
 create mode 100644 include/LocklessList.h

diff --git a/include/LocklessList.h b/include/LocklessList.h
new file mode 100644
index 00000000000..f97b219be0a
--- /dev/null
+++ b/include/LocklessList.h
@@ -0,0 +1,86 @@
+/*
+ * LocklessList.h - list with lockless push and pop
+ *
+ * Copyright (c) 2016 Javier Serrano Polo <javier@jasp.net>
+ *
+ * This file is part of LMMS - http://lmms.io
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public
+ * License along with this program (see COPYING); if not, write to the
+ * Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301 USA.
+ *
+ */
+
+#ifndef LOCKLESS_LIST_H
+#define LOCKLESS_LIST_H
+
+#include <QAtomicPointer>
+
+template<typename T>
+class LocklessList
+{
+public:
+	struct Element
+	{
+		T value;
+		Element * next;
+	} ;
+
+	void push( T value )
+	{
+		Element * e = new Element;
+		e->value = value;
+
+		do
+		{
+#if QT_VERSION >= 0x050000
+			e->next = m_first.loadAcquire();
+#else
+			e->next = m_first;
+#endif
+		}
+		while( !m_first.testAndSetOrdered( e->next, e ) );
+	}
+
+	Element * popList()
+	{
+		return m_first.fetchAndStoreOrdered( NULL );
+	}
+
+	Element * first()
+	{
+#if QT_VERSION >= 0x050000
+		return m_first.loadAcquire();
+#else
+		return m_first;
+#endif
+	}
+
+	void setFirst( Element * e )
+	{
+#if QT_VERSION >= 0x050000
+		m_first.storeRelease( e );
+#else
+		m_first = e;
+#endif
+	}
+
+
+private:
+	QAtomicPointer<Element> m_first;
+
+} ;
+
+
+#endif
diff --git a/include/Mixer.h b/include/Mixer.h
index f3c849c3ee1..1621457136e 100644
--- a/include/Mixer.h
+++ b/include/Mixer.h
@@ -37,6 +37,7 @@
 
 
 #include "lmms_basics.h"
+#include "LocklessList.h"
 #include "Note.h"
 #include "fifo_buffer.h"
 #include "MixerProfiler.h"
@@ -379,7 +380,8 @@ class EXPORT Mixer : public QObject
 
 	// playhandle stuff
 	PlayHandleList m_playHandles;
-	PlayHandleList m_newPlayHandles;	// place where new playhandles are added temporarily
+	// place where new playhandles are added temporarily
+	LocklessList<PlayHandle *> m_newPlayHandles;
 	ConstPlayHandleList m_playHandlesToRemove;
 
 
diff --git a/src/core/Mixer.cpp b/src/core/Mixer.cpp
index de89d2bd28d..9cbcb2d737d 100644
--- a/src/core/Mixer.cpp
+++ b/src/core/Mixer.cpp
@@ -61,6 +61,9 @@
 
 
 
+typedef LocklessList<PlayHandle *>::Element LocklessListElement;
+
+
 static __thread bool s_renderingThread;
 
 
@@ -412,8 +415,13 @@ const surroundSampleFrame * Mixer::renderNextBuffer()
 	song->processNextBuffer();
 
 	// add all play-handles that have to be added
-	m_playHandles += m_newPlayHandles;
-	m_newPlayHandles.clear();
+	for( LocklessListElement * e = m_newPlayHandles.popList(); e; )
+	{
+		m_playHandles += e->value;
+		LocklessListElement * next = e->next;
+		delete e;
+		e = next;
+	}
 
 	// STAGE 1: run and render all play handles
 	MixerWorkerThread::fillJobQueue<PlayHandleList>( m_playHandles );
@@ -635,10 +643,8 @@ bool Mixer::addPlayHandle( PlayHandle* handle )
 {
 	if( criticalXRuns() == false )
 	{
-		requestChangeInModel();
-			m_newPlayHandles.append( handle );
-			handle->audioPort()->addPlayHandle( handle );
-		doneChangeInModel();
+		m_newPlayHandles.push( handle );
+		handle->audioPort()->addPlayHandle( handle );
 		return true;
 	}
 
@@ -664,16 +670,26 @@ void Mixer::removePlayHandle( PlayHandle * _ph )
 		bool removedFromList = false;
 		// Check m_newPlayHandles first because doing it the other way around
 		// creates a race condition
-		PlayHandleList::Iterator it =
-				qFind( m_newPlayHandles.begin(),
-						m_newPlayHandles.end(), _ph );
-		if( it != m_newPlayHandles.end() )
+		for( LocklessListElement * e = m_newPlayHandles.first(),
+				* ePrev = NULL; e; ePrev = e, e = e->next )
 		{
-			m_newPlayHandles.erase( it );
-			removedFromList = true;
+			if( e->value == _ph )
+			{
+				if( ePrev )
+				{
+					ePrev->next = e->next;
+				}
+				else
+				{
+					m_newPlayHandles.setFirst( e->next );
+				}
+				delete e;
+				removedFromList = true;
+				break;
+			}
 		}
 		// Now check m_playHandles
-		it = qFind( m_playHandles.begin(),
+		PlayHandleList::Iterator it = qFind( m_playHandles.begin(),
 					m_playHandles.end(), _ph );
 		if( it != m_playHandles.end() )
 		{