Added code.

.gitignore

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+/msvc/.vs/
+*.exe
+*.pdb
+*.recipe
+*.ilk
+*.log
+*.tlog
+*.o
+*.lastbuildstate
+*.obj
+*.idb
+*.user

README.txt

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+This is a simple multiple-reader, single-writer mutex class for C++14. Reader
+locks can be upgraded and then downgraded without being released. It uses a
+queue (allocated on the stack) to prevent writer starvation and to resolve lock
+acquisition conflicts. Writer lock acquisition requests are always fulfilled in
+FIFO order.
+
+
+Usage examples:
+
+helios::rwmutex m;
+
+void reader(){
+    helios::rwlock l(m);
+    //reader code
+}
+
+void conditional_writer(){
+    helios::rwlock l(m);
+    //reader code
+    if (/*condition*/)
+        return;
+    l.upgrade();
+    //note: calling l.upgrade() again would throw an exception
+    //writer code
+}
+
+void nested_writer(){
+    helios::rwlock l(m);
+    //reader code
+    if (/*condition*/)
+        return;
+    {
+        helios::rwlock_writer_region r(l);
+        //writer code
+    }
+    //some more reader code
+}
+
+void unconditional_writer(){
+    helios::wlock l(m);
+    //writer code
+}

build_test.sh

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+#!/bin/sh
+c++ src/rwmutex.cpp test/test_concurrency.cpp -o test_concurrency

msvc/rwmutex_test.sln

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msvc/rwmutex_test/rwmutex_test.vcxproj

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msvc/rwmutex_test/rwmutex_test.vcxproj.filters

@@ -0,0 +1,30 @@
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src/rwmutex.cpp

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+#include "rwmutex.hpp"
+#include <stdexcept>
+
+namespace helios{
+
+rwlock::rwlock(rwmutex &m): m(m){
+	node.id = std::this_thread::get_id();
+	m.lock_reader(node);
+	state = 1;
+}
+
+rwlock::~rwlock(){
+	if (state == 1)
+		m.unlock_reader(node);
+	else
+		m.unlock_writer(node);
+}
+
+void rwlock::upgrade(){
+	if (state != 1)
+		throw std::runtime_error("incorrect use");
+	m.upgrade();
+	state = 2;
+}
+
+void rwlock::downgrade(){
+	if (state != 2)
+		throw std::runtime_error("incorrect use");
+	m.upgrade();
+	state = 1;
+}
+
+void rwmutex::lock_reader(rwlock_node &node){
+	UL lg(m);
+	push(node);
+	while (state == 2)
+		cv.wait(lg);
+	state = 1;
+}
+
+void rwmutex::unlock_reader(rwlock_node &node){
+	UL lg(m);
+	remove(node);
+	if (!queue_size)
+		state = 0;
+	cv.notify_all();
+}
+
+void rwmutex::upgrade(){
+	auto id = std::this_thread::get_id();
+	UL lg(m);
+	writers_waiting++;
+	cv.notify_all();
+	while (queue_head->id != id)
+		cv.wait(lg);
+	while (queue_size > writers_waiting && state != 0)
+		cv.wait(lg);
+	writers_waiting--;
+	state = 2;
+}
+
+void rwmutex::downgrade(){
+	UL lg(m);
+	state = 1;
+	cv.notify_all();
+}
+
+void rwmutex::lock_writer(rwlock_node &node){
+	auto id = std::this_thread::get_id();
+	UL lg(m);
+	push(node);
+	writers_waiting++;
+	cv.notify_all();
+	while (queue_head->id != id)
+		cv.wait(lg);
+	while (queue_size > writers_waiting && state != 0)
+		cv.wait(lg);
+	writers_waiting--;
+	state = 2;
+}
+
+void rwmutex::unlock_writer(rwlock_node &node){
+	UL lg(m);
+	remove(node);
+	state = !!queue_size;
+	cv.notify_all();
+}
+
+void rwmutex::push(rwlock_node &node){
+	queue_size++;
+	node.next = nullptr;
+	if (queue_size == 1){
+		queue_head = &node;
+		queue_tail = &node;
+		node.previous = nullptr;
+		return;
+	}
+	queue_tail->next = &node;
+	node.previous = queue_tail;
+	queue_tail = &node;
+}
+
+void rwmutex::remove(rwlock_node &node){
+	queue_size--;
+	if (node.previous)
+		node.previous->next = node.next;
+	else
+		queue_head = node.next;
+	if (node.next)
+		node.next->previous = node.previous;
+	else
+		queue_tail = node.previous;
+}
+
+}