From e91a91816496f60694e96dbd705f5b3ca49e33dc Mon Sep 17 00:00:00 2001
From: historic_bruno <ben@wildfiregames.com>
Date: Mon, 3 Dec 2012 13:24:12 +0000
Subject: [PATCH] Fixes race condition in EnsureMonotonic on 32-bit builds,
 which was causing unreliable timer behavior, fixes #1729

This was SVN commit r12927.
---
 source/lib/timer.cpp                          | 30 +++++--------------
 source/tools/atlas/GameInterface/GameLoop.cpp |  6 ++--
 2 files changed, 10 insertions(+), 26 deletions(-)

diff --git a/source/lib/timer.cpp b/source/lib/timer.cpp
index 227d67fdab..7cdbb924dc 100644
--- a/source/lib/timer.cpp
+++ b/source/lib/timer.cpp
@@ -34,6 +34,7 @@
 #include <cstdarg>
 
 #include "lib/module_init.h"
+#include "lib/posix/posix_pthread.h"
 #include "lib/posix/posix_time.h"
 # include "lib/sysdep/cpu.h"
 #if OS_WIN
@@ -81,33 +82,16 @@ void timer_LatchStartTime()
 #endif
 }
 
-
+static pthread_mutex_t ensure_monotonic_mutex = PTHREAD_MUTEX_INITIALIZER;
 // NB: does not guarantee strict monotonicity - callers must avoid
 // dividing by the difference of two equal times.
-// (this avoids having to update maxRepresentation when
-// updating newTime to the (more recent) oldTime)
 static void EnsureMonotonic(double& newTime)
 {
-	i64 newRepresentation;
-	memcpy(&newRepresentation, &newTime, sizeof(newRepresentation));
-
-	// representation of the latest time reported by any thread
-	static volatile i64 maxRepresentation;	// initially 0.0
-
-retry:
-	const i64 oldRepresentation = maxRepresentation;	// latch
-	double oldTime;
-	memcpy(&oldTime, &oldRepresentation, sizeof(oldTime));
-	// a previous or concurrent call got a more recent time -
-	// return that and avoid updating maxRepresentation.
-	if(newTime < oldTime)
-	{
-		newTime = oldTime;
-		return;
-	}
-
-	if(!cpu_CAS64(&maxRepresentation, oldRepresentation, newRepresentation))
-		goto retry;
+	pthread_mutex_lock(&ensure_monotonic_mutex);
+	static double maxTime;
+	maxTime = std::max(maxTime, newTime);
+	newTime = maxTime;
+	pthread_mutex_unlock(&ensure_monotonic_mutex);
 }
 
 
diff --git a/source/tools/atlas/GameInterface/GameLoop.cpp b/source/tools/atlas/GameInterface/GameLoop.cpp
index 484a4d2853..1c4d44f791 100644
--- a/source/tools/atlas/GameInterface/GameLoop.cpp
+++ b/source/tools/atlas/GameInterface/GameLoop.cpp
@@ -142,11 +142,11 @@ static void* RunEngine(void* data)
 
 		// Calculate frame length
 		{
-			double time = timer_Time();
+			const double time = timer_Time();
 			static double last_time = time;
-			// TODO: why is this sometimes negative, if timer_Time ensures monotonic results?
-			float realFrameLength = std::max(0.0f, (float)(time-last_time));
+			const double realFrameLength = time-last_time;
 			last_time = time;
+			ENSURE(realFrameLength >= 0.0);
 			// TODO: filter out big jumps, e.g. when having done a lot of slow
 			// processing in the last frame
 			state.realFrameLength = realFrameLength;