From 1786d9973558c0538ecb3f32cd695613c789c658 Mon Sep 17 00:00:00 2001
From: Glenn Maynard <gmaynard@users.sourceforge.net>
Date: Tue, 19 Dec 2006 08:00:45 +0000
Subject: [PATCH] GetPositionSeconds(&tm) returns the clock time timestamp
 associated with the returned sound time.  This is tricky: we may be
 interrupted, causing timing error.

We tried working around this by trying to hint the scheduler that we're in a
period where we don't want to be interrupted, even though we're in a thread
that's not normally high priority, by boosting the priority temporarily.  This
worked in Windows, but not in general; it's too far out of the expectations of
schedulers and generally just made things worse.

Let's look at this like an interruption-based lockless algorithm: try it, see
if it succeeded, and if it failed, try again.

Retrying even once should be a rare exception, but failsafe anyway, so a bug
in a sound driver won't hang.
---
 stepmania/src/RageSound.cpp | 39 ++++++++++++++++++++++++++++---------
 1 file changed, 30 insertions(+), 9 deletions(-)

diff --git a/stepmania/src/RageSound.cpp b/stepmania/src/RageSound.cpp
index de2d5a5283..51e9e7d66a 100644
--- a/stepmania/src/RageSound.cpp
+++ b/stepmania/src/RageSound.cpp
@@ -26,7 +26,6 @@
 #include "RageUtil.h"
 #include "RageLog.h"
 #include "PrefsManager.h"
-#include "arch/ArchHooks/ArchHooks.h"
 #include "RageSoundUtil.h"
 
 #include "RageSoundReader_Pan.h"
@@ -656,18 +655,40 @@ float RageSound::GetPositionSeconds( bool *bApproximate, RageTimer *pTimestamp )
 {
 	LockMut( m_Mutex );
 
-	if( pTimestamp )
+	if( pTimestamp == NULL )
+	{
+		const int64_t iPositionFrames = GetPositionSecondsInternal( bApproximate );
+		return iPositionFrames / float(samplerate());
+	}
+	
+	/*
+	 * We may have unpredictable scheduling delays between updating the timestamp
+	 * and reading the sound position.  If we're preempted while doing this and
+	 * it may have caused the timestamp to not match the returned time, retry.
+	 *
+	 * As a failsafe, only allow a few attempts.  If this has to try more than
+	 * a few times, then probably we have thread contention that's causing more
+	 * severe performance problems, anyway.
+	 */
+	int iTries = 3;
+	int64_t iPositionFrames;
+	do
 	{
-		HOOKS->EnterTimeCriticalSection();
 		pTimestamp->Touch();
+		iPositionFrames = GetPositionSecondsInternal( bApproximate );
+	} while( --iTries && pTimestamp->Ago() > 0.002f );
+
+	if( iTries == 0 )
+	{
+		static bool bLogged = false;
+		if( !bLogged )
+		{
+			bLogged = true;
+			LOG->Warn( "RageSound::GetPositionSeconds: too many tries" );
+		}
 	}
 
-	const int64_t iPositionFrames = GetPositionSecondsInternal( bApproximate );
-	const float fPosition = iPositionFrames / float(samplerate());
-	if( pTimestamp )
-		HOOKS->ExitTimeCriticalSection();
-
-	return fPosition;
+	return iPositionFrames / float(samplerate());
 }