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Date: Thu, 27 Dec 2007 07:38:25 +0000
From: John Bartholomew
Subject: [SDL] SDL Semaphore implementation broken on Windows?
Hi,
Over the past couple of days, I've been battling with SDL, SDL_Mixer and SMPEG to try to find an audio hang bug. I believe I've found the problem, which I think is a race condition inside SDL's semaphore implementation (at least the Windows implementation). The semaphore code uses Windows' built in semaphore functions, but it also maintains a separate count value. This count value is updated with bare increment and decrement operations in SemPost and SemWaitTimeout - no locking primitives to protect them.
In tracking down the apparent audio bug, I found that at some point a semaphore's count value was being decremented to -1, which is clearly not a valid value for it to take.
I'm still not certain exactly what sequence of operations is occuring for this to happen, but I believe that overall it's a race condition between a thread calling SemPost (which increments the count) and the thread on the other end calling SemWait (which decrements it).
I will try to make a test case to verify this, but I'm not sure if I'll be able to (threading errors being difficult to reproduce even in the best circumstances).
However, assuming this is the cause of my problems, there is a very
simple fix:
Windows provides InterlockedIncrement() and InterlockedDecrement()
functions to perform increments and decrements which are guaranteed to be atomic. So the fix is in thread/win32/SDL_syssem.c: replace occurrences of --sem->count with InterlockedDecrement(&sem->count); and replace occurrences of ++sem->count with InterlockedIncrement(&sem->count);
This is using SDL v1.2.12, built with VC++ 2008 Express, running on a
Core 2 duo processor.
author | Sam Lantinga <slouken@libsdl.org> |
---|---|
date | Tue, 17 Feb 2009 05:39:18 +0000 |
parents | f12379c41042 |
children |
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<HTML ><HEAD ><TITLE >Multi-threaded Programming</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+ "><LINK REL="HOME" TITLE="SDL Library Documentation" HREF="index.html"><LINK REL="UP" TITLE="SDL Reference" HREF="reference.html"><LINK REL="PREVIOUS" TITLE="SDL_CDtrack" HREF="sdlcdtrack.html"><LINK REL="NEXT" TITLE="SDL_CreateThread" HREF="sdlcreatethread.html"><META NAME="KEYWORD" CONTENT="threads"><META NAME="KEYWORD" CONTENT="function"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFF8DC" TEXT="#000000" LINK="#0000ee" VLINK="#551a8b" ALINK="#ff0000" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >SDL Library Documentation</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="sdlcdtrack.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="sdlcreatethread.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><H1 ><A NAME="THREAD" ></A >Chapter 12. Multi-threaded Programming</H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT ><A HREF="sdlcreatethread.html" >SDL_CreateThread</A > -- Creates a new thread of execution that shares its parent's properties.</DT ><DT ><A HREF="sdlthreadid.html" >SDL_ThreadID</A > -- Get the 32-bit thread identifier for the current thread.</DT ><DT ><A HREF="sdlgetthreadid.html" >SDL_GetThreadID</A > -- Get the SDL thread ID of a SDL_Thread</DT ><DT ><A HREF="sdlwaitthread.html" >SDL_WaitThread</A > -- Wait for a thread to finish.</DT ><DT ><A HREF="sdlkillthread.html" >SDL_KillThread</A > -- Gracelessly terminates the thread.</DT ><DT ><A HREF="sdlcreatemutex.html" >SDL_CreateMutex</A > -- Create a mutex</DT ><DT ><A HREF="sdldestroymutex.html" >SDL_DestroyMutex</A > -- Destroy a mutex</DT ><DT ><A HREF="sdlmutexp.html" >SDL_mutexP</A > -- Lock a mutex</DT ><DT ><A HREF="sdlmutexv.html" >SDL_mutexV</A > -- Unlock a mutex</DT ><DT ><A HREF="sdlcreatesemaphore.html" >SDL_CreateSemaphore</A > -- Creates a new semaphore and assigns an initial value to it.</DT ><DT ><A HREF="sdldestroysemaphore.html" >SDL_DestroySemaphore</A > -- Destroys a semaphore that was created by <A HREF="sdlcreatesemaphore.html" >SDL_CreateSemaphore</A >.</DT ><DT ><A HREF="sdlsemwait.html" >SDL_SemWait</A > -- Lock a semaphore and suspend the thread if the semaphore value is zero.</DT ><DT ><A HREF="sdlsemtrywait.html" >SDL_SemTryWait</A > -- Attempt to lock a semaphore but don't suspend the thread.</DT ><DT ><A HREF="sdlsemwaittimeout.html" >SDL_SemWaitTimeout</A > -- Lock a semaphore, but only wait up to a specified maximum time.</DT ><DT ><A HREF="sdlsempost.html" >SDL_SemPost</A > -- Unlock a semaphore.</DT ><DT ><A HREF="sdlsemvalue.html" >SDL_SemValue</A > -- Return the current value of a semaphore.</DT ><DT ><A HREF="sdlcreatecond.html" >SDL_CreateCond</A > -- Create a condition variable</DT ><DT ><A HREF="sdldestroycond.html" >SDL_DestroyCond</A > -- Destroy a condition variable</DT ><DT ><A HREF="sdlcondsignal.html" >SDL_CondSignal</A > -- Restart a thread wait on a condition variable</DT ><DT ><A HREF="sdlcondbroadcast.html" >SDL_CondBroadcast</A > -- Restart all threads waiting on a condition variable</DT ><DT ><A HREF="sdlcondwait.html" >SDL_CondWait</A > -- Wait on a condition variable</DT ><DT ><A HREF="sdlcondwaittimeout.html" >SDL_CondWaitTimeout</A > -- Wait on a condition variable, with timeout</DT ></DL ></DIV ><P >SDL provides functions for creating threads, mutexes, semphores and condition variables.</P ><P >In general, you must be very aware of concurrency and data integrity issues when writing multi-threaded programs. Some good guidelines include: <P ></P ><UL ><LI ><P >Don't call SDL video/event functions from separate threads</P ></LI ><LI ><P >Don't use any library functions in separate threads</P ></LI ><LI ><P >Don't perform any memory management in separate threads</P ></LI ><LI ><P >Lock global variables which may be accessed by multiple threads</P ></LI ><LI ><P >Never terminate threads, always set a flag and wait for them to quit</P ></LI ><LI ><P >Think very carefully about all possible ways your code may interact</P ></LI ></UL ></P ><DIV CLASS="NOTE" ><BLOCKQUOTE CLASS="NOTE" ><P ><B >Note: </B >SDL's threading is not implemented on MacOS, due to that lack of preemptive thread support (Mac OS X dos nt suffer from this problem)</P ></BLOCKQUOTE ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="sdlcdtrack.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="sdlcreatethread.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >SDL_CDtrack</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="reference.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >SDL_CreateThread</TD ></TR ></TABLE ></DIV ></BODY ></HTML >