5a537261a6
In my example case, there are 2 threads - one is a worker thread created by OpenThreads::Thread. The other thread is the main thread i.e. the thread that is intrinsically created when you execute the application. The crucial problem is that for the main thread, OpenThreads::Thread::CurrentThread() will return null.
I'll demonstrate this by breaking ReentrantMutex::lock() into sub-statements:
1.) if (_threadHoldingMutex==OpenThreads::Thread::CurrentThread())
2.) if (_lockCount>0){
3.)
OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_lockCountMutex);
++_lockCount;
return 0;
4.)
int result = Mutex::lock();
if (result==0)
{
OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_lockCountMutex);
5.)
_threadHoldingMutex = OpenThreads::Thread::CurrentThread();
_lockCount = 1;
return result;
An error will occur in the following case:
1) The worker thread calls lock(), it gets to the start of statement 5.
2) The main thread calls lock(). Statement 1 is evaluated as true as _threadHoldingMutex is null, and OpenThreads::Thread::CurrentThread() returns null.
3) The worker thread executes statement 5.
4) The main thread executes statement 2 and evaluates it as true, because the worker thread has set _lockCount to 1. The main thread executes statement 3, and now can access the mutexed-data at the same time as the worker thread!
The simple solution to this is to always protect access to _lockCount and _threadHoldingMutex using _lockCountMutex. I have done this in the file I am submitting."
126 lines
3.5 KiB
C++
126 lines
3.5 KiB
C++
/* -*-c++-*- OpenThreads - Copyright (C) 1998-2007 Robert Osfield
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*
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* This library is open source and may be redistributed and/or modified under
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* the terms of the OpenSceneGraph Public License (OSGPL) version 0.0 or
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* (at your option) any later version. The full license is in LICENSE file
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* included with this distribution, and on the openscenegraph.org website.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* OpenSceneGraph Public License for more details.
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*/
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#ifndef _OPENTHREADS_REENTRANTMUTEX_
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#define _OPENTHREADS_REENTRANTMUTEX_
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#include <OpenThreads/Thread>
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#include <OpenThreads/Mutex>
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#include <OpenThreads/ScopedLock>
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namespace OpenThreads {
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class ReentrantMutex : public OpenThreads::Mutex
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{
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public:
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ReentrantMutex():
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_threadHoldingMutex(0),
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_lockCount(0) {}
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virtual ~ReentrantMutex() {}
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virtual int lock()
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{
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{
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OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_lockCountMutex);
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if (_threadHoldingMutex==OpenThreads::Thread::CurrentThread() && _lockCount>0)
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{
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++_lockCount;
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return 0;
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}
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}
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int result = Mutex::lock();
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if (result==0)
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{
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OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_lockCountMutex);
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_threadHoldingMutex = OpenThreads::Thread::CurrentThread();
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_lockCount = 1;
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}
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return result;
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}
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virtual int unlock()
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{
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OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_lockCountMutex);
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#if 0
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if (_threadHoldingMutex==OpenThreads::Thread::CurrentThread() && _lockCount>0)
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{
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--_lockCount;
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if (_lockCount<=0)
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{
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_threadHoldingMutex = 0;
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return Mutex::unlock();
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}
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}
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else
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{
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osg::notify(osg::NOTICE)<<"Error: ReentrantMutex::unlock() - unlocking from the wrong thread."<<std::endl;
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}
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#else
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if (_lockCount>0)
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{
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--_lockCount;
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if (_lockCount<=0)
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{
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_threadHoldingMutex = 0;
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return Mutex::unlock();
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}
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}
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#endif
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return 0;
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}
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virtual int trylock()
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{
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{
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OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_lockCountMutex);
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if (_threadHoldingMutex==OpenThreads::Thread::CurrentThread() && _lockCount>0)
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{
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++_lockCount;
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return 0;
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}
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}
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int result = Mutex::trylock();
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if (result==0)
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{
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OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_lockCountMutex);
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_threadHoldingMutex = OpenThreads::Thread::CurrentThread();
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_lockCount = 1;
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}
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return result;
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}
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private:
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ReentrantMutex(const ReentrantMutex&):OpenThreads::Mutex() {}
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ReentrantMutex& operator =(const ReentrantMutex&) { return *(this); }
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OpenThreads::Thread* _threadHoldingMutex;
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OpenThreads::Mutex _lockCountMutex;
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unsigned int _lockCount;
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};
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}
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#endif
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