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."
This change has been done to make it easier for OpenSceneGraph users to check out the svn via https
without any conflicts introduced with a http externals.
Robert Osfield