I've been looking at the discussion from 2006 ("[osg-users]
osgDB/Reentrant Mutex not threadsafe ?") about this, and having looked
closely at OpenThreads::ReentrantMutex it's still not thread safe in the
following situation:
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.
N.B. The most robust solution would be to simply not use ReentrantMutex!
On Windows ReentrantMutex is unnecessary as critical sections already
allow recursion. On pthreads we need only call
pthread_mutexattr_settype(&mutex_attr,PTHREAD_MUTEX_RECURSIVE) when
initialising a mutex to make it allow recursion. However I don't know
about sproc.______________________________________________________________________ This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
ReentrantMutex
Description: ReentrantMutex
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