From: SourceForge.net (noreply_at_[hidden])
Date: 2006-08-11 09:32:27
Bugs item #1538697, was opened at 2006-08-11 13:32
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Category: threads
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: David Vest (davevest)
Assigned to: Mike Glassford (glassfordm)
Summary: Possible problem with condition releasing
Initial Comment:
I think this is a bug but since I have only just
started using the threads library it could be a
misunderstanding on my part.
Since the actual application is quite complex I've
created an imaginary scenario that shows the problem.
Scenario: A single 'throttle' manager that only lets
three threads at a time perform some work. When a
thread wants to work it must successfully addRef and
when it has finished it releases.
Problem: If all available 'slots' are used I am using
a condition in the addRef to wait for a release before
continuing. Therefore I would expect that after the
wait I can successfully assert that a slot is
available. However, sometimes this does not appear to
be true and I can only assume that sometimes the
condition is not working properly.
Work around: In CThrottle::release remove the 'unlock'
so that the notify_one occurs while the method still
has a lock on the m_mutexChangeState.
Have I misunderstood how condition is supposed to work?
Thanks,
Dave.
The code:
#include <boost\thread\thread.hpp>
#include <boost\thread\recursive_mutex.hpp>
#include <boost/thread/condition.hpp>
#include <boost/shared_ptr.hpp>
#include <windows.h>
#include <crtdbg.h>
#include <vector>
class CThrottle
{
int m_nThrottle;
public:
CThrottle() : m_nThrottle(0) {}
void addRef()
{
boost::mutex::scoped_lock lockIncrement
( m_mutexIncrement );
boost::recursive_mutex::scoped_lock
lockChangeState( m_mutexChangeState );
if ( !isAllowed() )
m_condition.wait(
lockChangeState );
_ASSERT( isAllowed() ); // PROBLEM
++m_nThrottle;
}
void release()
{
boost::recursive_mutex::scoped_lock
lockChangeState( m_mutexChangeState );
--m_nThrottle;
lockChangeState.unlock();
m_condition.notify_one();
}
bool isAllowed()
{
boost::recursive_mutex::scoped_lock
lockChangeState( m_mutexChangeState );
return m_nThrottle < 3;
}
private:
boost::mutex m_mutexIncrement;
boost::recursive_mutex m_mutexChangeState;
boost::condition m_condition;
};
class CTestThread
{
public:
CTestThread( CThrottle * pThrottle ) :
m_pThrottle( pThrottle ) {}
void operator()()
{
m_pThrottle->addRef();
std::cout << "Do something\n";
m_pThrottle->release();
}
CThrottle * m_pThrottle;
};
typedef boost::shared_ptr<CTestThread> _TestThreadPtr;
int main(int argc, _TCHAR* argv[])
{
CThrottle * pThrottle = new CThrottle();
boost::thread_group groupThreads;
std::vector<_TestThreadPtr> list;
for( int n=0; n < 10000; ++n )
{
_TestThreadPtr ptrTest( new CTestThread
( pThrottle ) );
groupThreads.create_thread(
*ptrTest.get() );
list.push_back( ptrTest );
}
groupThreads.join_all();
delete pThrottle;
return 0;
}
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