boost::condition question

22 Feb 2002

      Hi,

I'm trying to create a mechanism that synchronizes access to
shared 
data on a per-operation basis. The idea is that some operations (E.g. 
read operations) can proceed concurrently, while others (write 
operations) must block all other threads. I thought boost::condition 
would do the trick. Since I do not have that much experience in 
multithreaded programming it is entirely possible that I do not fully 
understand boost::condition or its allowable usage.

I tried to put operations in an "Operation Group". All
operations 
belonging to the same group are allowed to execute concurrently, 
while all others are blocked (E.g. all read operations are in the 
same group). It is also possible that an operation doesn't
specify an 
Operation Group, in which case a temporary group is assigned to the 
operation for each thread that calls it. Write operations are 
constructed this way.

Below is an example of what I mean (OperationLevelLockable is defined 
below):

class SomeClass : private OperationLevelLockable
{
public:
int ReadData()
{
	Lock	lock( this,  READ_OP_GROUP );

	return ( mData );
}

void WriteData( int data )
{
	Lock	lock( this );	// assign temp group to operation,
                                // so each write
 				// operation blocks all other accesses
	mData = data;
}

private:
	int	mData;
};

I'm developing on Windows 2000 on a multiprocessor machine.

This is the code I use to test my idea:

const int32 OPERATION_GROUP_1 = 1;
const int32 OPERATION_GROUP_2 = 2;
const int NUM_THREADS = 4;

class TestOperationLevelLockable : public MTTestCase
{
public:
	TestOperationLevelLockable() :
		mShared( this ), mCurrentGroup( 0 ),
                mThreadCount( 0 )
	{
		// Create two sets of threads, each performing a
                // different operation
		// When the TestOperationLevelLockable object is
                // constructed all
 		// registered threads are created an start running
		RegisterThread( this, 
&TestOperationLevelLockable::Operation1Thread, NUM_THREADS );
		RegisterThread( this, 
&TestOperationLevelLockable::Operation2Thread, NUM_THREADS );
	}

	void CallFromGroup( int32 opGroup )
	{
		boost::mutex::scoped_lock	lock( mMutex );

		if ( 0 == mCurrentGroup )
		{
			mCurrentGroup = opGroup;
		}

		if ( ( mCurrentGroup != opGroup ) &&
                     ( mThreadCount < NUM_THREADS ) )
		{
			ReportFailure( "OperationLevelLockable failed 
to lock out operations of different group", __FILE__, __LINE__ );
		}

		++mThreadCount;
	}

protected:
	class MTObject : public OperationLevelLockable
	{
	public:
		MTObject( TestOperationLevelLockable* target ) :
			mTarget( target ) {}

		void CallFromGroup( int32 opGroup )
		{
			mTarget->CallFromGroup( opGroup );
		}
	private:
		TestOperationLevelLockable* mTarget;
	};

	MTObject			mShared;
	int32				mCurrentGroup;
	int				mThreadCount;
	boost::mutex	mMutex;

	void DoMTTest()
	{
	}

	void Operation1Thread()
	{
		MTObject::Lock		lock( mShared,
                                              OPERATION_GROUP_1 );

		mShared.CallFromGroup( OPERATION_GROUP_1 );

		SleepThread( 30 ); // seconds, to keep the thread busy
	}

	void Operation2Thread()
	{
		MTObject::Lock		lock( mShared,
                                              OPERATION_GROUP_2 );

		mShared.CallFromGroup( OPERATION_GROUP_2 );

		SleepThread( 30 ); // seconds, to keep the thread busy
	}
};

And here is OperationLevelLockable:

class OperationLevelLockable
{
	class AccessCounter;
public:
	OperationLevelLockable(){}

	typedef	Internal::DefaultTMIntType			
			IntType;

	template<typename T>
	struct MakeSharedType
	{
		typedef	T volatile				
						Type;
	};

	class Lock;
	friend class Lock;

	class Lock
	{
	public:
		Lock( OperationLevelLockable& obj ) :
			mCounter( obj.mOpAccessCounter )
		{
		   mCounter.EnterOperation( Lock::TempOpGroup() );
		}

		Lock( OperationLevelLockable& obj, int32 opGroup ) :
			mCounter( obj.mOpAccessCounter )
		{
		   mCounter.EnterOperation( opGroup );
		}

		~Lock()
		{
			mCounter.LeaveOperation();
		}

	private:
		Lock( Lock const& );
		Lock& operator=( Lock const& );

		static IntType TempOpGroup()
		{
		   static volatile IntType opGroup =
                                      Internal::kFIRST_AUTO_GROUP;

		   IntType newGroup = 
OperationLevelLockable::AtomicIncrement( opGroup );

		   if ( newGroup >= Internal::kLAST_AUTO_GROUP )
		   {
		     OperationLevelLockable::AtomicAssign( opGroup, 
Internal::kFIRST_AUTO_GROUP );
		   }

		   return ( newGroup );
		}

		AccessCounter&	mCounter;
	};

static inline IntType AtomicIncrement( volatile IntType& lval );
static inline IntType AtomicDecrement( volatile IntType& lval );
static inline void AtomicAssign( IntType& lval, IntType rVal );
static inline void AtomicAssign( volatile IntType& lval, IntType 
rVal );

private:

	class AccessCounter :
		private 
boost::base_from_member<boost::recursive_mutex>
	{
	public:

		AccessCounter() :
			mLock( member ), mRefCnt( 0 ),
			mOpGroup( Internal::kANY_GROUP )  {}

		bool IsOpGroupActive( IntType opGroup ) const
		{
		   return ( ( Internal::kANY_GROUP == mOpGroup ) ||
                            ( opGroup == mOpGroup ) );
		}

		void LeaveOperation()
		{
			if ( 0 == 
OperationLevelLockable::AtomicDecrement( mRefCnt ) )
			{
			   SetOpGroup( Internal::kANY_GROUP );
			   mCanOpProceed.notify_all();
			}
		}

		void EnterOperation( IntType opGroup )
		{
		   mCanOpProceed.wait( mLock,
                                       CanOpProceed( *this,
                                                     opGroup ) );
		   SetOpGroup( opGroup );
		   OperationLevelLockable::AtomicIncrement( mRefCnt );
		}

	private:

		AccessCounter( AccessCounter const& );
		AccessCounter& operator=( AccessCounter const& );

		struct CanOpProceed
		{
			CanOpProceed( AccessCounter const& counter,
                                      IntType opGroup ) :
				mOpGroup( opGroup ),
                                mCounter( counter ) {}

			bool operator()() const
			{
			  return ( 
                             mCounter.IsOpGroupActive( mOpGroup )
                          );
			}

		private:		
			CanOpProceed& operator=( CanOpProceed 
const& );

		int32 mOpGroup;
		AccessCounter const& mCounter;
		};

		void SetOpGroup( IntType opGroup )
		{
		 OperationLevelLockable::AtomicAssign( mOpGroup,
                                                       opGroup );
		}

	boost::recursive_mutex::scoped_lock  mLock;
	boost::condition		     mCanOpProceed;
	volatile IntType                     mRefCnt;
	volatile IntType                     mOpGroup;
	};

	OperationLevelLockable( OperationLevelLockable const& );
	OperationLevelLockable& operator=( OperationLevelLockable 
const& );

	AccessCounter  mOpAccessCounter;
};

The idea is that the first operation that is executed acquires the 
lock and blocks out all threads not belonging to the same Operation 
Group. The original blocking works fine. Only threads executing in 
the same Operation Group are allowed to continue. When the last 
thread of the group leaves the operation in 
OperationLevelLockable::AccessCounter::LeaveOperation it releases all 
waiting threads and a thread from the other group is allowed to run. 
However when this thread finishes no new threads are allowed to run 
anymore and the application hangs.

Am I using boost::condition incorrectly. Should I use something else? 
It the test code wrong? Are there any other errors in the code that 
prevent it from behaving as expected. Should I use another technique 
to achieve the same?

I hope there is someone out there who has the hart to read through 
the entire posting and help me. I hope I have given enough 
information.

Thx.

Sven Van Echelpoel.

boost::condition question

svanechel