wrsync.cpp

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#include "wrSync.h"

wrSync::wrSync() 
{
	// Initially no readers want access, no writers want access, and 
	// no threads are accessing the resource
	m_nWaitingReaders = m_nWaitingWriters = m_nActive = 0;

	KeInitializeSemaphore(&m_hsemReaders,0,MAXLONG);
	KeInitializeSemaphore(&m_hsemWriters,0,MAXLONG);

	KeInitializeSpinLock(&m_SpinLock);
}

wrSync::~wrSync() 
{
	m_nWaitingReaders = m_nWaitingWriters = m_nActive = 0;
}

void wrSync::WaitToRead() 
{
	// Ensure exclusive access to the member variables
	KIRQL irql;
	KeAcquireSpinLock(&m_SpinLock,&irql);

	// Are there writers waiting or is a writer writing?
	bool fResourceWritePending = (m_nWaitingWriters || (m_nActive < 0));

	if (fResourceWritePending) 
	{
		// This reader must wait, increment the count of waiting readers
		m_nWaitingReaders++;
	} 
	else 
	{
		// This reader can read, increment the count of active readers
		m_nActive++;
	}

	// Allow other threads to attempt reading/writing
	KeReleaseSpinLock(&m_SpinLock,irql);

	if (fResourceWritePending) 
	{
		// This thread must wait
		KeWaitForSingleObject(&m_hsemReaders,Executive,KernelMode,FALSE, NULL);
	}
}

void wrSync::WaitToWrite() 
{
	// Ensure exclusive access to the member variables
	KIRQL irql;
	KeAcquireSpinLock(&m_SpinLock,&irql);

	// Are there any threads accessing the resource?
	bool fResourceOwned = (m_nActive != 0);

	if (fResourceOwned) 
	{
		// This writer must wait, increment the count of waiting writers
		m_nWaitingWriters++;
	} 
	else 
	{
		// This writer can write, decrement the count of active writers
		m_nActive = -1;
	}

	// Allow other threads to attempt reading/writing
	KeReleaseSpinLock(&m_SpinLock,irql);

	if (fResourceOwned) 
	{
		// This thread must wait
		KeWaitForSingleObject(&m_hsemWriters,Executive,KernelMode,FALSE, NULL);
	}
}

void wrSync::Done() 
{
	// Ensure exclusive access to the member variables
	KIRQL irql;
	KeAcquireSpinLock(&m_SpinLock,&irql);

	if (m_nActive > 0) 
	{
		// Readers have control so a reader must be done
		m_nActive--;
	} 
	else 
	{
		// Writers have control so a writer must be done
		m_nActive++;
	}

	PKSEMAPHORE hsem = NULL;  // Assume no threads are waiting
	LONG lCount = 1;     // Assume only 1 waiter wakes; always true for writers

	if (m_nActive == 0) 
	{
		// No thread has access, who should wake up?
		// NOTE: It is possible that readers could never get access
		//       if there are always writers wanting to write

		if (m_nWaitingWriters > 0) 
		{
			// Writers are waiting and they take priority over readers
			m_nActive = -1;         // A writer will get access
			m_nWaitingWriters--;    // One less writer will be waiting
			hsem = &m_hsemWriters;   // Writers wait on this semaphore
			// NOTE: The semaphore will release only 1 writer thread
		} 
		else if (m_nWaitingReaders > 0) 
		{
			// Readers are waiting and no writers are waiting
			m_nActive = m_nWaitingReaders;   // All readers will get access
			m_nWaitingReaders = 0;           // No readers will be waiting
			hsem = &m_hsemReaders;            // Readers wait on this semaphore
			lCount = m_nActive;              // Semaphore releases all readers
		} 
		else 
		{
			// There are no threads waiting at all; no semaphore gets released
		}
	}

	// Allow other threads to attempt reading/writing
	KeReleaseSpinLock(&m_SpinLock,irql);

	if (hsem != NULL) 
	{  
		// Some threads are to be released
		KeReleaseSemaphore(hsem,NULL,lCount,TRUE);
	}
}