Subj : Re: Optimization to Jeffrey Richter's COptex (Windows)?
To   : comp.programming.threads
From : Joseph Seigh
Date : Wed Jan 26 2005 08:04 pm

On Wed, 26 Jan 2005 14:47:17 -0800, SenderX <xxx@xxx.com> wrote:

>> http://www.microsoft.com/msj/0198/win320198.aspx
>> http://www.microsoft.com/msj/0198/win32textfigs.htm#fig2
>
> This is crap. Use the mutex logic that Alex suggested.
>
>
The coptex waiting logic should be changed to something like

__inline void CoptexSpin(coptex_t *lock) {
	DWORD SpinCount;

	if (lock->m_lLockCount != 0
		&& lock->m_dwThreadId != GetCurrentThreadId())

	{
		SpinCount = lock->m_dwSpinCount;
		while ((SpinCount-- > 0) && lock->m_lLockCount != 0)
			__asm pause;
	}

	return;
}

void CoptexEnter(coptex_t *lock) {

	CoptexSpin(lock);
	if (InterlockedIncrement(&(lock->m_lLockCount)) == 1) {
		lock->m_dwThreadId = GetCurrentThreadId();
	}

	else if (lock->m_dwThreadId != GetCurrentThreadId()) {
		WaitForSingleObject(lock->hEvent, INFINITE);
		lock->m_dwThreadId = GetCurrentThreadId();
	}

	lock->m_lRecurseCount++;

	return;
}

BOOL CoptexTryEnter(coptex_t *lock) {

	CoptexSpin(lock);
	if (InterlockedCompareExchange(&(lock->m_lLockCount), 1, 0) == 0) {
		lock->m_dwThreadId = GetCurrentThreadId();
		lock->m_lRecurseCount = 1;
		return TRUE;
	}

	else if (lock->m_dwThreadId == GetCurrentThreadId()) {
		lock->m_lRecurseCount++;
		return TRUE;
	}

	return FALSE;
}

I'm assuming GetCurrentThreadId or an equivalent intrinsic is very
efficient.  With an unconditional increment of the lockcount you
end up with the threadid final check after a memory barrier anyway
so paranoids like Alexander should be happy. :)

This is not an adaptive lock so it's not going to scale as well
as one.  One might be better off using other logic.


-- 
Joe Seigh

.