This is xnu-8019. See this file in:
/*
 * Copyright (c) 1998-2000 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
 * unlawful or unlicensed copies of an Apple operating system, or to
 * circumvent, violate, or enable the circumvention or violation of, any
 * terms of an Apple operating system software license agreement.
 *
 * Please obtain a copy of the License at
 * http://www.opensource.apple.com/apsl/ and read it before using this file.
 *
 * The Original Code and all software distributed under the License are
 * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
 * Please see the License for the specific language governing rights and
 * limitations under the License.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
 */
/* IOSyncer.cpp created by wgulland on 2000-02-02 */

#include <IOKit/IOLib.h>
#include <IOKit/IOSyncer.h>

OSDefineMetaClassAndStructors(IOSyncer, OSObject)

IOSyncer * IOSyncer::create(bool twoRetains)
{
	IOSyncer * me = new IOSyncer;

	if (me && !me->init(twoRetains)) {
		me->release();
		return NULL;
	}

	return me;
}

bool
IOSyncer::init(bool twoRetains)
{
	if (!OSObject::init()) {
		return false;
	}

	if (!(guardLock = IOSimpleLockAlloc())) {
		return false;
	}

	IOSimpleLockInit(guardLock);

	if (twoRetains) {
		retain();
	}

	fResult = kIOReturnSuccess;

	reinit();

	return true;
}

void
IOSyncer::reinit()
{
	IOInterruptState is = IOSimpleLockLockDisableInterrupt(guardLock);
	threadMustStop = true;
	IOSimpleLockUnlockEnableInterrupt(guardLock, is);
}

void
IOSyncer::free()
{
	// just in case a thread is blocked here:
	privateSignal();

	if (guardLock != NULL) {
		IOSimpleLockFree(guardLock);
	}

	OSObject::free();
}

IOReturn
IOSyncer::wait(bool autoRelease)
{
	IOInterruptState is = IOSimpleLockLockDisableInterrupt(guardLock);

	if (threadMustStop) {
		assert_wait((void *) &threadMustStop, false);
		IOSimpleLockUnlockEnableInterrupt(guardLock, is);
		thread_block(THREAD_CONTINUE_NULL);
	} else {
		IOSimpleLockUnlockEnableInterrupt(guardLock, is);
	}

	IOReturn result = fResult; // Pick up before auto deleting!

	if (autoRelease) {
		release();
	}

	return result;
}

void
IOSyncer::signal(IOReturn res, bool autoRelease)
{
	fResult = res;
	privateSignal();
	if (autoRelease) {
		release();
	}
}

void
IOSyncer::privateSignal()
{
	if (threadMustStop) {
		IOInterruptState is = IOSimpleLockLockDisableInterrupt(guardLock);
		threadMustStop = false;
		thread_wakeup_one((void *) &threadMustStop);
		IOSimpleLockUnlockEnableInterrupt(guardLock, is);
	}
}