This is xnu-11215.1.10. 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@
 */
/* Copyright (c) 1997 Apple Computer, Inc.  All rights reserved.
 * Copyright (c) 1994-1996 NeXT Software, Inc.  All rights reserved.
 *
 * AppleIOPSSafeCondLock.m. Lock object with exported condition variable,
 *	kernel version.
 *
 * HISTORY
 * 1997-11-
 * 01-Aug-91    Doug Mitchell at NeXT
 *      Created.
 */

#include <IOKit/IOConditionLock.h>

#define super OSObject
OSDefineMetaClassAndStructors(IOConditionLock, OSObject)

bool
IOConditionLock::initWithCondition(int inCondition, bool inIntr)
{
	if (!super::init()) {
		return false;
	}

	cond_interlock = IOLockAlloc();
	sleep_interlock = IOLockAlloc();

	condition = inCondition;
	want_lock    = false;
	waiting      = false;
	interruptible = (inIntr) ? THREAD_INTERRUPTIBLE : THREAD_UNINT;

	return cond_interlock && sleep_interlock;
}

IOConditionLock *
IOConditionLock::withCondition(int condition, bool intr)
{
	IOConditionLock *me = new IOConditionLock;

	if (me && !me->initWithCondition(condition, intr)) {
		me->release();
		return NULL;
	}

	return me;
}
void
IOConditionLock::free()
{
	if (cond_interlock) {
		IOLockFree(cond_interlock);
	}
	if (sleep_interlock) {
		IOLockFree(sleep_interlock);
	}
	super::free();
}

bool
IOConditionLock::getInterruptible() const
{
	return interruptible;
}

int
IOConditionLock:: getCondition() const
{
	return condition;
}

int
IOConditionLock:: setCondition(int inCondition)
{
	int old = condition;

	condition = inCondition;
	thread_wakeup_one((void *) &condition);

	return old;
}

void
IOConditionLock::unlock()
{
	IOTakeLock(sleep_interlock);

	thread_wakeup_one((void *) &condition);

	want_lock = false;
	if (waiting) {
		waiting = false;
		IOLockWakeup(sleep_interlock, this, /* one-thread */ false); // Wakeup everybody
	}

	IOUnlock(sleep_interlock);
}

void
IOConditionLock::unlockWith(int inCondition)
{
	IOTakeLock(sleep_interlock);
	IOTakeLock(cond_interlock);

	condition = inCondition;

	IOUnlock(cond_interlock);
	IOUnlock(sleep_interlock);

	unlock();
}

bool
IOConditionLock::tryLock()
{
	bool result;

	IOTakeLock(sleep_interlock);

	result = !want_lock;
	if (result) {
		want_lock = true;
	}

	IOUnlock(sleep_interlock);

	return result;
}

int
IOConditionLock::lock()
{
	int thread_res = THREAD_AWAKENED;

	IOTakeLock(sleep_interlock);

	/* Try to acquire the want_lock bit. */
	while (want_lock && thread_res == THREAD_AWAKENED) {
		waiting = true;
		thread_res = IOLockSleep(sleep_interlock, (void *) this, interruptible);
	}
	if (thread_res == THREAD_AWAKENED) {
		want_lock = true;
	}

	IOUnlock(sleep_interlock);

	return thread_res;
}

int
IOConditionLock::lockWhen(int inCondition)
{
	int thread_res;

	do{
		/* First get the actual lock */
		thread_res = lock();
		if (thread_res != THREAD_AWAKENED) {
			break; // Failed to acquire lock
		}
		if (inCondition == condition) {
			break; // Hold lock and condition is expected value
		}
		/*
		 * Need to hold a IOTakeLock when we call thread_sleep().
		 * Both _cond_interlock and want_lock must be held to
		 * change _condition.
		 */
		IOTakeLock(cond_interlock);
		unlock(); // Release lock and sleep

		/*
		 * this is the critical section on a multi in which
		 * another thread could hold _sleep_interlock, but they
		 * can't change _condition. Holding _cond_interlock here
		 * (until after assert_wait() is called from
		 * thread_sleep()) ensures that we'll be notified
		 * of changes in _condition.
		 */
		assert_wait((void *) &condition, interruptible); /* assert event */
		IOUnlock(cond_interlock);                /* release the lock */
		thread_res = thread_block(THREAD_CONTINUE_NULL); /* block ourselves */
	} while (thread_res == THREAD_AWAKENED);

	return thread_res;
}