task.c

/*
 * Copyright (C) 1998-2000  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE CONSORTIUM
 * DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL
 * INTERNET SOFTWARE CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING
 * FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
 * NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION
 * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

/* $Id: task.c,v 1.74 2000/08/30 23:47:14 bwelling Exp $ */

/*
 * Principal Author: Bob Halley
 */

/*
 * XXXRTH  Need to document the states a task can be in, and the rules
 * for changing states.
 */

#include <config.h>

#include <isc/condition.h>
#include <isc/event.h>
#include <isc/mem.h>
#include <isc/platform.h>
#include <isc/string.h>
#include <isc/task.h>
#include <isc/thread.h>
#include <isc/util.h>

#ifndef ISC_PLATFORM_USETHREADS
#include "task_p.h"
#endif

#define ISC_TASK_NAMES 1

#ifdef ISC_TASK_TRACE
#define XTRACE(m)		fprintf(stderr, "task %p thread %lu: %s\n", \
				       task, isc_thread_self(), (m))
#define XTTRACE(t, m)		fprintf(stderr, "task %p thread %lu: %s\n", \
				       (t), isc_thread_self(), (m))
#define XTHREADTRACE(m)		fprintf(stderr, "thread %lu: %s\n", \
				       isc_thread_self(), (m))
#else
#define XTRACE(m)
#define XTTRACE(t, m)
#define XTHREADTRACE(m)
#endif

/***
 *** Types.
 ***/

typedef enum {
	task_state_idle, task_state_ready, task_state_running,
	task_state_done
} task_state_t;

#define TASK_MAGIC			0x5441534BU	/* TASK. */
#define VALID_TASK(t)			((t) != NULL && \
					 (t)->magic == TASK_MAGIC)

struct isc_task {
	/* Not locked. */
	unsigned int			magic;
	isc_taskmgr_t *			manager;
	isc_mutex_t			lock;
	/* Locked by task lock. */
	task_state_t			state;
	unsigned int			references;
	isc_eventlist_t			events;
	isc_eventlist_t			on_shutdown;
	unsigned int			quantum;
	unsigned int			flags;
#ifdef ISC_TASK_NAMES
	char				name[16];
	void *				tag;
#endif
	/* Locked by task manager lock. */
	LINK(isc_task_t)		link;
	LINK(isc_task_t)		ready_link;
};

#define TASK_F_SHUTTINGDOWN		0x01

#define TASK_SHUTTINGDOWN(t)		(((t)->flags & TASK_F_SHUTTINGDOWN) \
					 != 0)

#define TASK_MANAGER_MAGIC		0x54534B4DU	/* TSKM. */
#define VALID_MANAGER(m)		((m) != NULL && \
					 (m)->magic == TASK_MANAGER_MAGIC)

struct isc_taskmgr {
	/* Not locked. */
	unsigned int			magic;
	isc_mem_t *			mctx;
	isc_mutex_t			lock;
	unsigned int			workers;
#ifdef ISC_PLATFORM_USETHREADS
	isc_thread_t *			threads;
#endif
	/* Locked by task manager lock. */
	unsigned int			default_quantum;
	LIST(isc_task_t)		tasks;
	LIST(isc_task_t)		ready_tasks;
#ifdef ISC_PLATFORM_USETHREADS
	isc_condition_t			work_available;
#endif
	isc_boolean_t			exiting;
#ifndef ISC_PLATFORM_USETHREADS
	unsigned int			refs;
#endif
};

#define DEFAULT_TASKMGR_QUANTUM		10
#define DEFAULT_DEFAULT_QUANTUM		5
#define FINISHED(m)			((m)->exiting && EMPTY((m)->tasks))

#ifndef ISC_PLATFORM_USETHREADS
static isc_taskmgr_t *taskmgr = NULL;
#endif

/***
 *** Tasks.
 ***/

static void
task_finished(isc_task_t *task) {
	isc_taskmgr_t *manager = task->manager;

	REQUIRE(EMPTY(task->events));
	REQUIRE(EMPTY(task->on_shutdown));
	REQUIRE(task->references == 0);
	REQUIRE(task->state == task_state_done);

	XTRACE("task_finished");

	LOCK(&manager->lock);
	UNLINK(manager->tasks, task, link);
#ifdef ISC_PLATFORM_USETHREADS
	if (FINISHED(manager)) {
		/*
		 * All tasks have completed and the
		 * task manager is exiting.  Wake up
		 * any idle worker threads so they
		 * can exit.
		 */
		BROADCAST(&manager->work_available);
	}
#endif
	UNLOCK(&manager->lock);

	DESTROYLOCK(&task->lock);
	task->magic = 0;
	isc_mem_put(manager->mctx, task, sizeof *task);
}

isc_result_t
isc_task_create(isc_taskmgr_t *manager, unsigned int quantum,
		isc_task_t **taskp)
{
	isc_task_t *task;
	isc_boolean_t exiting;

	REQUIRE(VALID_MANAGER(manager));
	REQUIRE(taskp != NULL && *taskp == NULL);

	task = isc_mem_get(manager->mctx, sizeof *task);
	if (task == NULL)
		return (ISC_R_NOMEMORY);
	XTRACE("create");
	task->manager = manager;
	if (isc_mutex_init(&task->lock) != ISC_R_SUCCESS) {
		isc_mem_put(manager->mctx, task, sizeof *task);
		UNEXPECTED_ERROR(__FILE__, __LINE__,
				 "isc_mutex_init() failed");
		return (ISC_R_UNEXPECTED);
	}
	task->state = task_state_idle;
	task->references = 1;
	INIT_LIST(task->events);
	INIT_LIST(task->on_shutdown);
	task->quantum = quantum;
	task->flags = 0;
#ifdef ISC_TASK_NAMES
	memset(task->name, 0, sizeof task->name);
	task->tag = NULL;
#endif
	INIT_LINK(task, link);
	INIT_LINK(task, ready_link);

	exiting = ISC_FALSE;
	LOCK(&manager->lock);
	if (!manager->exiting) {
		if (task->quantum == 0)
			task->quantum = manager->default_quantum;
		APPEND(manager->tasks, task, link);
	} else
		exiting = ISC_TRUE;
	UNLOCK(&manager->lock);

	if (exiting) {
		DESTROYLOCK(&task->lock);
		isc_mem_put(manager->mctx, task, sizeof *task);
		return (ISC_R_SHUTTINGDOWN);
	}

	task->magic = TASK_MAGIC;
	*taskp = task;

	return (ISC_R_SUCCESS);
}

void
isc_task_attach(isc_task_t *source, isc_task_t **targetp) {

	/*
	 * Attach *targetp to source.
	 */

	REQUIRE(VALID_TASK(source));
	REQUIRE(targetp != NULL && *targetp == NULL);

	XTTRACE(source, "attach");

	LOCK(&source->lock);
	source->references++;
	UNLOCK(&source->lock);

	*targetp = source;
}

static inline isc_boolean_t
task_shutdown(isc_task_t *task) {
	isc_boolean_t was_idle = ISC_FALSE;
	isc_event_t *event, *prev;

	/*
	 * Caller must be holding the task's lock.
	 */

	XTRACE("task_shutdown");

	if (! TASK_SHUTTINGDOWN(task)) {
		XTRACE("shutting down");
		task->flags |= TASK_F_SHUTTINGDOWN;
		if (task->state == task_state_idle) {
			INSIST(EMPTY(task->events));
			task->state = task_state_ready;
			was_idle = ISC_TRUE;
		}
		INSIST(task->state == task_state_ready ||
		       task->state == task_state_running);
		/*
		 * Note that we post shutdown events LIFO.
		 */
		for (event = TAIL(task->on_shutdown);
		     event != NULL;
		     event = prev) {
			prev = PREV(event, ev_link);
			DEQUEUE(task->on_shutdown, event, ev_link);
			ENQUEUE(task->events, event, ev_link);
		}
	}

	return (was_idle);
}

static inline void
task_ready(isc_task_t *task) {
	isc_taskmgr_t *manager = task->manager;

	REQUIRE(VALID_MANAGER(manager));
	REQUIRE(task->state == task_state_ready);

	XTRACE("task_ready");

	LOCK(&manager->lock);

	ENQUEUE(manager->ready_tasks, task, ready_link);
#ifdef ISC_PLATFORM_USETHREADS
	SIGNAL(&manager->work_available);
#endif

	UNLOCK(&manager->lock);
}

static inline isc_boolean_t
task_detach(isc_task_t *task) {

	/*
	 * Caller must be holding the task lock.
	 */

	REQUIRE(task->references > 0);

	XTRACE("detach");

	task->references--;
	if (task->references == 0 && task->state == task_state_idle) {
		INSIST(EMPTY(task->events));
		/*
		 * There are no references to this task, and no
		 * pending events.  We could try to optimize and
		 * either initiate shutdown or clean up the task,
		 * depending on its state, but it's easier to just
		 * make the task ready and allow run() or the event
		 * loop to deal with shutting down and termination.
		 */
		task->state = task_state_ready;
		return (ISC_TRUE);
	}

	return (ISC_FALSE);
}

void
isc_task_detach(isc_task_t **taskp) {
	isc_task_t *task;
	isc_boolean_t was_idle;

	/*
	 * Detach *taskp from its task.
	 */

	REQUIRE(taskp != NULL);
	task = *taskp;
	REQUIRE(VALID_TASK(task));

	XTRACE("isc_task_detach");

	LOCK(&task->lock);
	was_idle = task_detach(task);
	UNLOCK(&task->lock);

	if (was_idle)
		task_ready(task);

	*taskp = NULL;
}

static inline isc_boolean_t
task_send(isc_task_t *task, isc_event_t **eventp) {
	isc_boolean_t was_idle = ISC_FALSE;
	isc_event_t *event;

	/*
	 * Caller must be holding the task lock.
	 */

	REQUIRE(eventp != NULL);
	event = *eventp;
	REQUIRE(event != NULL);
	REQUIRE(event->ev_type > 0);
	REQUIRE(task->state != task_state_done);

	XTRACE("task_send");

	if (task->state == task_state_idle) {
		was_idle = ISC_TRUE;
		INSIST(EMPTY(task->events));
		task->state = task_state_ready;
	}
	INSIST(task->state == task_state_ready ||
	       task->state == task_state_running);
	ENQUEUE(task->events, event, ev_link);
	*eventp = NULL;

	return (was_idle);
}

void
isc_task_send(isc_task_t *task, isc_event_t **eventp) {
	isc_boolean_t was_idle;

	/*
	 * Send '*event' to 'task'.
	 */

	REQUIRE(VALID_TASK(task));

	XTRACE("isc_task_send");

	/*
	 * We're trying hard to hold locks for as short a time as possible.
	 * We're also trying to hold as few locks as possible.  This is why
	 * some processing is deferred until after the lock is released.
	 */
	LOCK(&task->lock);
	was_idle = task_send(task, eventp);
	UNLOCK(&task->lock);

	if (was_idle) {
		/*
		 * We need to add this task to the ready queue.
		 *
		 * We've waited until now to do it because making a task
		 * ready requires locking the manager.  If we tried to do
		 * this while holding the task lock, we could deadlock.
		 *
		 * We've changed the state to ready, so no one else will
		 * be trying to add this task to the ready queue.  The
		 * only way to leave the ready state is by executing the
		 * task.  It thus doesn't matter if events are added,
		 * removed, or a shutdown is started in the interval
		 * between the time we released the task lock, and the time
		 * we add the task to the ready queue.
		 */
		task_ready(task);
	}
}

void
isc_task_sendanddetach(isc_task_t **taskp, isc_event_t **eventp) {
	isc_boolean_t idle1, idle2;
	isc_task_t *task;

	/*
	 * Send '*event' to '*taskp' and then detach '*taskp' from its
	 * task.
	 */

	REQUIRE(taskp != NULL);
	task = *taskp;
	REQUIRE(VALID_TASK(task));

	XTRACE("isc_task_sendanddetach");

	LOCK(&task->lock);
	idle1 = task_send(task, eventp);
	idle2 = task_detach(task);
	UNLOCK(&task->lock);

	/*
	 * If idle1, then idle2 shouldn't be true as well since we're holding
	 * the task lock, and thus the task cannot switch from ready back to
	 * idle.
	 */
	INSIST(!(idle1 && idle2));

	if (idle1 || idle2)
		task_ready(task);

	*taskp = NULL;
}

#define PURGE_OK(event)	(((event)->ev_attributes & ISC_EVENTATTR_NOPURGE) == 0)

static unsigned int
dequeue_events(isc_task_t *task, void *sender, isc_eventtype_t first,
	       isc_eventtype_t last, void *tag,
	       isc_eventlist_t *events, isc_boolean_t purging)
{
	isc_event_t *event, *next_event;
	unsigned int count = 0;

	REQUIRE(VALID_TASK(task));
	REQUIRE(last >= first);

	XTRACE("dequeue_events");

	/*
	 * Events matching 'sender', whose type is >= first and <= last, and
	 * whose tag is 'tag' will be dequeued.  If 'purging', matching events
	 * which are marked as unpurgable will not be dequeued.
	 *
	 * sender == NULL means "any sender", and tag == NULL means "any tag".
	 */

	LOCK(&task->lock);

	for (event = HEAD(task->events); event != NULL; event = next_event) {
		next_event = NEXT(event, ev_link);
		if (event->ev_type >= first && event->ev_type <= last &&
		    (sender == NULL || event->ev_sender == sender) &&
		    (tag == NULL || event->ev_tag == tag) &&
		    (!purging || PURGE_OK(event))) {
			DEQUEUE(task->events, event, ev_link);
			ENQUEUE(*events, event, ev_link);
			count++;
		}
	}

	UNLOCK(&task->lock);

	return (count);
}

unsigned int
isc_task_purgerange(isc_task_t *task, void *sender, isc_eventtype_t first,
		    isc_eventtype_t last, void *tag)
{
	unsigned int count;
	isc_eventlist_t events;
	isc_event_t *event, *next_event;

	/*
	 * Purge events from a task's event queue.
	 */

	XTRACE("isc_task_purgerange");

	ISC_LIST_INIT(events);

	count = dequeue_events(task, sender, first, last, tag, &events,
			       ISC_TRUE);

	for (event = HEAD(events); event != NULL; event = next_event) {
		next_event = NEXT(event, ev_link);
		isc_event_free(&event);
	}

	/*
	 * Note that purging never changes the state of the task.
	 */

	return (count);
}

unsigned int
isc_task_purge(isc_task_t *task, void *sender, isc_eventtype_t type,
	       void *tag)
{
	/*
	 * Purge events from a task's event queue.
	 */

	XTRACE("isc_task_purge");

	return (isc_task_purgerange(task, sender, type, type, tag));
}

isc_boolean_t
isc_task_purgeevent(isc_task_t *task, isc_event_t *event) {
	isc_event_t *curr_event, *next_event;

	/*
	 * Purge 'event' from a task's event queue.
	 *
	 * XXXRTH:  WARNING:  This method may be removed before beta.
	 */

	REQUIRE(VALID_TASK(task));

	/*
	 * If 'event' is on the task's event queue, it will be purged,
	 * unless it is marked as unpurgeable.  'event' does not have to be
	 * on the task's event queue; in fact, it can even be an invalid
	 * pointer.  Purging only occurs if the event is actually on the task's
	 * event queue.
	 *
	 * Purging never changes the state of the task.
	 */

	LOCK(&task->lock);
	for (curr_event = HEAD(task->events);
	     curr_event != NULL;
	     curr_event = next_event) {
		next_event = NEXT(curr_event, ev_link);
		if (curr_event == event && PURGE_OK(event)) {
			DEQUEUE(task->events, curr_event, ev_link);
			break;
		}
	}
	UNLOCK(&task->lock);

	if (curr_event == NULL)
		return (ISC_FALSE);

	isc_event_free(&curr_event);

	return (ISC_TRUE);
}

unsigned int
isc_task_unsendrange(isc_task_t *task, void *sender, isc_eventtype_t first,
		     isc_eventtype_t last, void *tag,
		     isc_eventlist_t *events)
{
	/*
	 * Remove events from a task's event queue.
	 */

	XTRACE("isc_task_unsendrange");

	return (dequeue_events(task, sender, first, last, tag, events,
			       ISC_FALSE));
}

unsigned int
isc_task_unsend(isc_task_t *task, void *sender, isc_eventtype_t type,
		void *tag, isc_eventlist_t *events)
{
	/*
	 * Remove events from a task's event queue.
	 */

	XTRACE("isc_task_unsend");

	return (dequeue_events(task, sender, type, type, tag, events,
			       ISC_FALSE));
}

isc_result_t
isc_task_onshutdown(isc_task_t *task, isc_taskaction_t action, const void *arg)
{
	isc_boolean_t disallowed = ISC_FALSE;
	isc_result_t result = ISC_R_SUCCESS;
	isc_event_t *event;

	/*
	 * Send a shutdown event with action 'action' and argument 'arg' when
	 * 'task' is shutdown.
	 */

	REQUIRE(VALID_TASK(task));
	REQUIRE(action != NULL);

	event = isc_event_allocate(task->manager->mctx,
				   NULL,
				   ISC_TASKEVENT_SHUTDOWN,
				   action,
				   arg,
				   sizeof *event);
	if (event == NULL)
		return (ISC_R_NOMEMORY);

	LOCK(&task->lock);
	if (TASK_SHUTTINGDOWN(task)) {
		disallowed = ISC_TRUE;
		result = ISC_R_SHUTTINGDOWN;
	} else
		ENQUEUE(task->on_shutdown, event, ev_link);
	UNLOCK(&task->lock);

	if (disallowed)
		isc_mem_put(task->manager->mctx, event, sizeof *event);

	return (result);
}

void
isc_task_shutdown(isc_task_t *task) {
	isc_boolean_t was_idle;

	/*
	 * Shutdown 'task'.
	 */

	REQUIRE(VALID_TASK(task));

	LOCK(&task->lock);
	was_idle = task_shutdown(task);
	UNLOCK(&task->lock);

	if (was_idle)
		task_ready(task);
}

void
isc_task_destroy(isc_task_t **taskp) {

	/*
	 * Destroy '*taskp'.
	 */

	REQUIRE(taskp != NULL);

	isc_task_shutdown(*taskp);
	isc_task_detach(taskp);
}

void
isc_task_setname(isc_task_t *task, const char *name, void *tag) {

	/*
	 * Name 'task'.
	 */

	REQUIRE(VALID_TASK(task));

#ifdef ISC_TASK_NAMES
	LOCK(&task->lock);
	memset(task->name, 0, sizeof(task->name));
	strncpy(task->name, name, sizeof(task->name) - 1);
	task->tag = tag;
	UNLOCK(&task->lock);
#else
	(void)name;
	(void)tag;
#endif

}

const char *
isc_task_getname(isc_task_t *task) {
	return (task->name);
}

void *
isc_task_gettag(isc_task_t *task) {
	return (task->tag);
}


/***
 *** Task Manager.
 ***/
static void
dispatch(isc_taskmgr_t *manager) {
	isc_task_t *task;
#ifndef ISC_PLATFORM_USETHREADS
	unsigned int total_dispatch_count = 0;
#endif

	REQUIRE(VALID_MANAGER(manager));

	/*
	 * Again we're trying to hold the lock for as short a time as possible
	 * and to do as little locking and unlocking as possible.
	 *
	 * In both while loops, the appropriate lock must be held before the
	 * while body starts.  Code which acquired the lock at the top of
	 * the loop would be more readable, but would result in a lot of
	 * extra locking.  Compare:
	 *
	 * Straightforward:
	 *
	 *	LOCK();
	 *	...
	 *	UNLOCK();
	 *	while (expression) {
	 *		LOCK();
	 *		...
	 *		UNLOCK();
	 *
	 *	       	Unlocked part here...
	 *
	 *		LOCK();
	 *		...
	 *		UNLOCK();
	 *	}
	 *
	 * Note how if the loop continues we unlock and then immediately lock.
	 * For N iterations of the loop, this code does 2N+1 locks and 2N+1
	 * unlocks.  Also note that the lock is not held when the while
	 * condition is tested, which may or may not be important, depending
	 * on the expression.
	 *
	 * As written:
	 *
	 *	LOCK();
	 *	while (expression) {
	 *		...
	 *		UNLOCK();
	 *
	 *	       	Unlocked part here...
	 *
	 *		LOCK();
	 *		...
	 *	}
	 *	UNLOCK();
	 *
	 * For N iterations of the loop, this code does N+1 locks and N+1
	 * unlocks.  The while expression is always protected by the lock.
	 */

	LOCK(&manager->lock);
	while (!FINISHED(manager)) {
#ifdef ISC_PLATFORM_USETHREADS
		/*
		 * For reasons similar to those given in the comment in
		 * isc_task_send() above, it is safe for us to dequeue
		 * the task while only holding the manager lock, and then
		 * change the task to running state while only holding the
		 * task lock.
		 */
		while (EMPTY(manager->ready_tasks) && !FINISHED(manager)) {
			XTHREADTRACE("wait");
			WAIT(&manager->work_available, &manager->lock);
			XTHREADTRACE("awake");
		}
#else
		if (total_dispatch_count >= DEFAULT_TASKMGR_QUANTUM ||
		    EMPTY(manager->ready_tasks))
			break;
#endif
		XTHREADTRACE("working");

		task = HEAD(manager->ready_tasks);
		if (task != NULL) {
			unsigned int dispatch_count = 0;
			isc_boolean_t done = ISC_FALSE;
			isc_boolean_t requeue = ISC_FALSE;
			isc_boolean_t finished = ISC_FALSE;
			isc_event_t *event;

			INSIST(VALID_TASK(task));

			/*
			 * Note we only unlock the manager lock if we actually
			 * have a task to do.  We must reacquire the manager
			 * lock before exiting the 'if (task != NULL)' block.
			 */
			DEQUEUE(manager->ready_tasks, task, ready_link);
			UNLOCK(&manager->lock);

			LOCK(&task->lock);
			INSIST(task->state == task_state_ready);
			task->state = task_state_running;
			XTRACE("running");
			do {
				if (!EMPTY(task->events)) {
					event = HEAD(task->events);
					DEQUEUE(task->events, event, ev_link);

					/*
					 * Execute the event action.
					 */
					XTRACE("execute action");
					if (event->ev_action != NULL) {
						UNLOCK(&task->lock);
						(event->ev_action)(task,event);
						LOCK(&task->lock);
					}
					dispatch_count++;
#ifndef ISC_PLATFORM_USETHREADS
					total_dispatch_count++;
#endif
				}

				if (task->references == 0 &&
				    EMPTY(task->events) &&
				    !TASK_SHUTTINGDOWN(task)) {
					isc_boolean_t was_idle;

					/*
					 * There are no references and no
					 * pending events for this task,
					 * which means it will not become
					 * runnable again via an external
					 * action (such as sending an event
					 * or detaching).
					 *
					 * We initiate shutdown to prevent
					 * it from becoming a zombie.
					 *
					 * We do this here instead of in
					 * the "if EMPTY(task->events)" block
					 * below because:
					 *
					 *	If we post no shutdown events,
					 *	we want the task to finish.
					 *
					 *	If we did post shutdown events,
					 *	will still want the task's
					 *	quantum to be applied.
					 */
					was_idle = task_shutdown(task);
					INSIST(!was_idle);
				}

				if (EMPTY(task->events)) {
					/*
					 * Nothing else to do for this task
					 * right now.
					 */
					XTRACE("empty");
					if (task->references == 0 &&
					    TASK_SHUTTINGDOWN(task)) {
						/*
						 * The task is done.
						 */
						XTRACE("done");
						finished = ISC_TRUE;
						task->state = task_state_done;
					} else
						task->state = task_state_idle;
					done = ISC_TRUE;
				} else if (dispatch_count >= task->quantum) {
					/*
					 * Our quantum has expired, but
					 * there is more work to be done.
					 * We'll requeue it to the ready
					 * queue later.
					 *
					 * We don't check quantum until
					 * dispatching at least one event,
					 * so the minimum quantum is one.
					 */
					XTRACE("quantum");
					task->state = task_state_ready;
					requeue = ISC_TRUE;
					done = ISC_TRUE;
				}
			} while (!done);
			UNLOCK(&task->lock);

			if (finished)
				task_finished(task);

			LOCK(&manager->lock);
			if (requeue) {
				/*
				 * We know we're awake, so we don't have
				 * to wakeup any sleeping threads if the
				 * ready queue is empty before we requeue.
				 *
				 * A possible optimization if the queue is
				 * empty is to 'goto' the 'if (task != NULL)'
				 * block, avoiding the ENQUEUE of the task
				 * and the subsequent immediate DEQUEUE
				 * (since it is the only executable task).
				 * We don't do this because then we'd be
				 * skipping the exit_requested check.  The
				 * cost of ENQUEUE is low anyway, especially
				 * when you consider that we'd have to do
				 * an extra EMPTY check to see if we could
				 * do the optimization.  If the ready queue
				 * were usually nonempty, the 'optimization'
				 * might even hurt rather than help.
				 */
				ENQUEUE(manager->ready_tasks, task,
					ready_link);
			}
		}
	}
	UNLOCK(&manager->lock);
}

#ifdef ISC_PLATFORM_USETHREADS
static isc_threadresult_t
#ifdef _WIN32
WINAPI
#endif
run(void *uap) {
	isc_taskmgr_t *manager = uap;

	XTHREADTRACE("start");

	dispatch(manager);

	XTHREADTRACE("exit");

	return ((isc_threadresult_t)0);
}
#endif

static void
manager_free(isc_taskmgr_t *manager) {
	isc_mem_t *mctx;

#ifdef ISC_PLATFORM_USETHREADS
	(void)isc_condition_destroy(&manager->work_available);
	isc_mem_put(manager->mctx, manager->threads,
		    manager->workers * sizeof (isc_thread_t));
#endif
	DESTROYLOCK(&manager->lock);
	manager->magic = 0;
	mctx = manager->mctx;
	isc_mem_put(mctx, manager, sizeof *manager);
	isc_mem_detach(&mctx);
}

isc_result_t
isc_taskmgr_create(isc_mem_t *mctx, unsigned int workers,
		   unsigned int default_quantum, isc_taskmgr_t **managerp)
{
	unsigned int i, started = 0;
	isc_taskmgr_t *manager;

	/*
	 * Create a new task manager.
	 */

	REQUIRE(workers > 0);
	REQUIRE(managerp != NULL && *managerp == NULL);