task.c 24.6 KB
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/*
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 * Copyright (C) 1998, 1999, 2000  Internet Software Consortium.
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 * 
 * 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.
 */
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/*
 * Principal Author: Bob Halley
 */

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

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#include <config.h>

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#include <string.h>

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#include <isc/assertions.h>
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#include <isc/boolean.h>
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#include <isc/thread.h>
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#include <isc/mutex.h>
#include <isc/condition.h>
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#include <isc/error.h>
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#include <isc/event.h>
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#include <isc/task.h>
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#include <isc/util.h>
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#define ISC_TASK_NAMES 1

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#ifdef ISC_TASK_TRACE
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#define XTRACE(m)		printf("task %p thread %lu: %s\n", \
				       task, isc_thread_self(), (m))
#define XTTRACE(t, m)		printf("task %p thread %lu: %s\n", \
				       (t), isc_thread_self(), (m))
#define XTHREADTRACE(m)		printf("thread %lu: %s\n", \
				       isc_thread_self(), (m))
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#else
#define XTRACE(m)
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#define XTTRACE(t, m)
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#define XTHREADTRACE(m)
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#endif
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/***
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 *** Types.
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 ***/

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

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

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

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#define TASK_F_SHUTTINGDOWN		0x01
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#define TASK_SHUTTINGDOWN(t)		(((t)->flags & TASK_F_SHUTTINGDOWN) \
					 != 0)
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#define TASK_MANAGER_MAGIC		0x54534B4DU	/* TSKM. */
#define VALID_MANAGER(m)		((m) != NULL && \
					 (m)->magic == TASK_MANAGER_MAGIC)

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struct isc_taskmgr {
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	/* Not locked. */
	unsigned int			magic;
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	isc_mem_t *			mctx;
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	isc_mutex_t			lock;
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	unsigned int			workers;
	isc_thread_t *			threads;
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	/* Locked by task manager lock. */
	unsigned int			default_quantum;
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	LIST(isc_task_t)		tasks;
	LIST(isc_task_t)		ready_tasks;
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	isc_condition_t			work_available;
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	isc_boolean_t			exiting;
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};
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#define DEFAULT_DEFAULT_QUANTUM		5
#define FINISHED(m)			((m)->exiting && EMPTY((m)->tasks))
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/***
 *** Tasks.
 ***/

static void
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task_finished(isc_task_t *task) {
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	isc_taskmgr_t *manager = task->manager;
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	REQUIRE(EMPTY(task->events));
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	REQUIRE(EMPTY(task->on_shutdown));
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	REQUIRE(task->references == 0);
	REQUIRE(task->state == task_state_done);

	XTRACE("task_finished");
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	LOCK(&manager->lock);
	UNLINK(manager->tasks, task, link);
	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);
	}
	UNLOCK(&manager->lock);
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	(void)isc_mutex_destroy(&task->lock);
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	task->magic = 0;
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	isc_mem_put(task->mctx, task, sizeof *task);
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}

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isc_result_t
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isc_task_create(isc_taskmgr_t *manager, isc_mem_t *mctx, unsigned int quantum,
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		isc_task_t **taskp)
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{
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	isc_task_t *task;
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	isc_boolean_t exiting;
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	REQUIRE(VALID_MANAGER(manager));
	REQUIRE(taskp != NULL && *taskp == NULL);

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

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

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

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

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	return (ISC_R_SUCCESS);
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}

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void
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isc_task_attach(isc_task_t *source, isc_task_t **targetp) {
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	/*
	 * Attach *targetp to source.
	 */
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	REQUIRE(VALID_TASK(source));
	REQUIRE(targetp != NULL && *targetp == NULL);
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	XTTRACE(source, "attach");

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

	*targetp = source;
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}

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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, link);
			DEQUEUE(task->on_shutdown, event, link);
			ENQUEUE(task->events, event, 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);
	SIGNAL(&manager->work_available);

	UNLOCK(&manager->lock);
}

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static inline isc_boolean_t
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task_detach(isc_task_t *task) {

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

	REQUIRE(task->references > 0);

	XTRACE("detach");

	task->references--;
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	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() to deal with
		 * shutting down and termination.
		 */
		task->state = task_state_ready;
		return (ISC_TRUE);
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	}

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	return (ISC_FALSE);
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}

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void
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isc_task_detach(isc_task_t **taskp) {
	isc_task_t *task;
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	isc_boolean_t was_idle;
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	/*
	 * Detach *taskp from its task.
	 */

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

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	XTRACE("isc_task_detach");

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

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	if (was_idle)
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		task_ready(task);
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	*taskp = NULL;
}

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isc_mem_t *
isc_task_mem(isc_task_t *task) {

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	/*
	 * Get the task's memory context.
	 */

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	REQUIRE(VALID_TASK(task));
	
	return (task->mctx);
}

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static inline isc_boolean_t
task_send(isc_task_t *task, isc_event_t **eventp) {
	isc_boolean_t was_idle = ISC_FALSE;
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	isc_event_t *event;
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	/*
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	 * Caller must be holding the task lock.
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	 */

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	REQUIRE(eventp != NULL);
	event = *eventp;
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	REQUIRE(event != NULL);
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	REQUIRE(event->sender != NULL);
	REQUIRE(event->type > 0);
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	REQUIRE(task->state != task_state_done);
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	XTRACE("task_send");

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	if (task->state == task_state_idle) {
		was_idle = ISC_TRUE;
		INSIST(EMPTY(task->events));
		task->state = task_state_ready;
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	}
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	INSIST(task->state == task_state_ready ||
	       task->state == task_state_running);
	ENQUEUE(task->events, event, link);
	*eventp = NULL;
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	return (was_idle);
}	
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void
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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);
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	was_idle = task_send(task, eventp);
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	UNLOCK(&task->lock);

	if (was_idle) {
		/*
		 * We need to add this task to the ready queue.
		 *
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		 * 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.
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		 *
		 * We've changed the state to ready, so no one else will
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		 * 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,
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		 * removed, or a shutdown is started in the interval
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		 * between the time we released the task lock, and the time
		 * we add the task to the ready queue.
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		 */
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		task_ready(task);
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	}
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}

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void
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isc_task_sendanddetach(isc_task_t **taskp, isc_event_t **eventp) {
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	isc_boolean_t idle1, idle2;
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	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");
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	LOCK(&task->lock);
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	idle1 = task_send(task, eventp);
	idle2 = task_detach(task);
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	UNLOCK(&task->lock);

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	/*
	 * 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));
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	if (idle1 || idle2)
		task_ready(task);
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	*taskp = NULL;
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}

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#define PURGE_OK(event)	(((event)->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)
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{
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	isc_event_t *event, *next_event;
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	unsigned int count = 0;
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	REQUIRE(VALID_TASK(task));
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	REQUIRE(last >= first);
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	XTRACE("dequeue_events");
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	/*
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	 * 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.
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	 *
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	 * sender == NULL means "any sender", and tag == NULL means "any tag".
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	 */

	LOCK(&task->lock);
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	for (event = HEAD(task->events); event != NULL; event = next_event) {
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		next_event = NEXT(event, link);
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		if (event->type >= first && event->type <= last &&
		    (sender == NULL || event->sender == sender) &&
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		    (tag == NULL || event->tag == tag) &&
		    (!purging || PURGE_OK(event))) {
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			DEQUEUE(task->events, event, link);
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			ENQUEUE(*events, event, link);
			count++;
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		}
	}
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	UNLOCK(&task->lock);

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	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) {
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		next_event = NEXT(event, link);
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		isc_event_free(&event);
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	}
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	/*
	 * Note that purging never changes the state of the task.
	 */

	return (count);
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}

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

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	XTRACE("isc_task_purge");

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

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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.
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	 *
	 * XXXRTH:  WARNING:  This method may be removed before beta.
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	 */

	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, link);
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		if (curr_event == event && PURGE_OK(event)) {
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			DEQUEUE(task->events, curr_event, link);
			break;
		}
	}
	UNLOCK(&task->lock);

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

	isc_event_free(&curr_event);

	return (ISC_TRUE);
}

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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));
}

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

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

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

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

	if (disallowed)
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		isc_mem_put(task->mctx, event, sizeof *event);
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	return (result);
}

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void
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isc_task_shutdown(isc_task_t *task) {
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	isc_boolean_t was_idle;
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	/*
	 * Shutdown 'task'.
	 */

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	REQUIRE(VALID_TASK(task));

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

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	if (was_idle)
		task_ready(task);
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}
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void
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isc_task_destroy(isc_task_t **taskp) {
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	/*
	 * Destroy '*taskp'.
	 */

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	REQUIRE(taskp != NULL);

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

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void
isc_task_setname(isc_task_t *task, 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
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}
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/***
 *** Task Manager.
 ***/

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static isc_threadresult_t
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#ifdef _WIN32
WINAPI
#endif
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run(void *uap) {
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	isc_taskmgr_t *manager = uap;
	isc_task_t *task;
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	XTHREADTRACE("start");
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	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)) {
		/*
		 * For reasons similar to those given in the comment in
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		 * isc_task_send() above, it is safe for us to dequeue
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		 * 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)) {
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			XTHREADTRACE("wait");
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			WAIT(&manager->work_available, &manager->lock);
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			XTHREADTRACE("awake");
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		}
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		XTHREADTRACE("working");
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		task = HEAD(manager->ready_tasks);
		if (task != NULL) {
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			unsigned int dispatch_count = 0;
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			isc_boolean_t done = ISC_FALSE;
			isc_boolean_t requeue = ISC_FALSE;
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			isc_boolean_t finished = ISC_FALSE;
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			isc_event_t *event;
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			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);
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			INSIST(task->state == task_state_ready);
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			task->state = task_state_running;
			XTRACE("running");
			do {
				if (!EMPTY(task->events)) {
					event = HEAD(task->events);
					DEQUEUE(task->events, event, link);
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					/*
					 * Execute the event action.
					 */
					XTRACE("execute action");
					if (event->action != NULL) {
						UNLOCK(&task->lock);
						(event->action)(task, event);
						LOCK(&task->lock);
					}
					dispatch_count++;
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				}
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				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);
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				}

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				if (EMPTY(task->events)) {
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					/*
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					 * Nothing else to do for this task
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					 * right now.
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					 */
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					XTRACE("empty");
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					if (task->references == 0 &&
					    TASK_SHUTTINGDOWN(task)) {
						/*
						 * The task is done.
						 */
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						XTRACE("done");
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						finished = ISC_TRUE;
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						task->state = task_state_done;
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					} else
						task->state = task_state_idle;
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					done = ISC_TRUE;
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				} 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.
					 */
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					XTRACE("quantum");
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					task->state = task_state_ready;
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					requeue = ISC_TRUE;
					done = ISC_TRUE;
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				}
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			} while (!done);
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			UNLOCK(&task->lock);

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			if (finished)
				task_finished(task);
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			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);

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	XTHREADTRACE("exit");
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	return ((isc_threadresult_t)0);
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}

static void
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manager_free(isc_taskmgr_t *manager) {
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	(void)isc_condition_destroy(&manager->work_available);
	(void)isc_mutex_destroy(&manager->lock);
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	isc_mem_put(manager->mctx, manager->threads,
		    manager->workers * sizeof (isc_thread_t));
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	manager->magic = 0;
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	isc_mem_put(manager->mctx, manager, sizeof *manager);
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}

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isc_result_t
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isc_taskmgr_create(isc_mem_t *mctx, unsigned int workers, 
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		   unsigned int default_quantum, isc_taskmgr_t **managerp)
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{
	unsigned int i, started = 0;
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	isc_taskmgr_t *manager;
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	isc_thread_t *threads;
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	/*
	 * Create a new task manager.
	 */

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	REQUIRE(workers > 0);
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	REQUIRE(managerp != NULL && *managerp == NULL);
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	manager = isc_mem_get(mctx, sizeof *manager);
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	if (manager == NULL)
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		return (ISC_R_NOMEMORY);
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	manager->magic = TASK_MANAGER_MAGIC;
	manager->mctx = mctx;
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	threads = isc_mem_get(mctx, workers * sizeof (isc_thread_t));
	if (threads == NULL) {
		isc_mem_put(mctx, manager, sizeof *manager);
		return (ISC_R_NOMEMORY);
	}
	manager->threads = threads;
	manager->workers = 0;
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	if (isc_mutex_init(&manager->lock) != ISC_R_SUCCESS) {
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		isc_mem_put(mctx, threads, workers * sizeof (isc_thread_t));
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		isc_mem_put(mctx, manager, sizeof *manager);
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		UNEXPECTED_ERROR(__FILE__, __LINE__,
				 "isc_mutex_init() failed");
		return (ISC_R_UNEXPECTED);
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	}
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	if (default_quantum == 0)
		default_quantum = DEFAULT_DEFAULT_QUANTUM;
	manager->default_quantum = default_quantum;
	INIT_LIST(manager->tasks);
	INIT_LIST(manager->ready_tasks);
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	if (isc_condition_init(&manager->work_available) != ISC_R_SUCCESS) {
		(void)isc_mutex_destroy(&manager->lock);
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		isc_mem_put(mctx, threads, workers * sizeof (isc_thread_t));
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		isc_mem_put(mctx, manager, sizeof *manager);
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		UNEXPECTED_ERROR(__FILE__, __LINE__,
				 "isc_condition_init() failed");
		return (ISC_R_UNEXPECTED);
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	}
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	manager->exiting = ISC_FALSE;
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	manager->workers = 0;

	LOCK(&manager->lock);
	/*
	 * Start workers.
	 */
	for (i = 0; i < workers; i++) {
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		if (isc_thread_create(run, manager,
				      &manager->threads[manager->workers]) == 
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		    ISC_R_SUCCESS) {
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			manager->workers++;
			started++;
		}
	}
	UNLOCK(&manager->lock);

	if (started == 0) {
		manager_free(manager);
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		return (ISC_R_NOTHREADS);
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	}		

	*managerp = manager;

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	return (ISC_R_SUCCESS);
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}

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void
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isc_taskmgr_destroy(isc_taskmgr_t **managerp) {
	isc_taskmgr_t *manager;
	isc_task_t *task;
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	unsigned int i;
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	/*
	 * Destroy '*managerp'.
	 */

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	REQUIRE(managerp != NULL);
	manager = *managerp;
	REQUIRE(VALID_MANAGER(manager));

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	XTHREADTRACE("isc_taskmgr_destroy");
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	/*
	 * Only one non-worker thread may ever call this routine.
	 * If a worker thread wants to initiate shutdown of the
	 * task manager, it should ask some non-worker thread to call
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	 * isc_taskmgr_destroy(), e.g. by signalling a condition variable
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	 * that the startup thread is sleeping on.
	 */

	/*
	 * Unlike elsewhere, we're going to hold this lock a long time.
	 * We need to do so, because otherwise the list of tasks could
	 * change while we were traversing it.
	 *
	 * This is also the only function where we will hold both the
	 * task manager lock and a task lock at the same time.
	 */

	LOCK(&manager->lock);

	/*
	 * Make sure we only get called once.
	 */
	INSIST(!manager->exiting);
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	manager->exiting = ISC_TRUE;
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	/*
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	 * Post shutdown event(s) to every task (if they haven't already been
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	 * posted).
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	 */
	for (task = HEAD(manager->tasks);
	     task != NULL;
	     task = NEXT(task, link)) {
		LOCK(&task->lock);
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		if (task_shutdown(task))
			ENQUEUE(manager->ready_tasks, task, ready_link);
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		UNLOCK(&task->lock);
	}

	/*
	 * Wake up any sleeping workers.  This ensures we get work done if
	 * there's work left to do, and if there are already no tasks left
	 * it will cause the workers to see manager->exiting.
	 */
	BROADCAST(&manager->work_available);
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	UNLOCK(&manager->lock);
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	/*
	 * Wait for all the worker threads to exit.
	 */
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	for (i = 0; i < manager->workers; i++)
		(void)isc_thread_join(manager->threads[i], NULL);
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	manager_free(manager);

	*managerp = NULL;
}