xref: /freebsd-src/sys/contrib/openzfs/module/zfs/bqueue.c (revision c7046f76c2c027b00c0e6ba57cfd28f1a78f5e23)

/*
 * CDDL HEADER START
 *
 * This file and its contents are supplied under the terms of the
 * Common Development and Distribution License ("CDDL"), version 1.0.
 * You may only use this file in accordance with the terms of version
 * 1.0 of the CDDL.
 *
 * A full copy of the text of the CDDL should have accompanied this
 * source.  A copy of the CDDL is also available via the Internet at
 * http://www.illumos.org/license/CDDL.
 *
 * CDDL HEADER END
 */
/*
 * Copyright (c) 2014, 2018 by Delphix. All rights reserved.
 */

#include	<sys/bqueue.h>
#include	<sys/zfs_context.h>

static inline bqueue_node_t *
obj2node(bqueue_t *q, void *data)
{
	return ((bqueue_node_t *)((char *)data + q->bq_node_offset));
}

/*
 * Initialize a blocking queue  The maximum capacity of the queue is set to
 * size.  Types that are stored in a bqueue must contain a bqueue_node_t,
 * and node_offset must be its offset from the start of the struct.
 * fill_fraction is a performance tuning value; when the queue is full, any
 * threads attempting to enqueue records will block.  They will block until
 * they're signaled, which will occur when the queue is at least 1/fill_fraction
 * empty.  Similar behavior occurs on dequeue; if the queue is empty, threads
 * block.  They will be signalled when the queue has 1/fill_fraction full, or
 * when bqueue_flush is called.  As a result, you must call bqueue_flush when
 * you enqueue your final record on a thread, in case the dequeueing threads are
 * currently blocked and that enqueue does not cause them to be awoken.
 * Alternatively, this behavior can be disabled (causing signaling to happen
 * immediately) by setting fill_fraction to any value larger than size.
 * Return 0 on success, or -1 on failure.
 */
int
bqueue_init(bqueue_t *q, uint_t fill_fraction, size_t size, size_t node_offset)
{
	if (fill_fraction == 0) {
		return (-1);
	}
	list_create(&q->bq_list, node_offset + sizeof (bqueue_node_t),
	    node_offset + offsetof(bqueue_node_t, bqn_node));
	cv_init(&q->bq_add_cv, NULL, CV_DEFAULT, NULL);
	cv_init(&q->bq_pop_cv, NULL, CV_DEFAULT, NULL);
	mutex_init(&q->bq_lock, NULL, MUTEX_DEFAULT, NULL);
	q->bq_node_offset = node_offset;
	q->bq_size = 0;
	q->bq_maxsize = size;
	q->bq_fill_fraction = fill_fraction;
	return (0);
}

/*
 * Destroy a blocking queue.  This function asserts that there are no
 * elements in the queue, and no one is blocked on the condition
 * variables.
 */
void
bqueue_destroy(bqueue_t *q)
{
	mutex_enter(&q->bq_lock);
	ASSERT0(q->bq_size);
	cv_destroy(&q->bq_add_cv);
	cv_destroy(&q->bq_pop_cv);
	list_destroy(&q->bq_list);
	mutex_exit(&q->bq_lock);
	mutex_destroy(&q->bq_lock);
}

static void
bqueue_enqueue_impl(bqueue_t *q, void *data, size_t item_size, boolean_t flush)
{
	ASSERT3U(item_size, >, 0);
	ASSERT3U(item_size, <=, q->bq_maxsize);
	mutex_enter(&q->bq_lock);
	obj2node(q, data)->bqn_size = item_size;
	while (q->bq_size && q->bq_size + item_size > q->bq_maxsize) {
		/*
		 * Wake up bqueue_dequeue() thread if already sleeping in order
		 * to prevent the deadlock condition
		 */
		cv_signal(&q->bq_pop_cv);
		cv_wait_sig(&q->bq_add_cv, &q->bq_lock);
	}
	q->bq_size += item_size;
	list_insert_tail(&q->bq_list, data);
	if (flush)
		cv_broadcast(&q->bq_pop_cv);
	else if (q->bq_size >= q->bq_maxsize / q->bq_fill_fraction)
		cv_signal(&q->bq_pop_cv);
	mutex_exit(&q->bq_lock);
}

/*
 * Add data to q, consuming size units of capacity.  If there is insufficient
 * capacity to consume size units, block until capacity exists.  Asserts size is
 * > 0.
 */
void
bqueue_enqueue(bqueue_t *q, void *data, size_t item_size)
{
	bqueue_enqueue_impl(q, data, item_size, B_FALSE);
}

/*
 * Enqueue an entry, and then flush the queue.  This forces the popping threads
 * to wake up, even if we're below the fill fraction.  We have this in a single
 * function, rather than having a separate call, because it prevents race
 * conditions between the enqueuing thread and the dequeueing thread, where the
 * enqueueing thread will wake up the dequeueing thread, that thread will
 * destroy the condvar before the enqueuing thread is done.
 */
void
bqueue_enqueue_flush(bqueue_t *q, void *data, size_t item_size)
{
	bqueue_enqueue_impl(q, data, item_size, B_TRUE);
}

/*
 * Take the first element off of q.  If there are no elements on the queue, wait
 * until one is put there.  Return the removed element.
 */
void *
bqueue_dequeue(bqueue_t *q)
{
	void *ret = NULL;
	size_t item_size;
	mutex_enter(&q->bq_lock);
	while (q->bq_size == 0) {
		cv_wait_sig(&q->bq_pop_cv, &q->bq_lock);
	}
	ret = list_remove_head(&q->bq_list);
	ASSERT3P(ret, !=, NULL);
	item_size = obj2node(q, ret)->bqn_size;
	q->bq_size -= item_size;
	if (q->bq_size <= q->bq_maxsize - (q->bq_maxsize / q->bq_fill_fraction))
		cv_signal(&q->bq_add_cv);
	mutex_exit(&q->bq_lock);
	return (ret);
}

/*
 * Returns true if the space used is 0.
 */
boolean_t
bqueue_empty(bqueue_t *q)
{
	return (q->bq_size == 0);
}