queue.h

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/*
 * Copyright (C) 2011-2013  Internet Systems Consortium, Inc. ("ISC")
 *
 * Permission to use, copy, modify, and/or 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 ISC DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS.  IN NO EVENT SHALL ISC 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.
 */

/* $Id$ */

/*
 * This is a generic implementation of a two-lock concurrent queue.
 * There are built-in mutex locks for the head and tail of the queue,
 * allowing elements to be safely added and removed at the same time.
 *
 * NULL is "end of list"
 * -1 is "not linked"
 */

#ifndef ISC_QUEUE_H
#define ISC_QUEUE_H 1
#include <isc/assertions.h>
#include <isc/boolean.h>
#include <isc/mutex.h>

#ifdef ISC_QUEUE_CHECKINIT
#define ISC_QLINK_INSIST(x) ISC_INSIST(x)
#else
#define ISC_QLINK_INSIST(x) (void)0
#endif

#define ISC_QLINK(type) struct { type *prev, *next; }

#define ISC_QLINK_INIT(elt, link) \
        do { \
                (elt)->link.next = (elt)->link.prev = (void *)(-1); \
        } while(0)

#define ISC_QLINK_LINKED(elt, link) ((void*)(elt)->link.next != (void*)(-1))

#define ISC_QUEUE(type) struct { \
        type *head, *tail; \
        isc_mutex_t headlock, taillock; \
}

#define ISC_QUEUE_INIT(queue, link) \
        do { \
                (void) isc_mutex_init(&(queue).taillock); \
                (void) isc_mutex_init(&(queue).headlock); \
                (queue).tail = (queue).head = NULL; \
        } while (0)

#define ISC_QUEUE_EMPTY(queue) ISC_TF((queue).head == NULL)

#define ISC_QUEUE_DESTROY(queue) \
        do { \
                ISC_QLINK_INSIST(ISC_QUEUE_EMPTY(queue)); \
                (void) isc_mutex_destroy(&(queue).taillock); \
                (void) isc_mutex_destroy(&(queue).headlock); \
        } while (0)

/*
 * queues are meant to separate the locks at either end.  For best effect, that
 * means keeping the ends separate - i.e. non-empty queues work best.
 *
 * a push to an empty queue has to take the pop lock to update
 * the pop side of the queue.
 * Popping the last entry has to take the push lock to update
 * the push side of the queue.
 *
 * The order is (pop, push), because a pop is presumably in the
 * latency path and a push is when we're done.
 *
 * We do an MT hot test in push to see if we need both locks, so we can
 * acquire them in order.  Hopefully that makes the case where we get
 * the push lock and find we need the pop lock (and have to release it) rare.
 *
 * > 1 entry - no collision, push works on one end, pop on the other
 *   0 entry - headlock race
 *     pop wins - return(NULL), push adds new as both head/tail
 *     push wins - updates head/tail, becomes 1 entry case.
 *   1 entry - taillock race
 *     pop wins - return(pop) sets head/tail NULL, becomes 0 entry case
 *     push wins - updates {head,tail}->link.next, pop updates head
 *                 with new ->link.next and doesn't update tail
 *
 */
#define ISC_QUEUE_PUSH(queue, elt, link) \
        do { \
                isc_boolean_t headlocked = ISC_FALSE; \
                ISC_QLINK_INSIST(!ISC_QLINK_LINKED(elt, link)); \
                if ((queue).head == NULL) { \
                        LOCK(&(queue).headlock); \
                        headlocked = ISC_TRUE; \
                } \
                LOCK(&(queue).taillock); \
                if ((queue).tail == NULL && !headlocked) { \
                        UNLOCK(&(queue).taillock); \
                        LOCK(&(queue).headlock); \
                        LOCK(&(queue).taillock); \
                        headlocked = ISC_TRUE; \
                } \
                (elt)->link.prev = (queue).tail; \
                (elt)->link.next = NULL; \
                if ((queue).tail != NULL) \
                        (queue).tail->link.next = (elt); \
                (queue).tail = (elt); \
                UNLOCK(&(queue).taillock); \
                if (headlocked) { \
                        if ((queue).head == NULL) \
                                (queue).head = (elt); \
                        UNLOCK(&(queue).headlock); \
                } \
        } while (0)

#define ISC_QUEUE_POP(queue, link, ret) \
        do { \
                LOCK(&(queue).headlock); \
                ret = (queue).head; \
                while (ret != NULL) { \
                        if (ret->link.next == NULL) { \
                                LOCK(&(queue).taillock); \
                                if (ret->link.next == NULL) { \
                                        (queue).head = (queue).tail = NULL; \
                                        UNLOCK(&(queue).taillock); \
                                        break; \
                                }\
                                UNLOCK(&(queue).taillock); \
                        } \
                        (queue).head = ret->link.next; \
                        (queue).head->link.prev = NULL; \
                        break; \
                } \
                UNLOCK(&(queue).headlock); \
                if (ret != NULL) \
                        (ret)->link.next = (ret)->link.prev = (void *)(-1); \
        } while(0)

#define ISC_QUEUE_UNLINK(queue, elt, link) \
        do { \
                ISC_QLINK_INSIST(ISC_QLINK_LINKED(elt, link)); \
                LOCK(&(queue).headlock); \
                LOCK(&(queue).taillock); \
                if ((elt)->link.prev == NULL) \
                        (queue).head = (elt)->link.next; \
                else \
                        (elt)->link.prev->link.next = (elt)->link.next; \
                if ((elt)->link.next == NULL) \
                        (queue).tail = (elt)->link.prev; \
                else \
                        (elt)->link.next->link.prev = (elt)->link.prev; \
                UNLOCK(&(queue).taillock); \
                UNLOCK(&(queue).headlock); \
                (elt)->link.next = (elt)->link.prev = (void *)(-1); \
        } while(0)

#endif /* ISC_QUEUE_H */

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