xnu-3789.31.2.tar.gz

[apple/xnu.git] / osfmk / ipc / ipc_mqueue.c

/*
 * Copyright (c) 2000-2007 Apple Inc. All rights reserved.
 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
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 * 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
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 * may not be used to create, or enable the creation or redistribution of,
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 * 
 * Please obtain a copy of the License at
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 * 
 * The Original Code and all software distributed under the License are
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 * @APPLE_OSREFERENCE_LICENSE_HEADER_END@
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/*
 * @OSF_FREE_COPYRIGHT@
 */
/* 
 * Mach Operating System
 * Copyright (c) 1991,1990,1989 Carnegie Mellon University
 * All Rights Reserved.
 * 
 * Permission to use, copy, modify and distribute this software and its
 * documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 * 
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
 * ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 * 
 * Carnegie Mellon requests users of this software to return to
 * 
 *  Software Distribution Coordinator  or  Software.Distribution@CS.CMU.EDU
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 * 
 * any improvements or extensions that they make and grant Carnegie Mellon
 * the rights to redistribute these changes.
 */
/*
 */
/*
 *      File:   ipc/ipc_mqueue.c
 *      Author: Rich Draves
 *      Date:   1989
 *
 *      Functions to manipulate IPC message queues.
 */
/*
 * NOTICE: This file was modified by SPARTA, Inc. in 2006 to introduce
 * support for mandatory and extensible security protections.  This notice
 * is included in support of clause 2.2 (b) of the Apple Public License,
 * Version 2.0.
 */
    

#include <mach/port.h>
#include <mach/message.h>
#include <mach/sync_policy.h>

#include <kern/assert.h>
#include <kern/counters.h>
#include <kern/sched_prim.h>
#include <kern/ipc_kobject.h>
#include <kern/ipc_mig.h>       /* XXX - for mach_msg_receive_continue */
#include <kern/misc_protos.h>
#include <kern/task.h>
#include <kern/thread.h>
#include <kern/waitq.h>

#include <ipc/ipc_mqueue.h>
#include <ipc/ipc_kmsg.h>
#include <ipc/ipc_port.h>
#include <ipc/ipc_pset.h>
#include <ipc/ipc_space.h>

#if MACH_FLIPC
#include <ipc/flipc.h>
#endif

#ifdef __LP64__
#include <vm/vm_map.h>
#endif

#include <sys/event.h>

extern char     *proc_name_address(void *p);

int ipc_mqueue_full;            /* address is event for queue space */
int ipc_mqueue_rcv;             /* address is event for message arrival */

/* forward declarations */
void ipc_mqueue_receive_results(wait_result_t result);
static void ipc_mqueue_peek_on_thread(
        ipc_mqueue_t        port_mq,
        mach_msg_option_t   option,
        thread_t            thread);

/*
 *      Routine:        ipc_mqueue_init
 *      Purpose:
 *              Initialize a newly-allocated message queue.
 */
void
ipc_mqueue_init(
        ipc_mqueue_t    mqueue,
        boolean_t       is_set,
        uint64_t        *reserved_link)
{
        if (is_set) {
                waitq_set_init(&mqueue->imq_set_queue,
                               SYNC_POLICY_FIFO|SYNC_POLICY_PREPOST,
                               reserved_link, NULL);
        } else {
                waitq_init(&mqueue->imq_wait_queue, SYNC_POLICY_FIFO);
                ipc_kmsg_queue_init(&mqueue->imq_messages);
                mqueue->imq_seqno = 0;
                mqueue->imq_msgcount = 0;
                mqueue->imq_qlimit = MACH_PORT_QLIMIT_DEFAULT;
                mqueue->imq_fullwaiters = FALSE;
#if MACH_FLIPC
                mqueue->imq_fport = FPORT_NULL;
#endif
        }
        klist_init(&mqueue->imq_klist);
}

void ipc_mqueue_deinit(
        ipc_mqueue_t            mqueue)
{
        boolean_t is_set = imq_is_set(mqueue);

        if (is_set)
                waitq_set_deinit(&mqueue->imq_set_queue);
        else
                waitq_deinit(&mqueue->imq_wait_queue);
}

/*
 *      Routine:        imq_reserve_and_lock
 *      Purpose:
 *              Atomically lock an ipc_mqueue_t object and reserve
 *              an appropriate number of prepost linkage objects for
 *              use in wakeup operations.
 *      Conditions:
 *              mq is unlocked
 */
void
imq_reserve_and_lock(ipc_mqueue_t mq, uint64_t *reserved_prepost)
{
        *reserved_prepost = waitq_prepost_reserve(&mq->imq_wait_queue, 0,
                                                  WAITQ_KEEP_LOCKED);

}


/*
 *      Routine:        imq_release_and_unlock
 *      Purpose:
 *              Unlock an ipc_mqueue_t object, re-enable interrupts,
 *              and release any unused prepost object reservations.
 *      Conditions:
 *              mq is locked
 */
void
imq_release_and_unlock(ipc_mqueue_t mq, uint64_t reserved_prepost)
{
        assert(imq_held(mq));
        waitq_unlock(&mq->imq_wait_queue);
        waitq_prepost_release_reserve(reserved_prepost);
}


/*
 *      Routine:        ipc_mqueue_member
 *      Purpose:
 *              Indicate whether the (port) mqueue is a member of
 *              this portset's mqueue.  We do this by checking
 *              whether the portset mqueue's waitq is an member of
 *              the port's mqueue waitq.
 *      Conditions:
 *              the portset's mqueue is not already a member
 *              this may block while allocating linkage structures.
 */

boolean_t
ipc_mqueue_member(
        ipc_mqueue_t            port_mqueue,
        ipc_mqueue_t            set_mqueue)
{
        struct waitq *port_waitq = &port_mqueue->imq_wait_queue;
        struct waitq_set *set_waitq = &set_mqueue->imq_set_queue;

        return waitq_member(port_waitq, set_waitq);

}

/*
 *      Routine:        ipc_mqueue_remove
 *      Purpose:
 *              Remove the association between the queue and the specified
 *              set message queue.
 */

kern_return_t
ipc_mqueue_remove(
        ipc_mqueue_t      mqueue,
        ipc_mqueue_t      set_mqueue)
{
        struct waitq *mq_waitq = &mqueue->imq_wait_queue;
        struct waitq_set *set_waitq = &set_mqueue->imq_set_queue;

        return waitq_unlink(mq_waitq, set_waitq);
}

/*
 *      Routine:        ipc_mqueue_remove_from_all
 *      Purpose:
 *              Remove the mqueue from all the sets it is a member of
 *      Conditions:
 *              Nothing locked.
 *      Returns:
 *              mqueue unlocked and set links deallocated
 */
void
ipc_mqueue_remove_from_all(ipc_mqueue_t mqueue)
{
        struct waitq *mq_waitq = &mqueue->imq_wait_queue;
        kern_return_t kr;

        imq_lock(mqueue);

        assert(waitq_valid(mq_waitq));
        kr = waitq_unlink_all_unlock(mq_waitq);
        /* mqueue unlocked and set links deallocated */
}

/*
 *      Routine:        ipc_mqueue_remove_all
 *      Purpose:
 *              Remove all the member queues from the specified set.
 *              Also removes the queue from any containing sets.
 *      Conditions:
 *              Nothing locked.
 *      Returns:
 *              mqueue unlocked all set links deallocated
 */
void
ipc_mqueue_remove_all(ipc_mqueue_t      mqueue)
{
        struct waitq_set *mq_setq = &mqueue->imq_set_queue;

        imq_lock(mqueue);
        assert(waitqs_is_set(mq_setq));
        waitq_set_unlink_all_unlock(mq_setq);
        /* mqueue unlocked set links deallocated */
}


/*
 *      Routine:        ipc_mqueue_add
 *      Purpose:
 *              Associate the portset's mqueue with the port's mqueue.
 *              This has to be done so that posting the port will wakeup
 *              a portset waiter.  If there are waiters on the portset
 *              mqueue and messages on the port mqueue, try to match them
 *              up now.
 *      Conditions:
 *              May block.
 */
kern_return_t
ipc_mqueue_add(
        ipc_mqueue_t    port_mqueue,
        ipc_mqueue_t    set_mqueue,
        uint64_t        *reserved_link,
        uint64_t        *reserved_prepost)
{
        struct waitq     *port_waitq = &port_mqueue->imq_wait_queue;
        struct waitq_set *set_waitq = &set_mqueue->imq_set_queue;
        ipc_kmsg_queue_t kmsgq;
        ipc_kmsg_t       kmsg, next;
        kern_return_t    kr;

        assert(reserved_link && *reserved_link != 0);

        imq_lock(port_mqueue);

        /*
         * The link operation is now under the same lock-hold as
         * message iteration and thread wakeup, but doesn't have to be...
         */
        kr = waitq_link(port_waitq, set_waitq, WAITQ_ALREADY_LOCKED, reserved_link);
        if (kr != KERN_SUCCESS) {
                imq_unlock(port_mqueue);
                return kr;
        }

        /*
         * Now that the set has been added to the port, there may be
         * messages queued on the port and threads waiting on the set
         * waitq.  Lets get them together.
         */
        kmsgq = &port_mqueue->imq_messages;
        for (kmsg = ipc_kmsg_queue_first(kmsgq);
             kmsg != IKM_NULL;
             kmsg = next) {
                next = ipc_kmsg_queue_next(kmsgq, kmsg);

                for (;;) {
                        thread_t th;
                        mach_msg_size_t msize;
                        spl_t th_spl;

                        th = waitq_wakeup64_identify_locked(
                                                port_waitq,
                                                IPC_MQUEUE_RECEIVE,
                                                THREAD_AWAKENED, &th_spl,
                                                reserved_prepost, WAITQ_ALL_PRIORITIES,
                                                WAITQ_KEEP_LOCKED);
                        /* waitq/mqueue still locked, thread locked */

                        if (th == THREAD_NULL)
                                goto leave;

                        /*
                         * If the receiver waited with a facility not directly
                         * related to Mach messaging, then it isn't prepared to get
                         * handed the message directly.  Just set it running, and
                         * go look for another thread that can.
                         */
                        if (th->ith_state != MACH_RCV_IN_PROGRESS) {
                                if (th->ith_state == MACH_PEEK_IN_PROGRESS) {
                                        /*
                                         * wakeup the peeking thread, but
                                         * continue to loop over the threads
                                         * waiting on the port's mqueue to see
                                         * if there are any actual receivers
                                         */
                                        ipc_mqueue_peek_on_thread(port_mqueue,
                                                                  th->ith_option,
                                                                  th);
                                }
                                thread_unlock(th);
                                splx(th_spl);
                                continue;
                        }

                        /*
                         * Found a receiver. see if they can handle the message
                         * correctly (the message is not too large for them, or
                         * they didn't care to be informed that the message was
                         * too large).  If they can't handle it, take them off
                         * the list and let them go back and figure it out and
                         * just move onto the next.
                         */
                        msize = ipc_kmsg_copyout_size(kmsg, th->map);
                        if (th->ith_rsize <
                                        (msize + REQUESTED_TRAILER_SIZE(thread_is_64bit(th), th->ith_option))) {
                                th->ith_state = MACH_RCV_TOO_LARGE;
                                th->ith_msize = msize;
                                if (th->ith_option & MACH_RCV_LARGE) {
                                        /*
                                         * let him go without message
                                         */
                                        th->ith_receiver_name = port_mqueue->imq_receiver_name;
                                        th->ith_kmsg = IKM_NULL;
                                        th->ith_seqno = 0;
                                        thread_unlock(th);
                                        splx(th_spl);
                                        continue; /* find another thread */
                                }
                        } else {
                                th->ith_state = MACH_MSG_SUCCESS;
                        }

                        /*
                         * This thread is going to take this message,
                         * so give it to him.
                         */
                        ipc_kmsg_rmqueue(kmsgq, kmsg);
#if MACH_FLIPC
                        mach_node_t  node = kmsg->ikm_node;
#endif
                        ipc_mqueue_release_msgcount(port_mqueue, IMQ_NULL);

                        th->ith_kmsg = kmsg;
                        th->ith_seqno = port_mqueue->imq_seqno++;
                        thread_unlock(th);
                        splx(th_spl);
#if MACH_FLIPC
            if (MACH_NODE_VALID(node) && FPORT_VALID(port_mqueue->imq_fport))
                flipc_msg_ack(node, port_mqueue, TRUE);
#endif
                        break;  /* go to next message */
                }
        }
 leave:
        imq_unlock(port_mqueue);
        return KERN_SUCCESS;
}

/*
 *      Routine:        ipc_mqueue_changed
 *      Purpose:
 *              Wake up receivers waiting in a message queue.
 *      Conditions:
 *              The message queue is locked.
 */

void
ipc_mqueue_changed(
        ipc_mqueue_t            mqueue)
{
        /* Indicate that this message queue is vanishing */
        knote_vanish(&mqueue->imq_klist);

        waitq_wakeup64_all_locked(&mqueue->imq_wait_queue,
                                  IPC_MQUEUE_RECEIVE,
                                  THREAD_RESTART,
                                  NULL,
                                  WAITQ_ALL_PRIORITIES,
                                  WAITQ_KEEP_LOCKED);
}


                

/*
 *      Routine:        ipc_mqueue_send
 *      Purpose:
 *              Send a message to a message queue.  The message holds a reference
 *              for the destination port for this message queue in the 
 *              msgh_remote_port field.
 *
 *              If unsuccessful, the caller still has possession of
 *              the message and must do something with it.  If successful,
 *              the message is queued, given to a receiver, or destroyed.
 *      Conditions:
 *              mqueue is locked.
 *      Returns:
 *              MACH_MSG_SUCCESS        The message was accepted.
 *              MACH_SEND_TIMED_OUT     Caller still has message.
 *              MACH_SEND_INTERRUPTED   Caller still has message.
 */
mach_msg_return_t
ipc_mqueue_send(
        ipc_mqueue_t            mqueue,
        ipc_kmsg_t              kmsg,
        mach_msg_option_t       option,
        mach_msg_timeout_t  send_timeout)
{
        int wresult;

        /*
         *  Don't block if:
         *      1) We're under the queue limit.
         *      2) Caller used the MACH_SEND_ALWAYS internal option.
         *      3) Message is sent to a send-once right.
         */
        if (!imq_full(mqueue) ||
            (!imq_full_kernel(mqueue) && 
             ((option & MACH_SEND_ALWAYS) ||
              (MACH_MSGH_BITS_REMOTE(kmsg->ikm_header->msgh_bits) ==
               MACH_MSG_TYPE_PORT_SEND_ONCE)))) {
                mqueue->imq_msgcount++;
                assert(mqueue->imq_msgcount > 0);
                imq_unlock(mqueue);
        } else {
                thread_t cur_thread = current_thread();
                uint64_t deadline;

                /* 
                 * We have to wait for space to be granted to us.
                 */
                if ((option & MACH_SEND_TIMEOUT) && (send_timeout == 0)) {
                        imq_unlock(mqueue);
                        return MACH_SEND_TIMED_OUT;
                }
                if (imq_full_kernel(mqueue)) {
                        imq_unlock(mqueue);
                        return MACH_SEND_NO_BUFFER;
                }
                mqueue->imq_fullwaiters = TRUE;

                if (option & MACH_SEND_TIMEOUT)
                        clock_interval_to_deadline(send_timeout, 1000*NSEC_PER_USEC, &deadline);
                else
                        deadline = 0;
                wresult = waitq_assert_wait64_locked(
                                                &mqueue->imq_wait_queue,
                                                IPC_MQUEUE_FULL,
                                                THREAD_ABORTSAFE,
                                                TIMEOUT_URGENCY_USER_NORMAL,
                                                deadline, TIMEOUT_NO_LEEWAY,
                                                cur_thread);

                imq_unlock(mqueue);
                
                if (wresult == THREAD_WAITING) {
                        wresult = thread_block(THREAD_CONTINUE_NULL);
                        counter(c_ipc_mqueue_send_block++);
                }
                
                switch (wresult) {

                case THREAD_AWAKENED:
                        /* 
                         * we can proceed - inherited msgcount from waker
                         * or the message queue has been destroyed and the msgcount
                         * has been reset to zero (will detect in ipc_mqueue_post()).
                         */
                        break;
                        
                case THREAD_TIMED_OUT:
                        assert(option & MACH_SEND_TIMEOUT);
                        return MACH_SEND_TIMED_OUT;
                        
                case THREAD_INTERRUPTED:
                        return MACH_SEND_INTERRUPTED;
                        
                case THREAD_RESTART:
                        /* mqueue is being destroyed */
                        return MACH_SEND_INVALID_DEST;
                default:
                        panic("ipc_mqueue_send");
                }
        }

        ipc_mqueue_post(mqueue, kmsg, option);
        return MACH_MSG_SUCCESS;
}

/*
 *      Routine:        ipc_mqueue_override_send
 *      Purpose:
 *              Set an override qos on the first message in the queue
 *              (if the queue is full). This is a send-possible override
 *              that will go away as soon as we drain a message from the
 *              queue.
 *
 *      Conditions:
 *              The message queue is not locked.
 *              The caller holds a reference on the message queue.
 */
extern void ipc_mqueue_override_send(
        ipc_mqueue_t        mqueue,
        mach_msg_priority_t override)
{
        boolean_t __unused full_queue_empty = FALSE;

        imq_lock(mqueue);
        assert(imq_valid(mqueue));
        assert(!imq_is_set(mqueue));
        
        if (imq_full(mqueue)) {
                ipc_kmsg_t first = ipc_kmsg_queue_first(&mqueue->imq_messages);

                if (first && ipc_kmsg_override_qos(&mqueue->imq_messages, first, override))
                        KNOTE(&mqueue->imq_klist, 0);
                if (!first)
                        full_queue_empty = TRUE;
        }
        imq_unlock(mqueue);

#if DEVELOPMENT || DEBUG
        if (full_queue_empty) {
                ipc_port_t port = ip_from_mq(mqueue);
                int dst_pid = 0;
                if (ip_active(port) && !port->ip_tempowner &&
                    port->ip_receiver_name && port->ip_receiver &&
                    port->ip_receiver != ipc_space_kernel) {
                        dst_pid = task_pid(port->ip_receiver->is_task);
                }
                printf("%s[%d] could not override mqueue (dst:%d) with 0x%x: "
                       "queue slots are full, but there are no messages!\n",
                       proc_name_address(current_task()->bsd_info),
                       task_pid(current_task()), dst_pid, override);
        }
#endif
}

/*
 *      Routine:        ipc_mqueue_release_msgcount
 *      Purpose:
 *              Release a message queue reference in the case where we
 *              found a waiter.
 *
 *      Conditions:
 *              The message queue is locked.
 *              The message corresponding to this reference is off the queue.
 *              There is no need to pass reserved preposts because this will
 *              never prepost to anyone
 */
void
ipc_mqueue_release_msgcount(ipc_mqueue_t port_mq, ipc_mqueue_t set_mq)
{
        (void)set_mq;
        assert(imq_held(port_mq));
        assert(port_mq->imq_msgcount > 1 || ipc_kmsg_queue_empty(&port_mq->imq_messages));

        port_mq->imq_msgcount--;

        if (!imq_full(port_mq) && port_mq->imq_fullwaiters) {
                /*
                 * boost the priority of the awoken thread
                 * (WAITQ_PROMOTE_PRIORITY) to ensure it uses
                 * the message queue slot we've just reserved.
                 *
                 * NOTE: this will never prepost
                 */
                if (waitq_wakeup64_one_locked(&port_mq->imq_wait_queue,
                                              IPC_MQUEUE_FULL,
                                              THREAD_AWAKENED,
                                              NULL,
                                              WAITQ_PROMOTE_PRIORITY,
                                              WAITQ_KEEP_LOCKED) != KERN_SUCCESS) {
                        port_mq->imq_fullwaiters = FALSE;
                } else {
                        /* gave away our slot - add reference back */
                        port_mq->imq_msgcount++;
                }
        }

        if (ipc_kmsg_queue_empty(&port_mq->imq_messages)) {
                /* no more msgs: invalidate the port's prepost object */
                waitq_clear_prepost_locked(&port_mq->imq_wait_queue);
        }
}

/*
 *      Routine:        ipc_mqueue_post
 *      Purpose:
 *              Post a message to a waiting receiver or enqueue it.  If a
 *              receiver is waiting, we can release our reserved space in
 *              the message queue.
 *
 *      Conditions:
 *              mqueue is unlocked
 *              If we need to queue, our space in the message queue is reserved.
 */
void
ipc_mqueue_post(
        ipc_mqueue_t               mqueue,
        ipc_kmsg_t                 kmsg,
        mach_msg_option_t __unused option)
{
        uint64_t reserved_prepost = 0;
        boolean_t destroy_msg = FALSE;

        ipc_kmsg_trace_send(kmsg, option);

        /*
         *      While the msg queue     is locked, we have control of the
         *  kmsg, so the ref in it for the port is still good.
         *
         *      Check for a receiver for the message.
         */
        imq_reserve_and_lock(mqueue, &reserved_prepost);

        /* we may have raced with port destruction! */
        if (!imq_valid(mqueue)) {
                destroy_msg = TRUE;
                goto out_unlock;
        }

        for (;;) {
                struct waitq *waitq = &mqueue->imq_wait_queue;
                spl_t th_spl;
                thread_t receiver;
                mach_msg_size_t msize;

                receiver = waitq_wakeup64_identify_locked(waitq,
                                                          IPC_MQUEUE_RECEIVE,
                                                          THREAD_AWAKENED,
                                                          &th_spl,
                                                          &reserved_prepost,
                                                          WAITQ_ALL_PRIORITIES,
                                                          WAITQ_KEEP_LOCKED);
                /* waitq still locked, thread locked */

                if (receiver == THREAD_NULL) {

                        /* 
                         * no receivers; queue kmsg if space still reserved
                         * Reservations are cancelled when the port goes inactive.
                         * note that this will enqueue the message for any
                         * "peeking" receivers. 
                         *
                         * Also, post the knote to wake up any threads waiting
                         * on that style of interface if this insertion is of
                         * note (first insertion, or adjusted override qos all
                         * the way to the head of the queue).
                         * 
                         * This is just for ports. portset knotes are stay-active,
                         * and their threads get awakened through the !MACH_RCV_IN_PROGRESS
                         * logic below).
                         */
                        if (mqueue->imq_msgcount > 0) {
                                if (ipc_kmsg_enqueue_qos(&mqueue->imq_messages, kmsg))
                                        KNOTE(&mqueue->imq_klist, 0);
                                break;
                        }

                        /*
                         * Otherwise, the message queue must belong to an inactive
                         * port, so just destroy the message and pretend it was posted.
                         */
                        destroy_msg = TRUE;
                        goto out_unlock;
                }
        
                /*
                 * If a thread is attempting a "peek" into the message queue
                 * (MACH_PEEK_IN_PROGRESS), then we enqueue the message and set the
                 * thread running.  A successful peek is essentially the same as
                 * message delivery since the peeking thread takes responsibility
                 * for delivering the message and (eventually) removing it from
                 * the mqueue.  Only one thread can successfully use the peek
                 * facility on any given port, so we exit the waitq loop after
                 * encountering such a thread.
                 */
                if (receiver->ith_state == MACH_PEEK_IN_PROGRESS && mqueue->imq_msgcount > 0) {
                        ipc_kmsg_enqueue_qos(&mqueue->imq_messages, kmsg);
                        ipc_mqueue_peek_on_thread(mqueue, receiver->ith_option, receiver);
                        thread_unlock(receiver);
                        splx(th_spl);
                        break; /* Message was posted, so break out of loop */
                }

                /*
                 * If the receiver waited with a facility not directly related
                 * to Mach messaging, then it isn't prepared to get handed the
                 * message directly. Just set it running, and go look for
                 * another thread that can.
                 */
                if (receiver->ith_state != MACH_RCV_IN_PROGRESS) {
                        thread_unlock(receiver);
                        splx(th_spl);
                        continue;
                }

        
                /*
                 * We found a waiting thread.
                 * If the message is too large or the scatter list is too small
                 * the thread we wake up will get that as its status.
                 */
                msize = ipc_kmsg_copyout_size(kmsg, receiver->map);
                if (receiver->ith_rsize <
                                (msize + REQUESTED_TRAILER_SIZE(thread_is_64bit(receiver), receiver->ith_option))) {
                        receiver->ith_msize = msize;
                        receiver->ith_state = MACH_RCV_TOO_LARGE;
                } else {
                        receiver->ith_state = MACH_MSG_SUCCESS;
                }

                /*
                 * If there is no problem with the upcoming receive, or the
                 * receiver thread didn't specifically ask for special too
                 * large error condition, go ahead and select it anyway.
                 */
                if ((receiver->ith_state == MACH_MSG_SUCCESS) ||
                    !(receiver->ith_option & MACH_RCV_LARGE)) {
                        receiver->ith_kmsg = kmsg;
                        receiver->ith_seqno = mqueue->imq_seqno++;
#if MACH_FLIPC
                        mach_node_t node = kmsg->ikm_node;
#endif
                        thread_unlock(receiver);
                        splx(th_spl);

                        /* we didn't need our reserved spot in the queue */
                        ipc_mqueue_release_msgcount(mqueue, IMQ_NULL);

#if MACH_FLIPC
            if (MACH_NODE_VALID(node) && FPORT_VALID(mqueue->imq_fport))
                flipc_msg_ack(node, mqueue, TRUE);
#endif
                        break;
                }

                /*
                 * Otherwise, this thread needs to be released to run
                 * and handle its error without getting the message.  We
                 * need to go back and pick another one.
                 */
                receiver->ith_receiver_name = mqueue->imq_receiver_name;
                receiver->ith_kmsg = IKM_NULL;
                receiver->ith_seqno = 0;
                thread_unlock(receiver);
                splx(th_spl);
        }

out_unlock:
        /* clear the waitq boost we may have been given */
        waitq_clear_promotion_locked(&mqueue->imq_wait_queue, current_thread());
        imq_release_and_unlock(mqueue, reserved_prepost);
        if (destroy_msg)
                ipc_kmsg_destroy(kmsg);

        current_task()->messages_sent++;
        return;
}


/* static */ void
ipc_mqueue_receive_results(wait_result_t saved_wait_result)
{
        thread_t                self = current_thread();
        mach_msg_option_t       option = self->ith_option;

        /*
         * why did we wake up?
         */
        switch (saved_wait_result) {
        case THREAD_TIMED_OUT:
                self->ith_state = MACH_RCV_TIMED_OUT;
                return;

        case THREAD_INTERRUPTED:
                self->ith_state = MACH_RCV_INTERRUPTED;
                return;

        case THREAD_RESTART:
                /* something bad happened to the port/set */
                self->ith_state = MACH_RCV_PORT_CHANGED;
                return;

        case THREAD_AWAKENED:
                /*
                 * We do not need to go select a message, somebody
                 * handed us one (or a too-large indication).
                 */
                switch (self->ith_state) {
                case MACH_RCV_SCATTER_SMALL:
                case MACH_RCV_TOO_LARGE:
                        /*
                         * Somebody tried to give us a too large
                         * message. If we indicated that we cared,
                         * then they only gave us the indication,
                         * otherwise they gave us the indication
                         * AND the message anyway.
                         */
                        if (option & MACH_RCV_LARGE) {
                                return;
                        }

                case MACH_MSG_SUCCESS:
                case MACH_PEEK_READY:
                        return;

                default:
                        panic("ipc_mqueue_receive_results: strange ith_state");
                }

        default:
                panic("ipc_mqueue_receive_results: strange wait_result");
        }
}

void
ipc_mqueue_receive_continue(
        __unused void *param,
        wait_result_t wresult)
{
        ipc_mqueue_receive_results(wresult);
        mach_msg_receive_continue();  /* hard-coded for now */
}

/*
 *      Routine:        ipc_mqueue_receive
 *      Purpose:
 *              Receive a message from a message queue.
 *
 *      Conditions:
 *              Our caller must hold a reference for the port or port set
 *              to which this queue belongs, to keep the queue
 *              from being deallocated.
 *
 *              The kmsg is returned with clean header fields
 *              and with the circular bit turned off through the ith_kmsg
 *              field of the thread's receive continuation state.
 *      Returns:
 *              MACH_MSG_SUCCESS        Message returned in ith_kmsg.
 *              MACH_RCV_TOO_LARGE      Message size returned in ith_msize.
 *              MACH_RCV_TIMED_OUT      No message obtained.
 *              MACH_RCV_INTERRUPTED    No message obtained.
 *              MACH_RCV_PORT_DIED      Port/set died; no message.
 *              MACH_RCV_PORT_CHANGED   Port moved into set; no msg.
 *
 */

void
ipc_mqueue_receive(
        ipc_mqueue_t            mqueue,
        mach_msg_option_t       option,
        mach_msg_size_t         max_size,
        mach_msg_timeout_t      rcv_timeout,
        int                     interruptible)
{
        wait_result_t           wresult;
        thread_t                self = current_thread();

        imq_lock(mqueue);
        wresult = ipc_mqueue_receive_on_thread(mqueue, option, max_size,
                                               rcv_timeout, interruptible,
                                               self);
        /* mqueue unlocked */
        if (wresult == THREAD_NOT_WAITING)
                return;

        if (wresult == THREAD_WAITING) {
                counter((interruptible == THREAD_ABORTSAFE) ? 
                        c_ipc_mqueue_receive_block_user++ :
                        c_ipc_mqueue_receive_block_kernel++);

                if (self->ith_continuation)
                        thread_block(ipc_mqueue_receive_continue);
                        /* NOTREACHED */

                wresult = thread_block(THREAD_CONTINUE_NULL);
        }
        ipc_mqueue_receive_results(wresult);
}

static int mqueue_process_prepost_receive(void *ctx, struct waitq *waitq,
                                          struct waitq_set *wqset)
{
        ipc_mqueue_t     port_mq, *pmq_ptr;

        (void)wqset;
        port_mq = (ipc_mqueue_t)waitq;

        /*
         * If there are no messages on this queue, skip it and remove
         * it from the prepost list
         */
        if (ipc_kmsg_queue_empty(&port_mq->imq_messages))
                return WQ_ITERATE_INVALIDATE_CONTINUE;

        /*
         * There are messages waiting on this port.
         * Instruct the prepost iteration logic to break, but keep the
         * waitq locked.
         */
        pmq_ptr = (ipc_mqueue_t *)ctx;
        if (pmq_ptr)
                *pmq_ptr = port_mq;
        return WQ_ITERATE_BREAK_KEEP_LOCKED;
}

/*
 *      Routine:        ipc_mqueue_receive_on_thread
 *      Purpose:
 *              Receive a message from a message queue using a specified thread.
 *              If no message available, assert_wait on the appropriate waitq.
 *
 *      Conditions:
 *              Assumes thread is self.
 *              Called with mqueue locked.
 *              Returns with mqueue unlocked.
 *              May have assert-waited. Caller must block in those cases.
 */
wait_result_t
ipc_mqueue_receive_on_thread(
        ipc_mqueue_t            mqueue,
        mach_msg_option_t       option,
        mach_msg_size_t         max_size,
        mach_msg_timeout_t      rcv_timeout,
        int                     interruptible,
        thread_t                thread)
{
        wait_result_t           wresult;
        uint64_t                deadline;

        /* called with mqueue locked */

        /* no need to reserve anything: we never prepost to anyone */

        if (!imq_valid(mqueue)) {
                /* someone raced us to destroy this mqueue/port! */
                imq_unlock(mqueue);
                /*
                 * ipc_mqueue_receive_results updates the thread's ith_state
                 * TODO: differentiate between rights being moved and
                 * rights/ports being destroyed (21885327)
                 */
                return THREAD_RESTART;
        }
        
        if (imq_is_set(mqueue)) {
                ipc_mqueue_t port_mq = IMQ_NULL;

                (void)waitq_set_iterate_preposts(&mqueue->imq_set_queue,
                                                 &port_mq,
                                 mqueue_process_prepost_receive);

                if (port_mq != IMQ_NULL) {
                        /*
                         * We get here if there is at least one message
                         * waiting on port_mq. We have instructed the prepost
                         * iteration logic to leave both the port_mq and the
                         * set mqueue locked.
                         *
                         * TODO: previously, we would place this port at the
                         *       back of the prepost list...
                         */
                        imq_unlock(mqueue);

                        /*
                         * Continue on to handling the message with just
                         * the port mqueue locked.
                         */
                        if (option & MACH_PEEK_MSG)
                                ipc_mqueue_peek_on_thread(port_mq, option, thread);
                        else
                                ipc_mqueue_select_on_thread(port_mq, mqueue, option,
                                                            max_size, thread);

                        imq_unlock(port_mq);
                        return THREAD_NOT_WAITING;
                }
        } else if (imq_is_queue(mqueue)) {
                ipc_kmsg_queue_t kmsgs;

                /*
                 * Receive on a single port. Just try to get the messages.
                 */
                kmsgs = &mqueue->imq_messages;
                if (ipc_kmsg_queue_first(kmsgs) != IKM_NULL) {
                        if (option & MACH_PEEK_MSG)
                                ipc_mqueue_peek_on_thread(mqueue, option, thread);
                        else
                                ipc_mqueue_select_on_thread(mqueue, IMQ_NULL, option,
                                                            max_size, thread);
                        imq_unlock(mqueue);
                        return THREAD_NOT_WAITING;
                }
        } else {
                panic("Unknown mqueue type 0x%x: likely memory corruption!\n",
                      mqueue->imq_wait_queue.waitq_type);
        }
        
        /*
         * Looks like we'll have to block.  The mqueue we will
         * block on (whether the set's or the local port's) is
         * still locked.
         */
        if (option & MACH_RCV_TIMEOUT) {
                if (rcv_timeout == 0) {
                        imq_unlock(mqueue);
                        thread->ith_state = MACH_RCV_TIMED_OUT;
                        return THREAD_NOT_WAITING;
                }
        }

        thread->ith_option = option;
        thread->ith_rsize = max_size;
        thread->ith_msize = 0;

        if (option & MACH_PEEK_MSG)
                thread->ith_state = MACH_PEEK_IN_PROGRESS;
        else
                thread->ith_state = MACH_RCV_IN_PROGRESS;

        if (option & MACH_RCV_TIMEOUT)
                clock_interval_to_deadline(rcv_timeout, 1000*NSEC_PER_USEC, &deadline);
        else
                deadline = 0;

        wresult = waitq_assert_wait64_locked(&mqueue->imq_wait_queue,
                                             IPC_MQUEUE_RECEIVE,
                                             interruptible,
                                             TIMEOUT_URGENCY_USER_NORMAL,
                                             deadline,
                                             TIMEOUT_NO_LEEWAY,
                                             thread);
        /* preposts should be detected above, not here */
        if (wresult == THREAD_AWAKENED)
                panic("ipc_mqueue_receive_on_thread: sleep walking");

        imq_unlock(mqueue);

        return wresult;
}


/*
 *      Routine:        ipc_mqueue_peek_on_thread
 *      Purpose:
 *              A receiver discovered that there was a message on the queue
 *              before he had to block. Tell a thread about the message queue,
 *              but don't pick off any messages.
 *      Conditions:
 *              port_mq locked
 *              at least one message on port_mq's message queue
 *
 *      Returns: (on thread->ith_state)
 *              MACH_PEEK_READY         ith_peekq contains a message queue
 */
void
ipc_mqueue_peek_on_thread(
        ipc_mqueue_t        port_mq,
        mach_msg_option_t   option,
        thread_t            thread)
{
        (void)option;
        assert(option & MACH_PEEK_MSG);
        assert(ipc_kmsg_queue_first(&port_mq->imq_messages) != IKM_NULL);

        /*
         * Take a reference on the mqueue's associated port:
         * the peeking thread will be responsible to release this reference
         * using ip_release_mq()
         */
        ip_reference_mq(port_mq);
        thread->ith_peekq = port_mq;
        thread->ith_state = MACH_PEEK_READY;
}

/*
 *      Routine:        ipc_mqueue_select_on_thread
 *      Purpose:
 *              A receiver discovered that there was a message on the queue
 *              before he had to block.  Pick the message off the queue and
 *              "post" it to thread.
 *      Conditions:
 *              mqueue locked.
 *              thread not locked.
 *              There is a message.
 *              No need to reserve prepost objects - it will never prepost
 *
 *      Returns:
 *              MACH_MSG_SUCCESS        Actually selected a message for ourselves.
 *              MACH_RCV_TOO_LARGE  May or may not have pull it, but it is large
 */
void
ipc_mqueue_select_on_thread(
        ipc_mqueue_t            port_mq,
        ipc_mqueue_t            set_mq,
        mach_msg_option_t       option,
        mach_msg_size_t         max_size,
        thread_t                thread)
{
        ipc_kmsg_t kmsg;
        mach_msg_return_t mr = MACH_MSG_SUCCESS;
        mach_msg_size_t msize;

        /*
         * Do some sanity checking of our ability to receive
         * before pulling the message off the queue.
         */
        kmsg = ipc_kmsg_queue_first(&port_mq->imq_messages);
        assert(kmsg != IKM_NULL);

        /*
         * If we really can't receive it, but we had the
         * MACH_RCV_LARGE option set, then don't take it off
         * the queue, instead return the appropriate error
         * (and size needed).
         */
        msize = ipc_kmsg_copyout_size(kmsg, thread->map);
        if (msize + REQUESTED_TRAILER_SIZE(thread_is_64bit(thread), option) > max_size) {
                mr = MACH_RCV_TOO_LARGE;
                if (option & MACH_RCV_LARGE) {
                        thread->ith_receiver_name = port_mq->imq_receiver_name;
                        thread->ith_kmsg = IKM_NULL;
                        thread->ith_msize = msize;
                        thread->ith_seqno = 0;
                        thread->ith_state = mr;
                        return;
                }
        }

        ipc_kmsg_rmqueue(&port_mq->imq_messages, kmsg);
#if MACH_FLIPC
    if (MACH_NODE_VALID(kmsg->ikm_node) && FPORT_VALID(port_mq->imq_fport))
        flipc_msg_ack(kmsg->ikm_node, port_mq, TRUE);
#endif
        ipc_mqueue_release_msgcount(port_mq, set_mq);
        thread->ith_seqno = port_mq->imq_seqno++;
        thread->ith_kmsg = kmsg;
        thread->ith_state = mr;

        current_task()->messages_received++;
        return;
}

/*
 *      Routine:        ipc_mqueue_peek_locked
 *      Purpose:
 *              Peek at a (non-set) message queue to see if it has a message
 *              matching the sequence number provided (if zero, then the
 *              first message in the queue) and return vital info about the
 *              message.
 *
 *      Conditions:
 *              The ipc_mqueue_t is locked by callers.
 *              Other locks may be held by callers, so this routine cannot block.
 *              Caller holds reference on the message queue.
 */
unsigned
ipc_mqueue_peek_locked(ipc_mqueue_t mq,
                       mach_port_seqno_t * seqnop,
                       mach_msg_size_t * msg_sizep,
                       mach_msg_id_t * msg_idp,
                       mach_msg_max_trailer_t * msg_trailerp,
                       ipc_kmsg_t *kmsgp)
{
        ipc_kmsg_queue_t kmsgq;
        ipc_kmsg_t kmsg;
        mach_port_seqno_t seqno, msgoff;
        unsigned res = 0;

        assert(!imq_is_set(mq));

        seqno = 0;
        if (seqnop != NULL)
                seqno = *seqnop;

        if (seqno == 0) {
                seqno = mq->imq_seqno;
                msgoff = 0;
        } else if (seqno >= mq->imq_seqno && 
                   seqno < mq->imq_seqno + mq->imq_msgcount) {
                msgoff = seqno - mq->imq_seqno;
        } else
                goto out;

        /* look for the message that would match that seqno */
        kmsgq = &mq->imq_messages;
        kmsg = ipc_kmsg_queue_first(kmsgq);
        while (msgoff-- && kmsg != IKM_NULL) {
                kmsg = ipc_kmsg_queue_next(kmsgq, kmsg);
        }
        if (kmsg == IKM_NULL)
                goto out;

        /* found one - return the requested info */
        if (seqnop != NULL)
                *seqnop = seqno;
        if (msg_sizep != NULL)
                *msg_sizep = kmsg->ikm_header->msgh_size;
        if (msg_idp != NULL)
                *msg_idp = kmsg->ikm_header->msgh_id;
        if (msg_trailerp != NULL)
                memcpy(msg_trailerp, 
                       (mach_msg_max_trailer_t *)((vm_offset_t)kmsg->ikm_header +
                                                  round_msg(kmsg->ikm_header->msgh_size)),
                       sizeof(mach_msg_max_trailer_t));
        if (kmsgp != NULL)
                *kmsgp = kmsg;

        res = 1;

out:
        return res;
}


/*
 *      Routine:        ipc_mqueue_peek
 *      Purpose:
 *              Peek at a (non-set) message queue to see if it has a message
 *              matching the sequence number provided (if zero, then the
 *              first message in the queue) and return vital info about the
 *              message.
 *
 *      Conditions:
 *              The ipc_mqueue_t is unlocked.
 *              Locks may be held by callers, so this routine cannot block.
 *              Caller holds reference on the message queue.
 */
unsigned
ipc_mqueue_peek(ipc_mqueue_t mq,
                mach_port_seqno_t * seqnop,
                mach_msg_size_t * msg_sizep,
                mach_msg_id_t * msg_idp,
                mach_msg_max_trailer_t * msg_trailerp,
                ipc_kmsg_t *kmsgp)
{
        unsigned res;

        imq_lock(mq);

        res = ipc_mqueue_peek_locked(mq, seqnop, msg_sizep, msg_idp,
                                     msg_trailerp, kmsgp);

        imq_unlock(mq);
        return res;
}

/*
 *      Routine:        ipc_mqueue_release_peek_ref
 *      Purpose:
 *              Release the reference on an mqueue's associated port which was
 *              granted to a thread in ipc_mqueue_peek_on_thread (on the
 *              MACH_PEEK_MSG thread wakeup path).
 *
 *      Conditions:
 *              The ipc_mqueue_t should be locked on entry.
 *              The ipc_mqueue_t will be _unlocked_ on return
 *                      (and potentially invalid!)
 *
 */
void ipc_mqueue_release_peek_ref(ipc_mqueue_t mq)
{
        assert(!imq_is_set(mq));
        assert(imq_held(mq));

        /*
         * clear any preposts this mq may have generated
         * (which would cause subsequent immediate wakeups)
         */
        waitq_clear_prepost_locked(&mq->imq_wait_queue);

        imq_unlock(mq);

        /*
         * release the port reference: we need to do this outside the lock
         * because we might be holding the last port reference!
         **/
        ip_release_mq(mq);
}

/*
 * peek at the contained port message queues, break prepost iteration as soon
 * as we spot a message on one of the message queues referenced by the set's
 * prepost list.  No need to lock each message queue, as only the head of each
 * queue is checked. If a message wasn't there before we entered here, no need
 * to find it (if we do, great).
 */
static int mqueue_peek_iterator(void *ctx, struct waitq *waitq,
                                struct waitq_set *wqset)
{
        ipc_mqueue_t port_mq = (ipc_mqueue_t)waitq;
        ipc_kmsg_queue_t kmsgs = &port_mq->imq_messages;

        (void)ctx;
        (void)wqset;
                
        if (ipc_kmsg_queue_first(kmsgs) != IKM_NULL)
                return WQ_ITERATE_BREAK; /* break out of the prepost iteration */

        return WQ_ITERATE_CONTINUE;
}

/*
 *      Routine:        ipc_mqueue_set_peek
 *      Purpose:
 *              Peek at a message queue set to see if it has any ports
 *              with messages.
 *
 *      Conditions:
 *              Locks may be held by callers, so this routine cannot block.
 *              Caller holds reference on the message queue.
 */
unsigned
ipc_mqueue_set_peek(ipc_mqueue_t mq)
{
        int ret;

        imq_lock(mq);

        /*
         * We may have raced with port destruction where the mqueue is marked
         * as invalid. In that case, even though we don't have messages, we
         * have an end-of-life event to deliver.
         */
        if (!imq_is_valid(mq))
                return 1;

        ret = waitq_set_iterate_preposts(&mq->imq_set_queue, NULL,
                                         mqueue_peek_iterator);

        imq_unlock(mq);

        return (ret == WQ_ITERATE_BREAK);
}

/*
 *      Routine:        ipc_mqueue_set_gather_member_names
 *      Purpose:
 *              Discover all ports which are members of a given port set.
 *              Because the waitq linkage mechanism was redesigned to save
 *              significan amounts of memory, it no longer keeps back-pointers
 *              from a port set to a port. Therefore, we must iterate over all
 *              ports within a given IPC space and individually query them to
 *              see if they are members of the given set. Port names of ports
 *              found to be members of the given set will be gathered into the
 *              provided 'names' array.  Actual returned names are limited to
 *              maxnames entries, but we keep counting the actual number of
 *              members to let the caller decide to retry if necessary.
 *
 *      Conditions:
 *              Locks may be held by callers, so this routine cannot block.
 *              Caller holds reference on the message queue (via port set).
 */
void
ipc_mqueue_set_gather_member_names(
        ipc_space_t space,
        ipc_mqueue_t set_mq,
        ipc_entry_num_t maxnames,
        mach_port_name_t *names,
        ipc_entry_num_t *actualp)
{
        ipc_entry_t table;
        ipc_entry_num_t tsize;
        struct waitq_set *wqset;
        ipc_entry_num_t actual = 0;

        assert(set_mq != IMQ_NULL);
        wqset = &set_mq->imq_set_queue;

        assert(space != IS_NULL);
        is_read_lock(space);
        if (!is_active(space)) {
                is_read_unlock(space);
                goto out;
        }

        if (!waitq_set_is_valid(wqset)) {
                is_read_unlock(space);
                goto out;
        }

        table = space->is_table;
        tsize = space->is_table_size;
        for (ipc_entry_num_t idx = 0; idx < tsize; idx++) {
                ipc_entry_t entry = &table[idx];

                /* only receive rights can be members of port sets */
                if ((entry->ie_bits & MACH_PORT_TYPE_RECEIVE) != MACH_PORT_TYPE_NONE) {
                        __IGNORE_WCASTALIGN(ipc_port_t port = (ipc_port_t)entry->ie_object);
                        ipc_mqueue_t mq = &port->ip_messages;

                        assert(IP_VALID(port));
                        if (ip_active(port) &&
                            waitq_member(&mq->imq_wait_queue, wqset)) {
                                if (actual < maxnames)
                                        names[actual] = mq->imq_receiver_name;
                                actual++;
                        }
                }
        }

        is_read_unlock(space);

out:
        *actualp = actual;
}


/*
 *      Routine:        ipc_mqueue_destroy_locked
 *      Purpose:
 *              Destroy a (non-set) message queue.
 *              Set any blocked senders running.
 *              Destroy the kmsgs in the queue.
 *      Conditions:
 *              mqueue locked
 *              Receivers were removed when the receive right was "changed"
 */
boolean_t
ipc_mqueue_destroy_locked(ipc_mqueue_t mqueue)
{
        ipc_kmsg_queue_t kmqueue;
        ipc_kmsg_t kmsg;
        boolean_t reap = FALSE;

        assert(!imq_is_set(mqueue));

        /*
         *      rouse all blocked senders
         *      (don't boost anyone - we're tearing this queue down)
         *      (never preposts)
         */
        mqueue->imq_fullwaiters = FALSE;
        waitq_wakeup64_all_locked(&mqueue->imq_wait_queue,
                                  IPC_MQUEUE_FULL,
                                  THREAD_RESTART,
                                  NULL,
                                  WAITQ_ALL_PRIORITIES,
                                  WAITQ_KEEP_LOCKED);

        /*
         * Move messages from the specified queue to the per-thread
         * clean/drain queue while we have the mqueue lock.
         */
        kmqueue = &mqueue->imq_messages;
        while ((kmsg = ipc_kmsg_dequeue(kmqueue)) != IKM_NULL) {
#if MACH_FLIPC
    if (MACH_NODE_VALID(kmsg->ikm_node) && FPORT_VALID(mqueue->imq_fport))
        flipc_msg_ack(kmsg->ikm_node, mqueue, TRUE);
#endif
                boolean_t first;
                first = ipc_kmsg_delayed_destroy(kmsg);
                if (first)
                        reap = first;
        }

        /*
         * Wipe out message count, both for messages about to be
         * reaped and for reserved space for (previously) woken senders.
         * This is the indication to them that their reserved space is gone
         * (the mqueue was destroyed).
         */
        mqueue->imq_msgcount = 0;

        /* invalidate the waitq for subsequent mqueue operations */
        waitq_invalidate_locked(&mqueue->imq_wait_queue);

        /* clear out any preposting we may have done */
        waitq_clear_prepost_locked(&mqueue->imq_wait_queue);

        /*
         * assert that we are destroying / invalidating a queue that's
         * not a member of any other queue.
         */
        assert(mqueue->imq_preposts == 0);
        assert(mqueue->imq_in_pset == 0);

        return reap;
}

/*
 *      Routine:        ipc_mqueue_set_qlimit
 *      Purpose:
 *              Changes a message queue limit; the maximum number
 *              of messages which may be queued.
 *      Conditions:
 *              Nothing locked.
 */

void
ipc_mqueue_set_qlimit(
         ipc_mqueue_t                   mqueue,
         mach_port_msgcount_t   qlimit)
{

         assert(qlimit <= MACH_PORT_QLIMIT_MAX);

         /* wake up senders allowed by the new qlimit */
         imq_lock(mqueue);
         if (qlimit > mqueue->imq_qlimit) {
                 mach_port_msgcount_t i, wakeup;

                 /* caution: wakeup, qlimit are unsigned */
                 wakeup = qlimit - mqueue->imq_qlimit;

                 for (i = 0; i < wakeup; i++) {
                        /*
                         * boost the priority of the awoken thread
                         * (WAITQ_PROMOTE_PRIORITY) to ensure it uses
                         * the message queue slot we've just reserved.
                         *
                         * NOTE: this will never prepost
                         */
                        if (waitq_wakeup64_one_locked(&mqueue->imq_wait_queue,
                                                      IPC_MQUEUE_FULL,
                                                      THREAD_AWAKENED,
                                                      NULL,
                                                      WAITQ_PROMOTE_PRIORITY,
                                                      WAITQ_KEEP_LOCKED) == KERN_NOT_WAITING) {
                                mqueue->imq_fullwaiters = FALSE;
                                break;
                        }
                        mqueue->imq_msgcount++;  /* give it to the awakened thread */
                 }
        }
        mqueue->imq_qlimit = qlimit;
        imq_unlock(mqueue);
}

/*
 *      Routine:        ipc_mqueue_set_seqno
 *      Purpose:
 *              Changes an mqueue's sequence number.
 *      Conditions:
 *              Caller holds a reference to the queue's containing object.
 */
void
ipc_mqueue_set_seqno(
        ipc_mqueue_t            mqueue,
        mach_port_seqno_t       seqno)
{
        imq_lock(mqueue);
        mqueue->imq_seqno = seqno;
        imq_unlock(mqueue);
}


/*
 *      Routine:        ipc_mqueue_copyin
 *      Purpose:
 *              Convert a name in a space to a message queue.
 *      Conditions:
 *              Nothing locked.  If successful, the caller gets a ref for
 *              for the object. This ref ensures the continued existence of
 *              the queue.
 *      Returns:
 *              MACH_MSG_SUCCESS        Found a message queue.
 *              MACH_RCV_INVALID_NAME   The space is dead.
 *              MACH_RCV_INVALID_NAME   The name doesn't denote a right.
 *              MACH_RCV_INVALID_NAME
 *                      The denoted right is not receive or port set.
 *              MACH_RCV_IN_SET         Receive right is a member of a set.
 */

mach_msg_return_t
ipc_mqueue_copyin(
        ipc_space_t             space,
        mach_port_name_t        name,
        ipc_mqueue_t            *mqueuep,
        ipc_object_t            *objectp)
{
        ipc_entry_t entry;
        ipc_object_t object;
        ipc_mqueue_t mqueue;

        is_read_lock(space);
        if (!is_active(space)) {
                is_read_unlock(space);
                return MACH_RCV_INVALID_NAME;
        }

        entry = ipc_entry_lookup(space, name);
        if (entry == IE_NULL) {
                is_read_unlock(space);
                return MACH_RCV_INVALID_NAME;
        }

        object = entry->ie_object;

        if (entry->ie_bits & MACH_PORT_TYPE_RECEIVE) {
                ipc_port_t port;

                __IGNORE_WCASTALIGN(port = (ipc_port_t) object);
                assert(port != IP_NULL);

                ip_lock(port);
                assert(ip_active(port));
                assert(port->ip_receiver_name == name);
                assert(port->ip_receiver == space);
                is_read_unlock(space);
                mqueue = &port->ip_messages;

        } else if (entry->ie_bits & MACH_PORT_TYPE_PORT_SET) {
                ipc_pset_t pset;

                __IGNORE_WCASTALIGN(pset = (ipc_pset_t) object);
                assert(pset != IPS_NULL);

                ips_lock(pset);
                assert(ips_active(pset));
                is_read_unlock(space);

                mqueue = &pset->ips_messages;
        } else {
                is_read_unlock(space);
                return MACH_RCV_INVALID_NAME;
        }

        /*
         *      At this point, the object is locked and active,
         *      the space is unlocked, and mqueue is initialized.
         */

        io_reference(object);
        io_unlock(object);

        *objectp = object;
        *mqueuep = mqueue;
        return MACH_MSG_SUCCESS;
}