xnu-792.6.22.tar.gz

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

/*
 * Copyright (c) 2000-2004 Apple Computer, Inc. All rights reserved.
 *
 * @APPLE_LICENSE_HEADER_START@
 * 
 * The contents of this file constitute Original Code as defined in and
 * are subject to the Apple Public Source License Version 1.1 (the
 * "License").  You may not use this file except in compliance with the
 * License.  Please obtain a copy of the License at
 * http://www.apple.com/publicsource and read it before using this file.
 * 
 * This Original Code and all software distributed under the License are
 * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
 * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
 * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
 * License for the specific language governing rights and limitations
 * under the License.
 * 
 * @APPLE_LICENSE_HEADER_END@
 */
/*
 * @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.
 */

#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/wait_queue.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>

#include <ddb/tr.h>

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

#define TR_ENABLE 0

/* forward declarations */
void ipc_mqueue_receive_results(wait_result_t result);

/*
 *      Routine:        ipc_mqueue_init
 *      Purpose:
 *              Initialize a newly-allocated message queue.
 */
void
ipc_mqueue_init(
        ipc_mqueue_t    mqueue,
        boolean_t       is_set)
{
        if (is_set) {
                wait_queue_set_init(&mqueue->imq_set_queue, SYNC_POLICY_FIFO);
        } else {
                wait_queue_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;
        }
}

/*
 *      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)
{
        wait_queue_t    port_waitq = &port_mqueue->imq_wait_queue;
        wait_queue_set_t set_waitq = &set_mqueue->imq_set_queue;

        return (wait_queue_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)
{
        wait_queue_t     mq_waitq = &mqueue->imq_wait_queue;
        wait_queue_set_t set_waitq = &set_mqueue->imq_set_queue;

        return wait_queue_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.
 */
void
ipc_mqueue_remove_from_all(
        ipc_mqueue_t    mqueue)
{
        wait_queue_t    mq_waitq = &mqueue->imq_wait_queue;

        wait_queue_unlink_all(mq_waitq);
        return;
}

/*
 *      Routine:        ipc_mqueue_remove_all
 *      Purpose:
 *              Remove all the member queues from the specified set.
 *      Conditions:
 *              Nothing locked.
 */
void
ipc_mqueue_remove_all(
        ipc_mqueue_t    mqueue)
{
        wait_queue_set_t        mq_setq = &mqueue->imq_set_queue;

        wait_queue_set_unlink_all(mq_setq);
        return;
}


/*
 *      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)
{
        wait_queue_t     port_waitq = &port_mqueue->imq_wait_queue;
        wait_queue_set_t set_waitq = &set_mqueue->imq_set_queue;
        ipc_kmsg_queue_t kmsgq;
        ipc_kmsg_t       kmsg, next;
        kern_return_t    kr;
        spl_t            s;

        kr = wait_queue_link(port_waitq, set_waitq);
        if (kr != KERN_SUCCESS)
                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.
         */
        s = splsched();
        imq_lock(port_mqueue);
        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;

                        th = wait_queue_wakeup64_identity_locked(
                                                port_waitq,
                                                IPC_MQUEUE_RECEIVE,
                                                THREAD_AWAKENED,
                                                FALSE);
                        /* waitq/mqueue still locked, thread locked */

                        if (th == THREAD_NULL)
                                goto leave;

                        /*
                         * 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.
                         */
                        if (th->ith_msize <
                            kmsg->ikm_header->msgh_size +
                            REQUESTED_TRAILER_SIZE(th->ith_option)) {
                                th->ith_state = MACH_RCV_TOO_LARGE;
                                th->ith_msize = kmsg->ikm_header->msgh_size;
                                if (th->ith_option & MACH_RCV_LARGE) {
                                        /*
                                         * let him go without message
                                         */
                                        th->ith_kmsg = IKM_NULL;
                                        th->ith_seqno = 0;
                                        thread_unlock(th);
                                        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);
                        ipc_mqueue_release_msgcount(port_mqueue);

                        th->ith_kmsg = kmsg;
                        th->ith_seqno = port_mqueue->imq_seqno++;
                        thread_unlock(th);
                        break;  /* go to next message */
                }
                        
        }
 leave:
        imq_unlock(port_mqueue);
        splx(s);
        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)
{
        wait_queue_wakeup64_all_locked(
                                &mqueue->imq_wait_queue,
                                IPC_MQUEUE_RECEIVE,
                                THREAD_RESTART,
                                FALSE);         /* unlock waitq? */
}


                

/*
 *      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:
 *              Nothing 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;
        spl_t s;

        /*
         *  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.
         */
        s = splsched();
        imq_lock(mqueue);

        if (!imq_full(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);
                splx(s);
        } 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);
                        splx(s);
                        return MACH_SEND_TIMED_OUT;
                }
                mqueue->imq_fullwaiters = TRUE;
                thread_lock(cur_thread);
                if (option & MACH_SEND_TIMEOUT)
                        clock_interval_to_deadline(send_timeout, 1000*NSEC_PER_USEC, &deadline);
                else
                        deadline = 0;
                wresult = wait_queue_assert_wait64_locked(
                                                &mqueue->imq_wait_queue,
                                                IPC_MQUEUE_FULL,
                                                THREAD_ABORTSAFE, deadline,
                                                cur_thread);
                thread_unlock(cur_thread);
                imq_unlock(mqueue);
                splx(s);
                
                if (wresult == THREAD_WAITING) {
                        wresult = thread_block(THREAD_CONTINUE_NULL);
                        counter(c_ipc_mqueue_send_block++);
                }
                
                switch (wresult) {
                case THREAD_TIMED_OUT:
                        assert(option & MACH_SEND_TIMEOUT);
                        return MACH_SEND_TIMED_OUT;
                        
                case THREAD_AWAKENED:
                        /* we can proceed - inherited msgcount from waker */
                        assert(mqueue->imq_msgcount > 0);
                        break;
                        
                case THREAD_INTERRUPTED:
                        return MACH_SEND_INTERRUPTED;
                        
                case THREAD_RESTART:
                default:
                        panic("ipc_mqueue_send");
                }
        }

        ipc_mqueue_post(mqueue, kmsg);
        return MACH_MSG_SUCCESS;
}

/*
 *      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.
 */
void
ipc_mqueue_release_msgcount(
        ipc_mqueue_t mqueue)    
{
        assert(imq_held(mqueue));
        assert(mqueue->imq_msgcount > 1 || ipc_kmsg_queue_empty(&mqueue->imq_messages));

        mqueue->imq_msgcount--;

        if (!imq_full(mqueue) && mqueue->imq_fullwaiters) {
                if (wait_queue_wakeup64_one_locked(
                                                &mqueue->imq_wait_queue,
                                                IPC_MQUEUE_FULL,
                                                THREAD_AWAKENED,
                                                FALSE) != KERN_SUCCESS) {
                        mqueue->imq_fullwaiters = FALSE;
                } else {
                        /* gave away our slot - add reference back */
                        mqueue->imq_msgcount++; 
                }
        }
}

/*
 *      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:
 *              If we need to queue, our space in the message queue is reserved.
 */
void
ipc_mqueue_post(
        register ipc_mqueue_t   mqueue,
        register ipc_kmsg_t             kmsg)
{

        spl_t s;

        /*
         *      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.
         */
        s = splsched();
        imq_lock(mqueue);
        for (;;) {
                wait_queue_t waitq = &mqueue->imq_wait_queue;
                thread_t receiver;

                receiver = wait_queue_wakeup64_identity_locked(
                                                        waitq,
                                                        IPC_MQUEUE_RECEIVE,
                                                        THREAD_AWAKENED,
                                                        FALSE);
                /* waitq still locked, thread locked */

                if (receiver == THREAD_NULL) {
                        /* 
                         * no receivers; queue kmsg
                         */
                        assert(mqueue->imq_msgcount > 0);
                        ipc_kmsg_enqueue_macro(&mqueue->imq_messages, kmsg);
                        break;
                }
                
                /*
                 * 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.
                 */
                if (receiver->ith_msize <
                    (kmsg->ikm_header->msgh_size) +
                    REQUESTED_TRAILER_SIZE(receiver->ith_option)) {
                        receiver->ith_msize = kmsg->ikm_header->msgh_size;
                        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++;
                        thread_unlock(receiver);

                        /* we didn't need our reserved spot in the queue */
                        ipc_mqueue_release_msgcount(mqueue);
                        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_kmsg = IKM_NULL;
                receiver->ith_seqno = 0;
                thread_unlock(receiver);
        }

        imq_unlock(mqueue);
        splx(s);
        
        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;
        kern_return_t           mr;

        /*
         * 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).
                 */
                mr = MACH_MSG_SUCCESS;

                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:
                        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.
 *
 *              If continuation is non-zero, then we might discard
 *              our kernel stack when we block.  We will continue
 *              after unblocking by executing continuation.
 *
 *              If resume is true, then we are resuming a receive
 *              operation after a blocked receive discarded our stack.
 *      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.
 *      Returns:
 *              MACH_MSG_SUCCESS        Message returned in kmsgp.
 *              MACH_RCV_TOO_LARGE      Message size returned in kmsgp.
 *              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)
{
        ipc_kmsg_queue_t        kmsgs;
        wait_result_t           wresult;
        thread_t                self;
        uint64_t                                deadline;
        spl_t                   s;

        s = splsched();
        imq_lock(mqueue);
        
        if (imq_is_set(mqueue)) {
                wait_queue_link_t wql;
                ipc_mqueue_t port_mq;
                queue_t q;

                q = &mqueue->imq_setlinks;

                /*
                 * If we are waiting on a portset mqueue, we need to see if
                 * any of the member ports have work for us.  If so, try to
                 * deliver one of those messages. By holding the portset's
                 * mqueue lock during the search, we tie up any attempts by
                 * mqueue_deliver or portset membership changes that may
                 * cross our path. But this is a lock order violation, so we
                 * have to do it "softly."  If we don't find a message waiting
                 * for us, we will assert our intention to wait while still
                 * holding that lock.  When we release the lock, the deliver/
                 * change will succeed and find us.
                 */
        search_set:
                queue_iterate(q, wql, wait_queue_link_t, wql_setlinks) {
                        port_mq = (ipc_mqueue_t)wql->wql_queue;
                        kmsgs = &port_mq->imq_messages;
                        
                        if (!imq_lock_try(port_mq)) {
                                imq_unlock(mqueue);
                                splx(s);
                                delay(1);
                                s = splsched();
                                imq_lock(mqueue);
                                goto search_set; /* start again at beginning - SMP */
                        }

                        /*
                         * If there is still a message to be had, we will
                         * try to select it (may not succeed because of size
                         * and options).  In any case, we deliver those
                         * results back to the user.
                         *
                         * We also move the port's linkage to the tail of the
                         * list for this set (fairness). Future versions will
                         * sort by timestamp or priority.
                         */
                        if (ipc_kmsg_queue_first(kmsgs) == IKM_NULL) {
                                imq_unlock(port_mq);
                                continue;
                        }
                        queue_remove(q, wql, wait_queue_link_t, wql_setlinks);
                        queue_enter(q, wql, wait_queue_link_t, wql_setlinks);
                        imq_unlock(mqueue);

                        ipc_mqueue_select(port_mq, option, max_size);
                        imq_unlock(port_mq);
                        splx(s);
                        return;
                        
                }

        } else {

                /*
                 * Receive on a single port. Just try to get the messages.
                 */
                kmsgs = &mqueue->imq_messages;
                if (ipc_kmsg_queue_first(kmsgs) != IKM_NULL) {
                        ipc_mqueue_select(mqueue, option, max_size);
                        imq_unlock(mqueue);
                        splx(s);
                        return;
                }
        }
                
        /*
         * 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.
         */
        self = current_thread();
        if (option & MACH_RCV_TIMEOUT) {
                if (rcv_timeout == 0) {
                        imq_unlock(mqueue);
                        splx(s);
                        self->ith_state = MACH_RCV_TIMED_OUT;
                        return;
                }
        }

        thread_lock(self);
        self->ith_state = MACH_RCV_IN_PROGRESS;
        self->ith_option = option;
        self->ith_msize = max_size;

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

        wresult = wait_queue_assert_wait64_locked(&mqueue->imq_wait_queue,
                                                                                                IPC_MQUEUE_RECEIVE,
                                                                                                interruptible, deadline,
                                                                                                self);
        thread_unlock(self);
        imq_unlock(mqueue);
        splx(s);

        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);
}


/*
 *      Routine:        ipc_mqueue_select
 *      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 himself.
 *      Conditions:
 *              mqueue locked.
 *              There is a message.
 *      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(
        ipc_mqueue_t            mqueue,
        mach_msg_option_t       option,
        mach_msg_size_t         max_size)
{
        thread_t self = current_thread();
        ipc_kmsg_t kmsg;
        mach_msg_return_t mr;
        mach_msg_size_t rcv_size;

        mr = MACH_MSG_SUCCESS;


        /*
         * Do some sanity checking of our ability to receive
         * before pulling the message off the queue.
         */
        kmsg = ipc_kmsg_queue_first(&mqueue->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).
         */
        rcv_size = ipc_kmsg_copyout_size(kmsg, self->map);
        if (rcv_size + REQUESTED_TRAILER_SIZE(option) > max_size) {
                mr = MACH_RCV_TOO_LARGE;
                if (option & MACH_RCV_LARGE) {
                        self->ith_kmsg = IKM_NULL;
                        self->ith_msize = rcv_size;
                        self->ith_seqno = 0;
                        self->ith_state = mr;
                        return;
                }
        }

        ipc_kmsg_rmqueue_first_macro(&mqueue->imq_messages, kmsg);
        ipc_mqueue_release_msgcount(mqueue);
        self->ith_seqno = mqueue->imq_seqno++;
        self->ith_kmsg = kmsg;
        self->ith_state = mr;

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

/*
 *      Routine:        ipc_mqueue_destroy
 *      Purpose:
 *              Destroy a message queue.  Set any blocked senders running.
 *              Destroy the kmsgs in the queue.
 *      Conditions:
 *              Nothing locked.
 *              Receivers were removed when the receive right was "changed"
 */
void
ipc_mqueue_destroy(
        ipc_mqueue_t    mqueue) 
{
        ipc_kmsg_queue_t kmqueue;
        ipc_kmsg_t kmsg;
        spl_t s;


        s = splsched();
        imq_lock(mqueue);
        /*
         *      rouse all blocked senders
         */
        mqueue->imq_fullwaiters = FALSE;
        wait_queue_wakeup64_all_locked(
                                &mqueue->imq_wait_queue,
                                IPC_MQUEUE_FULL,
                                THREAD_AWAKENED,
                                FALSE);

        kmqueue = &mqueue->imq_messages;

        while ((kmsg = ipc_kmsg_dequeue(kmqueue)) != IKM_NULL) {
                imq_unlock(mqueue);
                splx(s);

                ipc_kmsg_destroy_dest(kmsg);

                s = splsched();
                imq_lock(mqueue);
        }
        imq_unlock(mqueue);
        splx(s);
}

/*
 *      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)
{
         spl_t s;

         assert(qlimit <= MACH_PORT_QLIMIT_MAX);

         /* wake up senders allowed by the new qlimit */
         s = splsched();
         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++) {
                         if (wait_queue_wakeup64_one_locked(
                                                        &mqueue->imq_wait_queue,
                                                        IPC_MQUEUE_FULL,
                                                        THREAD_AWAKENED,
                                                        FALSE) == KERN_NOT_WAITING) {
                                         mqueue->imq_fullwaiters = FALSE;
                                         break;
                         }
                         mqueue->imq_msgcount++;  /* give it to the awakened thread */
                 }
         }
        mqueue->imq_qlimit = qlimit;
        imq_unlock(mqueue);
        splx(s);
}

/*
 *      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)
{
        spl_t s;

        s = splsched();
        imq_lock(mqueue);
        mqueue->imq_seqno = seqno;
        imq_unlock(mqueue);
        splx(s);
}


/*
 *      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 (!space->is_active) {
                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;

                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;

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

                ips_lock(pset);
                assert(ips_active(pset));
                assert(pset->ips_local_name == name);
                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;
}