[IA64-SGI] get XPC to cleanly disengage from remote memory references

[linux-2.6] / arch / ia64 / sn / kernel / xpc_channel.c

/*
 * This file is subject to the terms and conditions of the GNU General Public
 * License.  See the file "COPYING" in the main directory of this archive
 * for more details.
 *
 * Copyright (c) 2004-2005 Silicon Graphics, Inc.  All Rights Reserved.
 */


/*
 * Cross Partition Communication (XPC) channel support.
 *
 *      This is the part of XPC that manages the channels and
 *      sends/receives messages across them to/from other partitions.
 *
 */


#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/cache.h>
#include <linux/interrupt.h>
#include <linux/slab.h>
#include <asm/sn/bte.h>
#include <asm/sn/sn_sal.h>
#include "xpc.h"


/*
 * Set up the initial values for the XPartition Communication channels.
 */
static void
xpc_initialize_channels(struct xpc_partition *part, partid_t partid)
{
        int ch_number;
        struct xpc_channel *ch;


        for (ch_number = 0; ch_number < part->nchannels; ch_number++) {
                ch = &part->channels[ch_number];

                ch->partid = partid;
                ch->number = ch_number;
                ch->flags = XPC_C_DISCONNECTED;

                ch->local_GP = &part->local_GPs[ch_number];
                ch->local_openclose_args =
                                        &part->local_openclose_args[ch_number];

                atomic_set(&ch->kthreads_assigned, 0);
                atomic_set(&ch->kthreads_idle, 0);
                atomic_set(&ch->kthreads_active, 0);

                atomic_set(&ch->references, 0);
                atomic_set(&ch->n_to_notify, 0);

                spin_lock_init(&ch->lock);
                sema_init(&ch->msg_to_pull_sema, 1);    /* mutex */
                sema_init(&ch->wdisconnect_sema, 0);    /* event wait */

                atomic_set(&ch->n_on_msg_allocate_wq, 0);
                init_waitqueue_head(&ch->msg_allocate_wq);
                init_waitqueue_head(&ch->idle_wq);
        }
}


/*
 * Setup the infrastructure necessary to support XPartition Communication
 * between the specified remote partition and the local one.
 */
enum xpc_retval
xpc_setup_infrastructure(struct xpc_partition *part)
{
        int ret, cpuid;
        struct timer_list *timer;
        partid_t partid = XPC_PARTID(part);


        /*
         * Zero out MOST of the entry for this partition. Only the fields
         * starting with `nchannels' will be zeroed. The preceding fields must
         * remain `viable' across partition ups and downs, since they may be
         * referenced during this memset() operation.
         */
        memset(&part->nchannels, 0, sizeof(struct xpc_partition) -
                                offsetof(struct xpc_partition, nchannels));

        /*
         * Allocate all of the channel structures as a contiguous chunk of
         * memory.
         */
        part->channels = kmalloc(sizeof(struct xpc_channel) * XPC_NCHANNELS,
                                                                GFP_KERNEL);
        if (part->channels == NULL) {
                dev_err(xpc_chan, "can't get memory for channels\n");
                return xpcNoMemory;
        }
        memset(part->channels, 0, sizeof(struct xpc_channel) * XPC_NCHANNELS);

        part->nchannels = XPC_NCHANNELS;


        /* allocate all the required GET/PUT values */

        part->local_GPs = xpc_kmalloc_cacheline_aligned(XPC_GP_SIZE,
                                        GFP_KERNEL, &part->local_GPs_base);
        if (part->local_GPs == NULL) {
                kfree(part->channels);
                part->channels = NULL;
                dev_err(xpc_chan, "can't get memory for local get/put "
                        "values\n");
                return xpcNoMemory;
        }
        memset(part->local_GPs, 0, XPC_GP_SIZE);

        part->remote_GPs = xpc_kmalloc_cacheline_aligned(XPC_GP_SIZE,
                                        GFP_KERNEL, &part->remote_GPs_base);
        if (part->remote_GPs == NULL) {
                kfree(part->channels);
                part->channels = NULL;
                kfree(part->local_GPs_base);
                part->local_GPs = NULL;
                dev_err(xpc_chan, "can't get memory for remote get/put "
                        "values\n");
                return xpcNoMemory;
        }
        memset(part->remote_GPs, 0, XPC_GP_SIZE);


        /* allocate all the required open and close args */

        part->local_openclose_args = xpc_kmalloc_cacheline_aligned(
                                        XPC_OPENCLOSE_ARGS_SIZE, GFP_KERNEL,
                                        &part->local_openclose_args_base);
        if (part->local_openclose_args == NULL) {
                kfree(part->channels);
                part->channels = NULL;
                kfree(part->local_GPs_base);
                part->local_GPs = NULL;
                kfree(part->remote_GPs_base);
                part->remote_GPs = NULL;
                dev_err(xpc_chan, "can't get memory for local connect args\n");
                return xpcNoMemory;
        }
        memset(part->local_openclose_args, 0, XPC_OPENCLOSE_ARGS_SIZE);

        part->remote_openclose_args = xpc_kmalloc_cacheline_aligned(
                                        XPC_OPENCLOSE_ARGS_SIZE, GFP_KERNEL,
                                        &part->remote_openclose_args_base);
        if (part->remote_openclose_args == NULL) {
                kfree(part->channels);
                part->channels = NULL;
                kfree(part->local_GPs_base);
                part->local_GPs = NULL;
                kfree(part->remote_GPs_base);
                part->remote_GPs = NULL;
                kfree(part->local_openclose_args_base);
                part->local_openclose_args = NULL;
                dev_err(xpc_chan, "can't get memory for remote connect args\n");
                return xpcNoMemory;
        }
        memset(part->remote_openclose_args, 0, XPC_OPENCLOSE_ARGS_SIZE);


        xpc_initialize_channels(part, partid);

        atomic_set(&part->nchannels_active, 0);
        atomic_set(&part->nchannels_engaged, 0);


        /* local_IPI_amo were set to 0 by an earlier memset() */

        /* Initialize this partitions AMO_t structure */
        part->local_IPI_amo_va = xpc_IPI_init(partid);

        spin_lock_init(&part->IPI_lock);

        atomic_set(&part->channel_mgr_requests, 1);
        init_waitqueue_head(&part->channel_mgr_wq);

        sprintf(part->IPI_owner, "xpc%02d", partid);
        ret = request_irq(SGI_XPC_NOTIFY, xpc_notify_IRQ_handler, SA_SHIRQ,
                                part->IPI_owner, (void *) (u64) partid);
        if (ret != 0) {
                kfree(part->channels);
                part->channels = NULL;
                kfree(part->local_GPs_base);
                part->local_GPs = NULL;
                kfree(part->remote_GPs_base);
                part->remote_GPs = NULL;
                kfree(part->local_openclose_args_base);
                part->local_openclose_args = NULL;
                kfree(part->remote_openclose_args_base);
                part->remote_openclose_args = NULL;
                dev_err(xpc_chan, "can't register NOTIFY IRQ handler, "
                        "errno=%d\n", -ret);
                return xpcLackOfResources;
        }

        /* Setup a timer to check for dropped IPIs */
        timer = &part->dropped_IPI_timer;
        init_timer(timer);
        timer->function = (void (*)(unsigned long)) xpc_dropped_IPI_check;
        timer->data = (unsigned long) part;
        timer->expires = jiffies + XPC_P_DROPPED_IPI_WAIT;
        add_timer(timer);

        /*
         * With the setting of the partition setup_state to XPC_P_SETUP, we're
         * declaring that this partition is ready to go.
         */
        part->setup_state = XPC_P_SETUP;


        /*
         * Setup the per partition specific variables required by the
         * remote partition to establish channel connections with us.
         *
         * The setting of the magic # indicates that these per partition
         * specific variables are ready to be used.
         */
        xpc_vars_part[partid].GPs_pa = __pa(part->local_GPs);
        xpc_vars_part[partid].openclose_args_pa =
                                        __pa(part->local_openclose_args);
        xpc_vars_part[partid].IPI_amo_pa = __pa(part->local_IPI_amo_va);
        cpuid = raw_smp_processor_id(); /* any CPU in this partition will do */
        xpc_vars_part[partid].IPI_nasid = cpuid_to_nasid(cpuid);
        xpc_vars_part[partid].IPI_phys_cpuid = cpu_physical_id(cpuid);
        xpc_vars_part[partid].nchannels = part->nchannels;
        xpc_vars_part[partid].magic = XPC_VP_MAGIC1;

        return xpcSuccess;
}


/*
 * Create a wrapper that hides the underlying mechanism for pulling a cacheline
 * (or multiple cachelines) from a remote partition.
 *
 * src must be a cacheline aligned physical address on the remote partition.
 * dst must be a cacheline aligned virtual address on this partition.
 * cnt must be an cacheline sized
 */
static enum xpc_retval
xpc_pull_remote_cachelines(struct xpc_partition *part, void *dst,
                                const void *src, size_t cnt)
{
        bte_result_t bte_ret;


        DBUG_ON((u64) src != L1_CACHE_ALIGN((u64) src));
        DBUG_ON((u64) dst != L1_CACHE_ALIGN((u64) dst));
        DBUG_ON(cnt != L1_CACHE_ALIGN(cnt));

        if (part->act_state == XPC_P_DEACTIVATING) {
                return part->reason;
        }

        bte_ret = xp_bte_copy((u64) src, (u64) ia64_tpa((u64) dst),
                                (u64) cnt, (BTE_NORMAL | BTE_WACQUIRE), NULL);
        if (bte_ret == BTE_SUCCESS) {
                return xpcSuccess;
        }

        dev_dbg(xpc_chan, "xp_bte_copy() from partition %d failed, ret=%d\n",
                XPC_PARTID(part), bte_ret);

        return xpc_map_bte_errors(bte_ret);
}


/*
 * Pull the remote per partititon specific variables from the specified
 * partition.
 */
enum xpc_retval
xpc_pull_remote_vars_part(struct xpc_partition *part)
{
        u8 buffer[L1_CACHE_BYTES * 2];
        struct xpc_vars_part *pulled_entry_cacheline =
                        (struct xpc_vars_part *) L1_CACHE_ALIGN((u64) buffer);
        struct xpc_vars_part *pulled_entry;
        u64 remote_entry_cacheline_pa, remote_entry_pa;
        partid_t partid = XPC_PARTID(part);
        enum xpc_retval ret;


        /* pull the cacheline that contains the variables we're interested in */

        DBUG_ON(part->remote_vars_part_pa !=
                                L1_CACHE_ALIGN(part->remote_vars_part_pa));
        DBUG_ON(sizeof(struct xpc_vars_part) != L1_CACHE_BYTES / 2);

        remote_entry_pa = part->remote_vars_part_pa +
                        sn_partition_id * sizeof(struct xpc_vars_part);

        remote_entry_cacheline_pa = (remote_entry_pa & ~(L1_CACHE_BYTES - 1));

        pulled_entry = (struct xpc_vars_part *) ((u64) pulled_entry_cacheline +
                                (remote_entry_pa & (L1_CACHE_BYTES - 1)));

        ret = xpc_pull_remote_cachelines(part, pulled_entry_cacheline,
                                        (void *) remote_entry_cacheline_pa,
                                        L1_CACHE_BYTES);
        if (ret != xpcSuccess) {
                dev_dbg(xpc_chan, "failed to pull XPC vars_part from "
                        "partition %d, ret=%d\n", partid, ret);
                return ret;
        }


        /* see if they've been set up yet */

        if (pulled_entry->magic != XPC_VP_MAGIC1 &&
                                pulled_entry->magic != XPC_VP_MAGIC2) {

                if (pulled_entry->magic != 0) {
                        dev_dbg(xpc_chan, "partition %d's XPC vars_part for "
                                "partition %d has bad magic value (=0x%lx)\n",
                                partid, sn_partition_id, pulled_entry->magic);
                        return xpcBadMagic;
                }

                /* they've not been initialized yet */
                return xpcRetry;
        }

        if (xpc_vars_part[partid].magic == XPC_VP_MAGIC1) {

                /* validate the variables */

                if (pulled_entry->GPs_pa == 0 ||
                                pulled_entry->openclose_args_pa == 0 ||
                                        pulled_entry->IPI_amo_pa == 0) {

                        dev_err(xpc_chan, "partition %d's XPC vars_part for "
                                "partition %d are not valid\n", partid,
                                sn_partition_id);
                        return xpcInvalidAddress;
                }

                /* the variables we imported look to be valid */

                part->remote_GPs_pa = pulled_entry->GPs_pa;
                part->remote_openclose_args_pa =
                                        pulled_entry->openclose_args_pa;
                part->remote_IPI_amo_va =
                                      (AMO_t *) __va(pulled_entry->IPI_amo_pa);
                part->remote_IPI_nasid = pulled_entry->IPI_nasid;
                part->remote_IPI_phys_cpuid = pulled_entry->IPI_phys_cpuid;

                if (part->nchannels > pulled_entry->nchannels) {
                        part->nchannels = pulled_entry->nchannels;
                }

                /* let the other side know that we've pulled their variables */

                xpc_vars_part[partid].magic = XPC_VP_MAGIC2;
        }

        if (pulled_entry->magic == XPC_VP_MAGIC1) {
                return xpcRetry;
        }

        return xpcSuccess;
}


/*
 * Get the IPI flags and pull the openclose args and/or remote GPs as needed.
 */
static u64
xpc_get_IPI_flags(struct xpc_partition *part)
{
        unsigned long irq_flags;
        u64 IPI_amo;
        enum xpc_retval ret;


        /*
         * See if there are any IPI flags to be handled.
         */

        spin_lock_irqsave(&part->IPI_lock, irq_flags);
        if ((IPI_amo = part->local_IPI_amo) != 0) {
                part->local_IPI_amo = 0;
        }
        spin_unlock_irqrestore(&part->IPI_lock, irq_flags);


        if (XPC_ANY_OPENCLOSE_IPI_FLAGS_SET(IPI_amo)) {
                ret = xpc_pull_remote_cachelines(part,
                                        part->remote_openclose_args,
                                        (void *) part->remote_openclose_args_pa,
                                        XPC_OPENCLOSE_ARGS_SIZE);
                if (ret != xpcSuccess) {
                        XPC_DEACTIVATE_PARTITION(part, ret);

                        dev_dbg(xpc_chan, "failed to pull openclose args from "
                                "partition %d, ret=%d\n", XPC_PARTID(part),
                                ret);

                        /* don't bother processing IPIs anymore */
                        IPI_amo = 0;
                }
        }

        if (XPC_ANY_MSG_IPI_FLAGS_SET(IPI_amo)) {
                ret = xpc_pull_remote_cachelines(part, part->remote_GPs,
                                                (void *) part->remote_GPs_pa,
                                                XPC_GP_SIZE);
                if (ret != xpcSuccess) {
                        XPC_DEACTIVATE_PARTITION(part, ret);

                        dev_dbg(xpc_chan, "failed to pull GPs from partition "
                                "%d, ret=%d\n", XPC_PARTID(part), ret);

                        /* don't bother processing IPIs anymore */
                        IPI_amo = 0;
                }
        }

        return IPI_amo;
}


/*
 * Allocate the local message queue and the notify queue.
 */
static enum xpc_retval
xpc_allocate_local_msgqueue(struct xpc_channel *ch)
{
        unsigned long irq_flags;
        int nentries;
        size_t nbytes;


        // >>> may want to check for ch->flags & XPC_C_DISCONNECTING between
        // >>> iterations of the for-loop, bail if set?

        // >>> should we impose a minumum #of entries? like 4 or 8?
        for (nentries = ch->local_nentries; nentries > 0; nentries--) {

                nbytes = nentries * ch->msg_size;
                ch->local_msgqueue = xpc_kmalloc_cacheline_aligned(nbytes,
                                                (GFP_KERNEL | GFP_DMA),
                                                &ch->local_msgqueue_base);
                if (ch->local_msgqueue == NULL) {
                        continue;
                }
                memset(ch->local_msgqueue, 0, nbytes);

                nbytes = nentries * sizeof(struct xpc_notify);
                ch->notify_queue = kmalloc(nbytes, (GFP_KERNEL | GFP_DMA));
                if (ch->notify_queue == NULL) {
                        kfree(ch->local_msgqueue_base);
                        ch->local_msgqueue = NULL;
                        continue;
                }
                memset(ch->notify_queue, 0, nbytes);

                spin_lock_irqsave(&ch->lock, irq_flags);
                if (nentries < ch->local_nentries) {
                        dev_dbg(xpc_chan, "nentries=%d local_nentries=%d, "
                                "partid=%d, channel=%d\n", nentries,
                                ch->local_nentries, ch->partid, ch->number);

                        ch->local_nentries = nentries;
                }
                spin_unlock_irqrestore(&ch->lock, irq_flags);
                return xpcSuccess;
        }

        dev_dbg(xpc_chan, "can't get memory for local message queue and notify "
                "queue, partid=%d, channel=%d\n", ch->partid, ch->number);
        return xpcNoMemory;
}


/*
 * Allocate the cached remote message queue.
 */
static enum xpc_retval
xpc_allocate_remote_msgqueue(struct xpc_channel *ch)
{
        unsigned long irq_flags;
        int nentries;
        size_t nbytes;


        DBUG_ON(ch->remote_nentries <= 0);

        // >>> may want to check for ch->flags & XPC_C_DISCONNECTING between
        // >>> iterations of the for-loop, bail if set?

        // >>> should we impose a minumum #of entries? like 4 or 8?
        for (nentries = ch->remote_nentries; nentries > 0; nentries--) {

                nbytes = nentries * ch->msg_size;
                ch->remote_msgqueue = xpc_kmalloc_cacheline_aligned(nbytes,
                                                (GFP_KERNEL | GFP_DMA),
                                                &ch->remote_msgqueue_base);
                if (ch->remote_msgqueue == NULL) {
                        continue;
                }
                memset(ch->remote_msgqueue, 0, nbytes);

                spin_lock_irqsave(&ch->lock, irq_flags);
                if (nentries < ch->remote_nentries) {
                        dev_dbg(xpc_chan, "nentries=%d remote_nentries=%d, "
                                "partid=%d, channel=%d\n", nentries,
                                ch->remote_nentries, ch->partid, ch->number);

                        ch->remote_nentries = nentries;
                }
                spin_unlock_irqrestore(&ch->lock, irq_flags);
                return xpcSuccess;
        }

        dev_dbg(xpc_chan, "can't get memory for cached remote message queue, "
                "partid=%d, channel=%d\n", ch->partid, ch->number);
        return xpcNoMemory;
}


/*
 * Allocate message queues and other stuff associated with a channel.
 *
 * Note: Assumes all of the channel sizes are filled in.
 */
static enum xpc_retval
xpc_allocate_msgqueues(struct xpc_channel *ch)
{
        unsigned long irq_flags;
        int i;
        enum xpc_retval ret;


        DBUG_ON(ch->flags & XPC_C_SETUP);

        if ((ret = xpc_allocate_local_msgqueue(ch)) != xpcSuccess) {
                return ret;
        }

        if ((ret = xpc_allocate_remote_msgqueue(ch)) != xpcSuccess) {
                kfree(ch->local_msgqueue_base);
                ch->local_msgqueue = NULL;
                kfree(ch->notify_queue);
                ch->notify_queue = NULL;
                return ret;
        }

        for (i = 0; i < ch->local_nentries; i++) {
                /* use a semaphore as an event wait queue */
                sema_init(&ch->notify_queue[i].sema, 0);
        }

        spin_lock_irqsave(&ch->lock, irq_flags);
        ch->flags |= XPC_C_SETUP;
        spin_unlock_irqrestore(&ch->lock, irq_flags);

        return xpcSuccess;
}


/*
 * Process a connect message from a remote partition.
 *
 * Note: xpc_process_connect() is expecting to be called with the
 * spin_lock_irqsave held and will leave it locked upon return.
 */
static void
xpc_process_connect(struct xpc_channel *ch, unsigned long *irq_flags)
{
        enum xpc_retval ret;


        DBUG_ON(!spin_is_locked(&ch->lock));

        if (!(ch->flags & XPC_C_OPENREQUEST) ||
                                !(ch->flags & XPC_C_ROPENREQUEST)) {
                /* nothing more to do for now */
                return;
        }
        DBUG_ON(!(ch->flags & XPC_C_CONNECTING));

        if (!(ch->flags & XPC_C_SETUP)) {
                spin_unlock_irqrestore(&ch->lock, *irq_flags);
                ret = xpc_allocate_msgqueues(ch);
                spin_lock_irqsave(&ch->lock, *irq_flags);

                if (ret != xpcSuccess) {
                        XPC_DISCONNECT_CHANNEL(ch, ret, irq_flags);
                }
                if (ch->flags & (XPC_C_CONNECTED | XPC_C_DISCONNECTING)) {
                        return;
                }

                DBUG_ON(!(ch->flags & XPC_C_SETUP));
                DBUG_ON(ch->local_msgqueue == NULL);
                DBUG_ON(ch->remote_msgqueue == NULL);
        }

        if (!(ch->flags & XPC_C_OPENREPLY)) {
                ch->flags |= XPC_C_OPENREPLY;
                xpc_IPI_send_openreply(ch, irq_flags);
        }

        if (!(ch->flags & XPC_C_ROPENREPLY)) {
                return;
        }

        DBUG_ON(ch->remote_msgqueue_pa == 0);

        ch->flags = (XPC_C_CONNECTED | XPC_C_SETUP);    /* clear all else */

        dev_info(xpc_chan, "channel %d to partition %d connected\n",
                ch->number, ch->partid);

        spin_unlock_irqrestore(&ch->lock, *irq_flags);
        xpc_create_kthreads(ch, 1);
        spin_lock_irqsave(&ch->lock, *irq_flags);
}


/*
 * Notify those who wanted to be notified upon delivery of their message.
 */
static void
xpc_notify_senders(struct xpc_channel *ch, enum xpc_retval reason, s64 put)
{
        struct xpc_notify *notify;
        u8 notify_type;
        s64 get = ch->w_remote_GP.get - 1;


        while (++get < put && atomic_read(&ch->n_to_notify) > 0) {

                notify = &ch->notify_queue[get % ch->local_nentries];

                /*
                 * See if the notify entry indicates it was associated with
                 * a message who's sender wants to be notified. It is possible
                 * that it is, but someone else is doing or has done the
                 * notification.
                 */
                notify_type = notify->type;
                if (notify_type == 0 ||
                                cmpxchg(&notify->type, notify_type, 0) !=
                                                                notify_type) {
                        continue;
                }

                DBUG_ON(notify_type != XPC_N_CALL);

                atomic_dec(&ch->n_to_notify);

                if (notify->func != NULL) {
                        dev_dbg(xpc_chan, "notify->func() called, notify=0x%p, "
                                "msg_number=%ld, partid=%d, channel=%d\n",
                                (void *) notify, get, ch->partid, ch->number);

                        notify->func(reason, ch->partid, ch->number,
                                                                notify->key);

                        dev_dbg(xpc_chan, "notify->func() returned, "
                                "notify=0x%p, msg_number=%ld, partid=%d, "
                                "channel=%d\n", (void *) notify, get,
                                ch->partid, ch->number);
                }
        }
}


/*
 * Free up message queues and other stuff that were allocated for the specified
 * channel.
 *
 * Note: ch->reason and ch->reason_line are left set for debugging purposes,
 * they're cleared when XPC_C_DISCONNECTED is cleared.
 */
static void
xpc_free_msgqueues(struct xpc_channel *ch)
{
        DBUG_ON(!spin_is_locked(&ch->lock));
        DBUG_ON(atomic_read(&ch->n_to_notify) != 0);

        ch->remote_msgqueue_pa = 0;
        ch->func = NULL;
        ch->key = NULL;
        ch->msg_size = 0;
        ch->local_nentries = 0;
        ch->remote_nentries = 0;
        ch->kthreads_assigned_limit = 0;
        ch->kthreads_idle_limit = 0;

        ch->local_GP->get = 0;
        ch->local_GP->put = 0;
        ch->remote_GP.get = 0;
        ch->remote_GP.put = 0;
        ch->w_local_GP.get = 0;
        ch->w_local_GP.put = 0;
        ch->w_remote_GP.get = 0;
        ch->w_remote_GP.put = 0;
        ch->next_msg_to_pull = 0;

        if (ch->flags & XPC_C_SETUP) {
                ch->flags &= ~XPC_C_SETUP;

                dev_dbg(xpc_chan, "ch->flags=0x%x, partid=%d, channel=%d\n",
                        ch->flags, ch->partid, ch->number);

                kfree(ch->local_msgqueue_base);
                ch->local_msgqueue = NULL;
                kfree(ch->remote_msgqueue_base);
                ch->remote_msgqueue = NULL;
                kfree(ch->notify_queue);
                ch->notify_queue = NULL;
        }
}


/*
 * spin_lock_irqsave() is expected to be held on entry.
 */
static void
xpc_process_disconnect(struct xpc_channel *ch, unsigned long *irq_flags)
{
        struct xpc_partition *part = &xpc_partitions[ch->partid];
        u32 channel_was_connected = (ch->flags & XPC_C_WASCONNECTED);


        DBUG_ON(!spin_is_locked(&ch->lock));

        if (!(ch->flags & XPC_C_DISCONNECTING)) {
                return;
        }

        DBUG_ON(!(ch->flags & XPC_C_CLOSEREQUEST));

        /* make sure all activity has settled down first */

        if (atomic_read(&ch->references) > 0) {
                return;
        }
        DBUG_ON(atomic_read(&ch->kthreads_assigned) != 0);

        if (part->act_state == XPC_P_DEACTIVATING) {
                /* can't proceed until the other side disengages from us */
                if (xpc_partition_engaged(1UL << ch->partid)) {
                        return;
                }

        } else {

                /* as long as the other side is up do the full protocol */

                if (!(ch->flags & XPC_C_RCLOSEREQUEST)) {
                        return;
                }

                if (!(ch->flags & XPC_C_CLOSEREPLY)) {
                        ch->flags |= XPC_C_CLOSEREPLY;
                        xpc_IPI_send_closereply(ch, irq_flags);
                }

                if (!(ch->flags & XPC_C_RCLOSEREPLY)) {
                        return;
                }
        }

        /* wake those waiting for notify completion */
        if (atomic_read(&ch->n_to_notify) > 0) {
                /* >>> we do callout while holding ch->lock */
                xpc_notify_senders(ch, ch->reason, ch->w_local_GP.put);
        }

        /* both sides are disconnected now */

        /* it's now safe to free the channel's message queues */
        xpc_free_msgqueues(ch);

        /* mark disconnected, clear all other flags except XPC_C_WDISCONNECT */
        ch->flags = (XPC_C_DISCONNECTED | (ch->flags & XPC_C_WDISCONNECT));

        atomic_dec(&part->nchannels_active);

        if (channel_was_connected) {
                dev_info(xpc_chan, "channel %d to partition %d disconnected, "
                        "reason=%d\n", ch->number, ch->partid, ch->reason);
        }

        /* wake the thread that is waiting for this channel to disconnect */
        if (ch->flags & XPC_C_WDISCONNECT) {
                spin_unlock_irqrestore(&ch->lock, *irq_flags);
                up(&ch->wdisconnect_sema);
                spin_lock_irqsave(&ch->lock, *irq_flags);
        }
}


/*
 * Process a change in the channel's remote connection state.
 */
static void
xpc_process_openclose_IPI(struct xpc_partition *part, int ch_number,
                                u8 IPI_flags)
{
        unsigned long irq_flags;
        struct xpc_openclose_args *args =
                                &part->remote_openclose_args[ch_number];
        struct xpc_channel *ch = &part->channels[ch_number];
        enum xpc_retval reason;



        spin_lock_irqsave(&ch->lock, irq_flags);


        if (IPI_flags & XPC_IPI_CLOSEREQUEST) {

                dev_dbg(xpc_chan, "XPC_IPI_CLOSEREQUEST (reason=%d) received "
                        "from partid=%d, channel=%d\n", args->reason,
                        ch->partid, ch->number);

                /*
                 * If RCLOSEREQUEST is set, we're probably waiting for
                 * RCLOSEREPLY. We should find it and a ROPENREQUEST packed
                 * with this RCLOSEREQUEST in the IPI_flags.
                 */

                if (ch->flags & XPC_C_RCLOSEREQUEST) {
                        DBUG_ON(!(ch->flags & XPC_C_DISCONNECTING));
                        DBUG_ON(!(ch->flags & XPC_C_CLOSEREQUEST));
                        DBUG_ON(!(ch->flags & XPC_C_CLOSEREPLY));
                        DBUG_ON(ch->flags & XPC_C_RCLOSEREPLY);

                        DBUG_ON(!(IPI_flags & XPC_IPI_CLOSEREPLY));
                        IPI_flags &= ~XPC_IPI_CLOSEREPLY;
                        ch->flags |= XPC_C_RCLOSEREPLY;

                        /* both sides have finished disconnecting */
                        xpc_process_disconnect(ch, &irq_flags);
                }

                if (ch->flags & XPC_C_DISCONNECTED) {
                        // >>> explain this section

                        if (!(IPI_flags & XPC_IPI_OPENREQUEST)) {
                                DBUG_ON(part->act_state !=
                                                        XPC_P_DEACTIVATING);
                                spin_unlock_irqrestore(&ch->lock, irq_flags);
                                return;
                        }

                        XPC_SET_REASON(ch, 0, 0);
                        ch->flags &= ~XPC_C_DISCONNECTED;

                        atomic_inc(&part->nchannels_active);
                        ch->flags |= (XPC_C_CONNECTING | XPC_C_ROPENREQUEST);
                }

                IPI_flags &= ~(XPC_IPI_OPENREQUEST | XPC_IPI_OPENREPLY);

                /*
                 * The meaningful CLOSEREQUEST connection state fields are:
                 *      reason = reason connection is to be closed
                 */

                ch->flags |= XPC_C_RCLOSEREQUEST;

                if (!(ch->flags & XPC_C_DISCONNECTING)) {
                        reason = args->reason;
                        if (reason <= xpcSuccess || reason > xpcUnknownReason) {
                                reason = xpcUnknownReason;
                        } else if (reason == xpcUnregistering) {
                                reason = xpcOtherUnregistering;
                        }

                        XPC_DISCONNECT_CHANNEL(ch, reason, &irq_flags);
                } else {
                        xpc_process_disconnect(ch, &irq_flags);
                }
        }


        if (IPI_flags & XPC_IPI_CLOSEREPLY) {

                dev_dbg(xpc_chan, "XPC_IPI_CLOSEREPLY received from partid=%d,"
                        " channel=%d\n", ch->partid, ch->number);

                if (ch->flags & XPC_C_DISCONNECTED) {
                        DBUG_ON(part->act_state != XPC_P_DEACTIVATING);
                        spin_unlock_irqrestore(&ch->lock, irq_flags);
                        return;
                }

                DBUG_ON(!(ch->flags & XPC_C_CLOSEREQUEST));
                DBUG_ON(!(ch->flags & XPC_C_RCLOSEREQUEST));

                ch->flags |= XPC_C_RCLOSEREPLY;

                if (ch->flags & XPC_C_CLOSEREPLY) {
                        /* both sides have finished disconnecting */
                        xpc_process_disconnect(ch, &irq_flags);
                }
        }


        if (IPI_flags & XPC_IPI_OPENREQUEST) {

                dev_dbg(xpc_chan, "XPC_IPI_OPENREQUEST (msg_size=%d, "
                        "local_nentries=%d) received from partid=%d, "
                        "channel=%d\n", args->msg_size, args->local_nentries,
                        ch->partid, ch->number);

                if ((ch->flags & (XPC_C_DISCONNECTING | XPC_C_WDISCONNECT)) ||
                                        part->act_state == XPC_P_DEACTIVATING) {
                        spin_unlock_irqrestore(&ch->lock, irq_flags);
                        return;
                }
                DBUG_ON(!(ch->flags & (XPC_C_DISCONNECTED |
                                                        XPC_C_OPENREQUEST)));
                DBUG_ON(ch->flags & (XPC_C_ROPENREQUEST | XPC_C_ROPENREPLY |
                                        XPC_C_OPENREPLY | XPC_C_CONNECTED));

                /*
                 * The meaningful OPENREQUEST connection state fields are:
                 *      msg_size = size of channel's messages in bytes
                 *      local_nentries = remote partition's local_nentries
                 */
                DBUG_ON(args->msg_size == 0);
                DBUG_ON(args->local_nentries == 0);

                ch->flags |= (XPC_C_ROPENREQUEST | XPC_C_CONNECTING);
                ch->remote_nentries = args->local_nentries;


                if (ch->flags & XPC_C_OPENREQUEST) {
                        if (args->msg_size != ch->msg_size) {
                                XPC_DISCONNECT_CHANNEL(ch, xpcUnequalMsgSizes,
                                                                &irq_flags);
                                spin_unlock_irqrestore(&ch->lock, irq_flags);
                                return;
                        }
                } else {
                        ch->msg_size = args->msg_size;

                        XPC_SET_REASON(ch, 0, 0);
                        ch->flags &= ~XPC_C_DISCONNECTED;

                        atomic_inc(&part->nchannels_active);
                }

                xpc_process_connect(ch, &irq_flags);
        }


        if (IPI_flags & XPC_IPI_OPENREPLY) {

                dev_dbg(xpc_chan, "XPC_IPI_OPENREPLY (local_msgqueue_pa=0x%lx, "
                        "local_nentries=%d, remote_nentries=%d) received from "
                        "partid=%d, channel=%d\n", args->local_msgqueue_pa,
                        args->local_nentries, args->remote_nentries,
                        ch->partid, ch->number);

                if (ch->flags & (XPC_C_DISCONNECTING | XPC_C_DISCONNECTED)) {
                        spin_unlock_irqrestore(&ch->lock, irq_flags);
                        return;
                }
                DBUG_ON(!(ch->flags & XPC_C_OPENREQUEST));
                DBUG_ON(!(ch->flags & XPC_C_ROPENREQUEST));
                DBUG_ON(ch->flags & XPC_C_CONNECTED);

                /*
                 * The meaningful OPENREPLY connection state fields are:
                 *      local_msgqueue_pa = physical address of remote
                 *                          partition's local_msgqueue
                 *      local_nentries = remote partition's local_nentries
                 *      remote_nentries = remote partition's remote_nentries
                 */
                DBUG_ON(args->local_msgqueue_pa == 0);
                DBUG_ON(args->local_nentries == 0);
                DBUG_ON(args->remote_nentries == 0);

                ch->flags |= XPC_C_ROPENREPLY;
                ch->remote_msgqueue_pa = args->local_msgqueue_pa;

                if (args->local_nentries < ch->remote_nentries) {
                        dev_dbg(xpc_chan, "XPC_IPI_OPENREPLY: new "
                                "remote_nentries=%d, old remote_nentries=%d, "
                                "partid=%d, channel=%d\n",
                                args->local_nentries, ch->remote_nentries,
                                ch->partid, ch->number);

                        ch->remote_nentries = args->local_nentries;
                }
                if (args->remote_nentries < ch->local_nentries) {
                        dev_dbg(xpc_chan, "XPC_IPI_OPENREPLY: new "
                                "local_nentries=%d, old local_nentries=%d, "
                                "partid=%d, channel=%d\n",
                                args->remote_nentries, ch->local_nentries,
                                ch->partid, ch->number);

                        ch->local_nentries = args->remote_nentries;
                }

                xpc_process_connect(ch, &irq_flags);
        }

        spin_unlock_irqrestore(&ch->lock, irq_flags);
}


/*
 * Attempt to establish a channel connection to a remote partition.
 */
static enum xpc_retval
xpc_connect_channel(struct xpc_channel *ch)
{
        unsigned long irq_flags;
        struct xpc_registration *registration = &xpc_registrations[ch->number];


        if (down_interruptible(&registration->sema) != 0) {
                return xpcInterrupted;
        }

        if (!XPC_CHANNEL_REGISTERED(ch->number)) {
                up(&registration->sema);
                return xpcUnregistered;
        }

        spin_lock_irqsave(&ch->lock, irq_flags);

        DBUG_ON(ch->flags & XPC_C_CONNECTED);
        DBUG_ON(ch->flags & XPC_C_OPENREQUEST);

        if (ch->flags & XPC_C_DISCONNECTING) {
                spin_unlock_irqrestore(&ch->lock, irq_flags);
                up(&registration->sema);
                return ch->reason;
        }


        /* add info from the channel connect registration to the channel */

        ch->kthreads_assigned_limit = registration->assigned_limit;
        ch->kthreads_idle_limit = registration->idle_limit;
        DBUG_ON(atomic_read(&ch->kthreads_assigned) != 0);
        DBUG_ON(atomic_read(&ch->kthreads_idle) != 0);
        DBUG_ON(atomic_read(&ch->kthreads_active) != 0);

        ch->func = registration->func;
        DBUG_ON(registration->func == NULL);
        ch->key = registration->key;

        ch->local_nentries = registration->nentries;

        if (ch->flags & XPC_C_ROPENREQUEST) {
                if (registration->msg_size != ch->msg_size) {
                        /* the local and remote sides aren't the same */

                        /*
                         * Because XPC_DISCONNECT_CHANNEL() can block we're
                         * forced to up the registration sema before we unlock
                         * the channel lock. But that's okay here because we're
                         * done with the part that required the registration
                         * sema. XPC_DISCONNECT_CHANNEL() requires that the
                         * channel lock be locked and will unlock and relock
                         * the channel lock as needed.
                         */
                        up(&registration->sema);
                        XPC_DISCONNECT_CHANNEL(ch, xpcUnequalMsgSizes,
                                                                &irq_flags);
                        spin_unlock_irqrestore(&ch->lock, irq_flags);
                        return xpcUnequalMsgSizes;
                }
        } else {
                ch->msg_size = registration->msg_size;

                XPC_SET_REASON(ch, 0, 0);
                ch->flags &= ~XPC_C_DISCONNECTED;

                atomic_inc(&xpc_partitions[ch->partid].nchannels_active);
        }

        up(&registration->sema);


        /* initiate the connection */

        ch->flags |= (XPC_C_OPENREQUEST | XPC_C_CONNECTING);
        xpc_IPI_send_openrequest(ch, &irq_flags);

        xpc_process_connect(ch, &irq_flags);

        spin_unlock_irqrestore(&ch->lock, irq_flags);

        return xpcSuccess;
}


/*
 * Clear some of the msg flags in the local message queue.
 */
static inline void
xpc_clear_local_msgqueue_flags(struct xpc_channel *ch)
{
        struct xpc_msg *msg;
        s64 get;


        get = ch->w_remote_GP.get;
        do {
                msg = (struct xpc_msg *) ((u64) ch->local_msgqueue +
                                (get % ch->local_nentries) * ch->msg_size);
                msg->flags = 0;
        } while (++get < (volatile s64) ch->remote_GP.get);
}


/*
 * Clear some of the msg flags in the remote message queue.
 */
static inline void
xpc_clear_remote_msgqueue_flags(struct xpc_channel *ch)
{
        struct xpc_msg *msg;
        s64 put;


        put = ch->w_remote_GP.put;
        do {
                msg = (struct xpc_msg *) ((u64) ch->remote_msgqueue +
                                (put % ch->remote_nentries) * ch->msg_size);
                msg->flags = 0;
        } while (++put < (volatile s64) ch->remote_GP.put);
}


static void
xpc_process_msg_IPI(struct xpc_partition *part, int ch_number)
{
        struct xpc_channel *ch = &part->channels[ch_number];
        int nmsgs_sent;


        ch->remote_GP = part->remote_GPs[ch_number];


        /* See what, if anything, has changed for each connected channel */

        xpc_msgqueue_ref(ch);

        if (ch->w_remote_GP.get == ch->remote_GP.get &&
                                ch->w_remote_GP.put == ch->remote_GP.put) {
                /* nothing changed since GPs were last pulled */
                xpc_msgqueue_deref(ch);
                return;
        }

        if (!(ch->flags & XPC_C_CONNECTED)){
                xpc_msgqueue_deref(ch);
                return;
        }


        /*
         * First check to see if messages recently sent by us have been
         * received by the other side. (The remote GET value will have
         * changed since we last looked at it.)
         */

        if (ch->w_remote_GP.get != ch->remote_GP.get) {

                /*
                 * We need to notify any senders that want to be notified
                 * that their sent messages have been received by their
                 * intended recipients. We need to do this before updating
                 * w_remote_GP.get so that we don't allocate the same message
                 * queue entries prematurely (see xpc_allocate_msg()).
                 */
                if (atomic_read(&ch->n_to_notify) > 0) {
                        /*
                         * Notify senders that messages sent have been
                         * received and delivered by the other side.
                         */
                        xpc_notify_senders(ch, xpcMsgDelivered,
                                                        ch->remote_GP.get);
                }

                /*
                 * Clear msg->flags in previously sent messages, so that
                 * they're ready for xpc_allocate_msg().
                 */
                xpc_clear_local_msgqueue_flags(ch);

                ch->w_remote_GP.get = ch->remote_GP.get;

                dev_dbg(xpc_chan, "w_remote_GP.get changed to %ld, partid=%d, "
                        "channel=%d\n", ch->w_remote_GP.get, ch->partid,
                        ch->number);

                /*
                 * If anyone was waiting for message queue entries to become
                 * available, wake them up.
                 */
                if (atomic_read(&ch->n_on_msg_allocate_wq) > 0) {
                        wake_up(&ch->msg_allocate_wq);
                }
        }


        /*
         * Now check for newly sent messages by the other side. (The remote
         * PUT value will have changed since we last looked at it.)
         */

        if (ch->w_remote_GP.put != ch->remote_GP.put) {
                /*
                 * Clear msg->flags in previously received messages, so that
                 * they're ready for xpc_get_deliverable_msg().
                 */
                xpc_clear_remote_msgqueue_flags(ch);

                ch->w_remote_GP.put = ch->remote_GP.put;

                dev_dbg(xpc_chan, "w_remote_GP.put changed to %ld, partid=%d, "
                        "channel=%d\n", ch->w_remote_GP.put, ch->partid,
                        ch->number);

                nmsgs_sent = ch->w_remote_GP.put - ch->w_local_GP.get;
                if (nmsgs_sent > 0) {
                        dev_dbg(xpc_chan, "msgs waiting to be copied and "
                                "delivered=%d, partid=%d, channel=%d\n",
                                nmsgs_sent, ch->partid, ch->number);

                        if (ch->flags & XPC_C_CONNECTCALLOUT) {
                                xpc_activate_kthreads(ch, nmsgs_sent);
                        }
                }
        }

        xpc_msgqueue_deref(ch);
}


void
xpc_process_channel_activity(struct xpc_partition *part)
{
        unsigned long irq_flags;
        u64 IPI_amo, IPI_flags;
        struct xpc_channel *ch;
        int ch_number;
        u32 ch_flags;


        IPI_amo = xpc_get_IPI_flags(part);

        /*
         * Initiate channel connections for registered channels.
         *
         * For each connected channel that has pending messages activate idle
         * kthreads and/or create new kthreads as needed.
         */

        for (ch_number = 0; ch_number < part->nchannels; ch_number++) {
                ch = &part->channels[ch_number];


                /*
                 * Process any open or close related IPI flags, and then deal
                 * with connecting or disconnecting the channel as required.
                 */

                IPI_flags = XPC_GET_IPI_FLAGS(IPI_amo, ch_number);

                if (XPC_ANY_OPENCLOSE_IPI_FLAGS_SET(IPI_flags)) {
                        xpc_process_openclose_IPI(part, ch_number, IPI_flags);
                }

                ch_flags = ch->flags;   /* need an atomic snapshot of flags */

                if (ch_flags & XPC_C_DISCONNECTING) {
                        spin_lock_irqsave(&ch->lock, irq_flags);
                        xpc_process_disconnect(ch, &irq_flags);
                        spin_unlock_irqrestore(&ch->lock, irq_flags);
                        continue;
                }

                if (part->act_state == XPC_P_DEACTIVATING) {
                        continue;
                }

                if (!(ch_flags & XPC_C_CONNECTED)) {
                        if (!(ch_flags & XPC_C_OPENREQUEST)) {
                                DBUG_ON(ch_flags & XPC_C_SETUP);
                                (void) xpc_connect_channel(ch);
                        } else {
                                spin_lock_irqsave(&ch->lock, irq_flags);
                                xpc_process_connect(ch, &irq_flags);
                                spin_unlock_irqrestore(&ch->lock, irq_flags);
                        }
                        continue;
                }


                /*
                 * Process any message related IPI flags, this may involve the
                 * activation of kthreads to deliver any pending messages sent
                 * from the other partition.
                 */

                if (XPC_ANY_MSG_IPI_FLAGS_SET(IPI_flags)) {
                        xpc_process_msg_IPI(part, ch_number);
                }
        }
}


/*
 * XPC's heartbeat code calls this function to inform XPC that a partition is
 * going down.  XPC responds by tearing down the XPartition Communication
 * infrastructure used for the just downed partition.
 *
 * XPC's heartbeat code will never call this function and xpc_partition_up()
 * at the same time. Nor will it ever make multiple calls to either function
 * at the same time.
 */
void
xpc_partition_going_down(struct xpc_partition *part, enum xpc_retval reason)
{
        unsigned long irq_flags;
        int ch_number;
        struct xpc_channel *ch;


        dev_dbg(xpc_chan, "deactivating partition %d, reason=%d\n",
                XPC_PARTID(part), reason);

        if (!xpc_part_ref(part)) {
                /* infrastructure for this partition isn't currently set up */
                return;
        }


        /* disconnect channels associated with the partition going down */

        for (ch_number = 0; ch_number < part->nchannels; ch_number++) {
                ch = &part->channels[ch_number];

                xpc_msgqueue_ref(ch);
                spin_lock_irqsave(&ch->lock, irq_flags);

                XPC_DISCONNECT_CHANNEL(ch, reason, &irq_flags);

                spin_unlock_irqrestore(&ch->lock, irq_flags);
                xpc_msgqueue_deref(ch);
        }

        xpc_wakeup_channel_mgr(part);

        xpc_part_deref(part);
}


/*
 * Teardown the infrastructure necessary to support XPartition Communication
 * between the specified remote partition and the local one.
 */
void
xpc_teardown_infrastructure(struct xpc_partition *part)
{
        partid_t partid = XPC_PARTID(part);


        /*
         * We start off by making this partition inaccessible to local
         * processes by marking it as no longer setup. Then we make it
         * inaccessible to remote processes by clearing the XPC per partition
         * specific variable's magic # (which indicates that these variables
         * are no longer valid) and by ignoring all XPC notify IPIs sent to
         * this partition.
         */

        DBUG_ON(atomic_read(&part->nchannels_engaged) != 0);
        DBUG_ON(atomic_read(&part->nchannels_active) != 0);
        DBUG_ON(part->setup_state != XPC_P_SETUP);
        part->setup_state = XPC_P_WTEARDOWN;

        xpc_vars_part[partid].magic = 0;


        free_irq(SGI_XPC_NOTIFY, (void *) (u64) partid);


        /*
         * Before proceding with the teardown we have to wait until all
         * existing references cease.
         */
        wait_event(part->teardown_wq, (atomic_read(&part->references) == 0));


        /* now we can begin tearing down the infrastructure */

        part->setup_state = XPC_P_TORNDOWN;

        /* in case we've still got outstanding timers registered... */
        del_timer_sync(&part->dropped_IPI_timer);

        kfree(part->remote_openclose_args_base);
        part->remote_openclose_args = NULL;
        kfree(part->local_openclose_args_base);
        part->local_openclose_args = NULL;
        kfree(part->remote_GPs_base);
        part->remote_GPs = NULL;
        kfree(part->local_GPs_base);
        part->local_GPs = NULL;
        kfree(part->channels);
        part->channels = NULL;
        part->local_IPI_amo_va = NULL;
}


/*
 * Called by XP at the time of channel connection registration to cause
 * XPC to establish connections to all currently active partitions.
 */
void
xpc_initiate_connect(int ch_number)
{
        partid_t partid;
        struct xpc_partition *part;
        struct xpc_channel *ch;


        DBUG_ON(ch_number < 0 || ch_number >= XPC_NCHANNELS);

        for (partid = 1; partid < XP_MAX_PARTITIONS; partid++) {
                part = &xpc_partitions[partid];

                if (xpc_part_ref(part)) {
                        ch = &part->channels[ch_number];

                        if (!(ch->flags & XPC_C_DISCONNECTING)) {
                                DBUG_ON(ch->flags & XPC_C_OPENREQUEST);
                                DBUG_ON(ch->flags & XPC_C_CONNECTED);
                                DBUG_ON(ch->flags & XPC_C_SETUP);

                                /*
                                 * Initiate the establishment of a connection
                                 * on the newly registered channel to the
                                 * remote partition.
                                 */
                                xpc_wakeup_channel_mgr(part);
                        }

                        xpc_part_deref(part);
                }
        }
}


void
xpc_connected_callout(struct xpc_channel *ch)
{
        unsigned long irq_flags;


        /* let the registerer know that a connection has been established */

        if (ch->func != NULL) {
                dev_dbg(xpc_chan, "ch->func() called, reason=xpcConnected, "
                        "partid=%d, channel=%d\n", ch->partid, ch->number);

                ch->func(xpcConnected, ch->partid, ch->number,
                                (void *) (u64) ch->local_nentries, ch->key);

                dev_dbg(xpc_chan, "ch->func() returned, reason=xpcConnected, "
                        "partid=%d, channel=%d\n", ch->partid, ch->number);
        }

        spin_lock_irqsave(&ch->lock, irq_flags);
        ch->flags |= XPC_C_CONNECTCALLOUT;
        spin_unlock_irqrestore(&ch->lock, irq_flags);
}


/*
 * Called by XP at the time of channel connection unregistration to cause
 * XPC to teardown all current connections for the specified channel.
 *
 * Before returning xpc_initiate_disconnect() will wait until all connections
 * on the specified channel have been closed/torndown. So the caller can be
 * assured that they will not be receiving any more callouts from XPC to the
 * function they registered via xpc_connect().
 *
 * Arguments:
 *
 *      ch_number - channel # to unregister.
 */
void
xpc_initiate_disconnect(int ch_number)
{
        unsigned long irq_flags;
        partid_t partid;
        struct xpc_partition *part;
        struct xpc_channel *ch;


        DBUG_ON(ch_number < 0 || ch_number >= XPC_NCHANNELS);

        /* initiate the channel disconnect for every active partition */
        for (partid = 1; partid < XP_MAX_PARTITIONS; partid++) {
                part = &xpc_partitions[partid];

                if (xpc_part_ref(part)) {
                        ch = &part->channels[ch_number];
                        xpc_msgqueue_ref(ch);

                        spin_lock_irqsave(&ch->lock, irq_flags);

                        if (!(ch->flags & XPC_C_DISCONNECTED)) {
                                ch->flags |= XPC_C_WDISCONNECT;

                                XPC_DISCONNECT_CHANNEL(ch, xpcUnregistering,
                                                                &irq_flags);
                        }

                        spin_unlock_irqrestore(&ch->lock, irq_flags);

                        xpc_msgqueue_deref(ch);
                        xpc_part_deref(part);
                }
        }

        xpc_disconnect_wait(ch_number);
}


/*
 * To disconnect a channel, and reflect it back to all who may be waiting.
 *
 * An OPEN is not allowed until XPC_C_DISCONNECTING is cleared by
 * xpc_process_disconnect(), and if set, XPC_C_WDISCONNECT is cleared by
 * xpc_disconnect_wait().
 *
 * THE CHANNEL IS TO BE LOCKED BY THE CALLER AND WILL REMAIN LOCKED UPON RETURN.
 */
void
xpc_disconnect_channel(const int line, struct xpc_channel *ch,
                        enum xpc_retval reason, unsigned long *irq_flags)
{
        u32 channel_was_connected = (ch->flags & XPC_C_CONNECTED);


        DBUG_ON(!spin_is_locked(&ch->lock));

        if (ch->flags & (XPC_C_DISCONNECTING | XPC_C_DISCONNECTED)) {
                return;
        }
        DBUG_ON(!(ch->flags & (XPC_C_CONNECTING | XPC_C_CONNECTED)));

        dev_dbg(xpc_chan, "reason=%d, line=%d, partid=%d, channel=%d\n",
                reason, line, ch->partid, ch->number);

        XPC_SET_REASON(ch, reason, line);

        ch->flags |= (XPC_C_CLOSEREQUEST | XPC_C_DISCONNECTING);
        /* some of these may not have been set */
        ch->flags &= ~(XPC_C_OPENREQUEST | XPC_C_OPENREPLY |
                        XPC_C_ROPENREQUEST | XPC_C_ROPENREPLY |
                        XPC_C_CONNECTING | XPC_C_CONNECTED);

        xpc_IPI_send_closerequest(ch, irq_flags);

        if (channel_was_connected) {
                ch->flags |= XPC_C_WASCONNECTED;
        }

        spin_unlock_irqrestore(&ch->lock, *irq_flags);

        /* wake all idle kthreads so they can exit */
        if (atomic_read(&ch->kthreads_idle) > 0) {
                wake_up_all(&ch->idle_wq);
        }

        /* wake those waiting to allocate an entry from the local msg queue */
        if (atomic_read(&ch->n_on_msg_allocate_wq) > 0) {
                wake_up(&ch->msg_allocate_wq);
        }

        spin_lock_irqsave(&ch->lock, *irq_flags);
}


void
xpc_disconnecting_callout(struct xpc_channel *ch)
{
        /*
         * Let the channel's registerer know that the channel is being
         * disconnected. We don't want to do this if the registerer was never
         * informed of a connection being made.
         */

        if (ch->func != NULL) {
                dev_dbg(xpc_chan, "ch->func() called, reason=xpcDisconnecting,"
                        " partid=%d, channel=%d\n", ch->partid, ch->number);

                ch->func(xpcDisconnecting, ch->partid, ch->number, NULL,
                                                                ch->key);

                dev_dbg(xpc_chan, "ch->func() returned, reason="
                        "xpcDisconnecting, partid=%d, channel=%d\n",
                        ch->partid, ch->number);
        }
}


/*
 * Wait for a message entry to become available for the specified channel,
 * but don't wait any longer than 1 jiffy.
 */
static enum xpc_retval
xpc_allocate_msg_wait(struct xpc_channel *ch)
{
        enum xpc_retval ret;


        if (ch->flags & XPC_C_DISCONNECTING) {
                DBUG_ON(ch->reason == xpcInterrupted);  // >>> Is this true?
                return ch->reason;
        }

        atomic_inc(&ch->n_on_msg_allocate_wq);
        ret = interruptible_sleep_on_timeout(&ch->msg_allocate_wq, 1);
        atomic_dec(&ch->n_on_msg_allocate_wq);

        if (ch->flags & XPC_C_DISCONNECTING) {
                ret = ch->reason;
                DBUG_ON(ch->reason == xpcInterrupted);  // >>> Is this true?
        } else if (ret == 0) {
                ret = xpcTimeout;
        } else {
                ret = xpcInterrupted;
        }

        return ret;
}


/*
 * Allocate an entry for a message from the message queue associated with the
 * specified channel.
 */
static enum xpc_retval
xpc_allocate_msg(struct xpc_channel *ch, u32 flags,
                        struct xpc_msg **address_of_msg)
{
        struct xpc_msg *msg;
        enum xpc_retval ret;
        s64 put;


        /* this reference will be dropped in xpc_send_msg() */
        xpc_msgqueue_ref(ch);

        if (ch->flags & XPC_C_DISCONNECTING) {
                xpc_msgqueue_deref(ch);
                return ch->reason;
        }
        if (!(ch->flags & XPC_C_CONNECTED)) {
                xpc_msgqueue_deref(ch);
                return xpcNotConnected;
        }


        /*
         * Get the next available message entry from the local message queue.
         * If none are available, we'll make sure that we grab the latest
         * GP values.
         */
        ret = xpcTimeout;

        while (1) {

                put = (volatile s64) ch->w_local_GP.put;
                if (put - (volatile s64) ch->w_remote_GP.get <
                                                        ch->local_nentries) {

                        /* There are available message entries. We need to try
                         * to secure one for ourselves. We'll do this by trying
                         * to increment w_local_GP.put as long as someone else
                         * doesn't beat us to it. If they do, we'll have to
                         * try again.
                         */
                        if (cmpxchg(&ch->w_local_GP.put, put, put + 1) ==
                                                                        put) {
                                /* we got the entry referenced by put */
                                break;
                        }
                        continue;       /* try again */
                }


                /*
                 * There aren't any available msg entries at this time.
                 *
                 * In waiting for a message entry to become available,
                 * we set a timeout in case the other side is not
                 * sending completion IPIs. This lets us fake an IPI
                 * that will cause the IPI handler to fetch the latest
                 * GP values as if an IPI was sent by the other side.
                 */
                if (ret == xpcTimeout) {
                        xpc_IPI_send_local_msgrequest(ch);
                }

                if (flags & XPC_NOWAIT) {
                        xpc_msgqueue_deref(ch);
                        return xpcNoWait;
                }

                ret = xpc_allocate_msg_wait(ch);
                if (ret != xpcInterrupted && ret != xpcTimeout) {
                        xpc_msgqueue_deref(ch);
                        return ret;
                }
        }


        /* get the message's address and initialize it */
        msg = (struct xpc_msg *) ((u64) ch->local_msgqueue +
                                (put % ch->local_nentries) * ch->msg_size);


        DBUG_ON(msg->flags != 0);
        msg->number = put;

        dev_dbg(xpc_chan, "w_local_GP.put changed to %ld; msg=0x%p, "
                "msg_number=%ld, partid=%d, channel=%d\n", put + 1,
                (void *) msg, msg->number, ch->partid, ch->number);

        *address_of_msg = msg;

        return xpcSuccess;
}


/*
 * Allocate an entry for a message from the message queue associated with the
 * specified channel. NOTE that this routine can sleep waiting for a message
 * entry to become available. To not sleep, pass in the XPC_NOWAIT flag.
 *
 * Arguments:
 *
 *      partid - ID of partition to which the channel is connected.
 *      ch_number - channel #.
 *      flags - see xpc.h for valid flags.
 *      payload - address of the allocated payload area pointer (filled in on
 *                return) in which the user-defined message is constructed.
 */
enum xpc_retval
xpc_initiate_allocate(partid_t partid, int ch_number, u32 flags, void **payload)
{
        struct xpc_partition *part = &xpc_partitions[partid];
        enum xpc_retval ret = xpcUnknownReason;
        struct xpc_msg *msg;


        DBUG_ON(partid <= 0 || partid >= XP_MAX_PARTITIONS);
        DBUG_ON(ch_number < 0 || ch_number >= part->nchannels);

        *payload = NULL;

        if (xpc_part_ref(part)) {
                ret = xpc_allocate_msg(&part->channels[ch_number], flags, &msg);
                xpc_part_deref(part);

                if (msg != NULL) {
                        *payload = &msg->payload;
                }
        }

        return ret;
}


/*
 * Now we actually send the messages that are ready to be sent by advancing
 * the local message queue's Put value and then send an IPI to the recipient
 * partition.
 */
static void
xpc_send_msgs(struct xpc_channel *ch, s64 initial_put)
{
        struct xpc_msg *msg;
        s64 put = initial_put + 1;
        int send_IPI = 0;


        while (1) {

                while (1) {
                        if (put == (volatile s64) ch->w_local_GP.put) {
                                break;
                        }

                        msg = (struct xpc_msg *) ((u64) ch->local_msgqueue +
                               (put % ch->local_nentries) * ch->msg_size);

                        if (!(msg->flags & XPC_M_READY)) {
                                break;
                        }

                        put++;
                }

                if (put == initial_put) {
                        /* nothing's changed */
                        break;
                }

                if (cmpxchg_rel(&ch->local_GP->put, initial_put, put) !=
                                                                initial_put) {
                        /* someone else beat us to it */
                        DBUG_ON((volatile s64) ch->local_GP->put < initial_put);
                        break;
                }

                /* we just set the new value of local_GP->put */

                dev_dbg(xpc_chan, "local_GP->put changed to %ld, partid=%d, "
                        "channel=%d\n", put, ch->partid, ch->number);

                send_IPI = 1;

                /*
                 * We need to ensure that the message referenced by
                 * local_GP->put is not XPC_M_READY or that local_GP->put
                 * equals w_local_GP.put, so we'll go have a look.
                 */
                initial_put = put;
        }

        if (send_IPI) {
                xpc_IPI_send_msgrequest(ch);
        }
}


/*
 * Common code that does the actual sending of the message by advancing the
 * local message queue's Put value and sends an IPI to the partition the
 * message is being sent to.
 */
static enum xpc_retval
xpc_send_msg(struct xpc_channel *ch, struct xpc_msg *msg, u8 notify_type,
                        xpc_notify_func func, void *key)
{
        enum xpc_retval ret = xpcSuccess;
        struct xpc_notify *notify = notify;
        s64 put, msg_number = msg->number;


        DBUG_ON(notify_type == XPC_N_CALL && func == NULL);
        DBUG_ON((((u64) msg - (u64) ch->local_msgqueue) / ch->msg_size) !=
                                        msg_number % ch->local_nentries);
        DBUG_ON(msg->flags & XPC_M_READY);

        if (ch->flags & XPC_C_DISCONNECTING) {
                /* drop the reference grabbed in xpc_allocate_msg() */
                xpc_msgqueue_deref(ch);
                return ch->reason;
        }

        if (notify_type != 0) {
                /*
                 * Tell the remote side to send an ACK interrupt when the
                 * message has been delivered.
                 */
                msg->flags |= XPC_M_INTERRUPT;

                atomic_inc(&ch->n_to_notify);

                notify = &ch->notify_queue[msg_number % ch->local_nentries];
                notify->func = func;
                notify->key = key;
                notify->type = notify_type;

                // >>> is a mb() needed here?

                if (ch->flags & XPC_C_DISCONNECTING) {
                        /*
                         * An error occurred between our last error check and
                         * this one. We will try to clear the type field from
                         * the notify entry. If we succeed then
                         * xpc_disconnect_channel() didn't already process
                         * the notify entry.
                         */
                        if (cmpxchg(&notify->type, notify_type, 0) ==
                                                                notify_type) {
                                atomic_dec(&ch->n_to_notify);
                                ret = ch->reason;
                        }

                        /* drop the reference grabbed in xpc_allocate_msg() */
                        xpc_msgqueue_deref(ch);
                        return ret;
                }
        }

        msg->flags |= XPC_M_READY;

        /*
         * The preceding store of msg->flags must occur before the following
         * load of ch->local_GP->put.
         */
        mb();

        /* see if the message is next in line to be sent, if so send it */

        put = ch->local_GP->put;
        if (put == msg_number) {
                xpc_send_msgs(ch, put);
        }

        /* drop the reference grabbed in xpc_allocate_msg() */
        xpc_msgqueue_deref(ch);
        return ret;
}


/*
 * Send a message previously allocated using xpc_initiate_allocate() on the
 * specified channel connected to the specified partition.
 *
 * This routine will not wait for the message to be received, nor will
 * notification be given when it does happen. Once this routine has returned
 * the message entry allocated via xpc_initiate_allocate() is no longer
 * accessable to the caller.
 *
 * This routine, although called by users, does not call xpc_part_ref() to
 * ensure that the partition infrastructure is in place. It relies on the
 * fact that we called xpc_msgqueue_ref() in xpc_allocate_msg().
 *
 * Arguments:
 *
 *      partid - ID of partition to which the channel is connected.
 *      ch_number - channel # to send message on.
 *      payload - pointer to the payload area allocated via
 *                      xpc_initiate_allocate().
 */
enum xpc_retval
xpc_initiate_send(partid_t partid, int ch_number, void *payload)
{
        struct xpc_partition *part = &xpc_partitions[partid];
        struct xpc_msg *msg = XPC_MSG_ADDRESS(payload);
        enum xpc_retval ret;


        dev_dbg(xpc_chan, "msg=0x%p, partid=%d, channel=%d\n", (void *) msg,
                partid, ch_number);

        DBUG_ON(partid <= 0 || partid >= XP_MAX_PARTITIONS);
        DBUG_ON(ch_number < 0 || ch_number >= part->nchannels);
        DBUG_ON(msg == NULL);

        ret = xpc_send_msg(&part->channels[ch_number], msg, 0, NULL, NULL);

        return ret;
}


/*
 * Send a message previously allocated using xpc_initiate_allocate on the
 * specified channel connected to the specified partition.
 *
 * This routine will not wait for the message to be sent. Once this routine
 * has returned the message entry allocated via xpc_initiate_allocate() is no
 * longer accessable to the caller.
 *
 * Once the remote end of the channel has received the message, the function
 * passed as an argument to xpc_initiate_send_notify() will be called. This
 * allows the sender to free up or re-use any buffers referenced by the
 * message, but does NOT mean the message has been processed at the remote
 * end by a receiver.
 *
 * If this routine returns an error, the caller's function will NOT be called.
 *
 * This routine, although called by users, does not call xpc_part_ref() to
 * ensure that the partition infrastructure is in place. It relies on the
 * fact that we called xpc_msgqueue_ref() in xpc_allocate_msg().
 *
 * Arguments:
 *
 *      partid - ID of partition to which the channel is connected.
 *      ch_number - channel # to send message on.
 *      payload - pointer to the payload area allocated via
 *                      xpc_initiate_allocate().
 *      func - function to call with asynchronous notification of message
 *                receipt. THIS FUNCTION MUST BE NON-BLOCKING.
 *      key - user-defined key to be passed to the function when it's called.
 */
enum xpc_retval
xpc_initiate_send_notify(partid_t partid, int ch_number, void *payload,
                                xpc_notify_func func, void *key)
{
        struct xpc_partition *part = &xpc_partitions[partid];
        struct xpc_msg *msg = XPC_MSG_ADDRESS(payload);
        enum xpc_retval ret;


        dev_dbg(xpc_chan, "msg=0x%p, partid=%d, channel=%d\n", (void *) msg,
                partid, ch_number);

        DBUG_ON(partid <= 0 || partid >= XP_MAX_PARTITIONS);
        DBUG_ON(ch_number < 0 || ch_number >= part->nchannels);
        DBUG_ON(msg == NULL);
        DBUG_ON(func == NULL);

        ret = xpc_send_msg(&part->channels[ch_number], msg, XPC_N_CALL,
                                                                func, key);
        return ret;
}


static struct xpc_msg *
xpc_pull_remote_msg(struct xpc_channel *ch, s64 get)
{
        struct xpc_partition *part = &xpc_partitions[ch->partid];
        struct xpc_msg *remote_msg, *msg;
        u32 msg_index, nmsgs;
        u64 msg_offset;
        enum xpc_retval ret;


        if (down_interruptible(&ch->msg_to_pull_sema) != 0) {
                /* we were interrupted by a signal */
                return NULL;
        }

        while (get >= ch->next_msg_to_pull) {

                /* pull as many messages as are ready and able to be pulled */

                msg_index = ch->next_msg_to_pull % ch->remote_nentries;

                DBUG_ON(ch->next_msg_to_pull >=
                                        (volatile s64) ch->w_remote_GP.put);
                nmsgs =  (volatile s64) ch->w_remote_GP.put -
                                                ch->next_msg_to_pull;
                if (msg_index + nmsgs > ch->remote_nentries) {
                        /* ignore the ones that wrap the msg queue for now */
                        nmsgs = ch->remote_nentries - msg_index;
                }

                msg_offset = msg_index * ch->msg_size;
                msg = (struct xpc_msg *) ((u64) ch->remote_msgqueue +
                                                                msg_offset);
                remote_msg = (struct xpc_msg *) (ch->remote_msgqueue_pa +
                                                                msg_offset);

                if ((ret = xpc_pull_remote_cachelines(part, msg, remote_msg,
                                nmsgs * ch->msg_size)) != xpcSuccess) {

                        dev_dbg(xpc_chan, "failed to pull %d msgs starting with"
                                " msg %ld from partition %d, channel=%d, "
                                "ret=%d\n", nmsgs, ch->next_msg_to_pull,
                                ch->partid, ch->number, ret);

                        XPC_DEACTIVATE_PARTITION(part, ret);

                        up(&ch->msg_to_pull_sema);
                        return NULL;
                }

                mb();   /* >>> this may not be needed, we're not sure */

                ch->next_msg_to_pull += nmsgs;
        }

        up(&ch->msg_to_pull_sema);

        /* return the message we were looking for */
        msg_offset = (get % ch->remote_nentries) * ch->msg_size;
        msg = (struct xpc_msg *) ((u64) ch->remote_msgqueue + msg_offset);

        return msg;
}


/*
 * Get a message to be delivered.
 */
static struct xpc_msg *
xpc_get_deliverable_msg(struct xpc_channel *ch)
{
        struct xpc_msg *msg = NULL;
        s64 get;


        do {
                if ((volatile u32) ch->flags & XPC_C_DISCONNECTING) {
                        break;
                }

                get = (volatile s64) ch->w_local_GP.get;
                if (get == (volatile s64) ch->w_remote_GP.put) {
                        break;
                }

                /* There are messages waiting to be pulled and delivered.
                 * We need to try to secure one for ourselves. We'll do this
                 * by trying to increment w_local_GP.get and hope that no one
                 * else beats us to it. If they do, we'll we'll simply have
                 * to try again for the next one.
                 */

                if (cmpxchg(&ch->w_local_GP.get, get, get + 1) == get) {
                        /* we got the entry referenced by get */

                        dev_dbg(xpc_chan, "w_local_GP.get changed to %ld, "
                                "partid=%d, channel=%d\n", get + 1,
                                ch->partid, ch->number);

                        /* pull the message from the remote partition */

                        msg = xpc_pull_remote_msg(ch, get);

                        DBUG_ON(msg != NULL && msg->number != get);
                        DBUG_ON(msg != NULL && (msg->flags & XPC_M_DONE));
                        DBUG_ON(msg != NULL && !(msg->flags & XPC_M_READY));

                        break;
                }

        } while (1);

        return msg;
}


/*
 * Deliver a message to its intended recipient.
 */
void
xpc_deliver_msg(struct xpc_channel *ch)
{
        struct xpc_msg *msg;


        if ((msg = xpc_get_deliverable_msg(ch)) != NULL) {

                /*
                 * This ref is taken to protect the payload itself from being
                 * freed before the user is finished with it, which the user
                 * indicates by calling xpc_initiate_received().
                 */
                xpc_msgqueue_ref(ch);

                atomic_inc(&ch->kthreads_active);

                if (ch->func != NULL) {
                        dev_dbg(xpc_chan, "ch->func() called, msg=0x%p, "
                                "msg_number=%ld, partid=%d, channel=%d\n",
                                (void *) msg, msg->number, ch->partid,
                                ch->number);

                        /* deliver the message to its intended recipient */
                        ch->func(xpcMsgReceived, ch->partid, ch->number,
                                        &msg->payload, ch->key);

                        dev_dbg(xpc_chan, "ch->func() returned, msg=0x%p, "
                                "msg_number=%ld, partid=%d, channel=%d\n",
                                (void *) msg, msg->number, ch->partid,
                                ch->number);
                }

                atomic_dec(&ch->kthreads_active);
        }
}


/*
 * Now we actually acknowledge the messages that have been delivered and ack'd
 * by advancing the cached remote message queue's Get value and if requested
 * send an IPI to the message sender's partition.
 */
static void
xpc_acknowledge_msgs(struct xpc_channel *ch, s64 initial_get, u8 msg_flags)
{
        struct xpc_msg *msg;
        s64 get = initial_get + 1;
        int send_IPI = 0;


        while (1) {

                while (1) {
                        if (get == (volatile s64) ch->w_local_GP.get) {
                                break;
                        }

                        msg = (struct xpc_msg *) ((u64) ch->remote_msgqueue +
                               (get % ch->remote_nentries) * ch->msg_size);

                        if (!(msg->flags & XPC_M_DONE)) {
                                break;
                        }

                        msg_flags |= msg->flags;
                        get++;
                }

                if (get == initial_get) {
                        /* nothing's changed */
                        break;
                }

                if (cmpxchg_rel(&ch->local_GP->get, initial_get, get) !=
                                                                initial_get) {
                        /* someone else beat us to it */
                        DBUG_ON((volatile s64) ch->local_GP->get <=
                                                                initial_get);
                        break;
                }

                /* we just set the new value of local_GP->get */

                dev_dbg(xpc_chan, "local_GP->get changed to %ld, partid=%d, "
                        "channel=%d\n", get, ch->partid, ch->number);

                send_IPI = (msg_flags & XPC_M_INTERRUPT);

                /*
                 * We need to ensure that the message referenced by
                 * local_GP->get is not XPC_M_DONE or that local_GP->get
                 * equals w_local_GP.get, so we'll go have a look.
                 */
                initial_get = get;
        }

        if (send_IPI) {
                xpc_IPI_send_msgrequest(ch);
        }
}


/*
 * Acknowledge receipt of a delivered message.
 *
 * If a message has XPC_M_INTERRUPT set, send an interrupt to the partition
 * that sent the message.
 *
 * This function, although called by users, does not call xpc_part_ref() to
 * ensure that the partition infrastructure is in place. It relies on the
 * fact that we called xpc_msgqueue_ref() in xpc_deliver_msg().
 *
 * Arguments:
 *
 *      partid - ID of partition to which the channel is connected.
 *      ch_number - channel # message received on.
 *      payload - pointer to the payload area allocated via
 *                      xpc_initiate_allocate().
 */
void
xpc_initiate_received(partid_t partid, int ch_number, void *payload)
{
        struct xpc_partition *part = &xpc_partitions[partid];
        struct xpc_channel *ch;
        struct xpc_msg *msg = XPC_MSG_ADDRESS(payload);
        s64 get, msg_number = msg->number;


        DBUG_ON(partid <= 0 || partid >= XP_MAX_PARTITIONS);
        DBUG_ON(ch_number < 0 || ch_number >= part->nchannels);

        ch = &part->channels[ch_number];

        dev_dbg(xpc_chan, "msg=0x%p, msg_number=%ld, partid=%d, channel=%d\n",
                (void *) msg, msg_number, ch->partid, ch->number);

        DBUG_ON((((u64) msg - (u64) ch->remote_msgqueue) / ch->msg_size) !=
                                        msg_number % ch->remote_nentries);
        DBUG_ON(msg->flags & XPC_M_DONE);

        msg->flags |= XPC_M_DONE;

        /*
         * The preceding store of msg->flags must occur before the following
         * load of ch->local_GP->get.
         */
        mb();

        /*
         * See if this message is next in line to be acknowledged as having
         * been delivered.
         */
        get = ch->local_GP->get;
        if (get == msg_number) {
                xpc_acknowledge_msgs(ch, get, msg->flags);
        }

        /* the call to xpc_msgqueue_ref() was done by xpc_deliver_msg()  */
        xpc_msgqueue_deref(ch);
}