kernel - usched_dfly revamp (6), reimplement shared spinlocks & misc others

[dragonfly.git] / sys / kern / lwkt_ipiq.c

/*
 * Copyright (c) 2003,2004 The DragonFly Project.  All rights reserved.
 * 
 * This code is derived from software contributed to The DragonFly Project
 * by Matthew Dillon <dillon@backplane.com>
 * 
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name of The DragonFly Project nor the names of its
 *    contributors may be used to endorse or promote products derived
 *    from this software without specific, prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * This module implements IPI message queueing and the MI portion of IPI
 * message processing.
 */

#include "opt_ddb.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/rtprio.h>
#include <sys/queue.h>
#include <sys/thread2.h>
#include <sys/sysctl.h>
#include <sys/ktr.h>
#include <sys/kthread.h>
#include <machine/cpu.h>
#include <sys/lock.h>
#include <sys/caps.h>

#include <vm/vm.h>
#include <vm/vm_param.h>
#include <vm/vm_kern.h>
#include <vm/vm_object.h>
#include <vm/vm_page.h>
#include <vm/vm_map.h>
#include <vm/vm_pager.h>
#include <vm/vm_extern.h>
#include <vm/vm_zone.h>

#include <machine/stdarg.h>
#include <machine/smp.h>
#include <machine/atomic.h>

#ifdef SMP
static __int64_t ipiq_count;    /* total calls to lwkt_send_ipiq*() */
static __int64_t ipiq_fifofull; /* number of fifo full conditions detected */
static __int64_t ipiq_avoided;  /* interlock with target avoids cpu ipi */
static __int64_t ipiq_passive;  /* passive IPI messages */
static __int64_t ipiq_cscount;  /* number of cpu synchronizations */
static int ipiq_debug;          /* set to 1 for debug */
#ifdef PANIC_DEBUG
static int      panic_ipiq_cpu = -1;
static int      panic_ipiq_count = 100;
#endif
#endif

#ifdef SMP
SYSCTL_QUAD(_lwkt, OID_AUTO, ipiq_count, CTLFLAG_RW, &ipiq_count, 0,
    "Number of IPI's sent");
SYSCTL_QUAD(_lwkt, OID_AUTO, ipiq_fifofull, CTLFLAG_RW, &ipiq_fifofull, 0,
    "Number of fifo full conditions detected");
SYSCTL_QUAD(_lwkt, OID_AUTO, ipiq_avoided, CTLFLAG_RW, &ipiq_avoided, 0,
    "Number of IPI's avoided by interlock with target cpu");
SYSCTL_QUAD(_lwkt, OID_AUTO, ipiq_passive, CTLFLAG_RW, &ipiq_passive, 0,
    "Number of passive IPI messages sent");
SYSCTL_QUAD(_lwkt, OID_AUTO, ipiq_cscount, CTLFLAG_RW, &ipiq_cscount, 0,
    "Number of cpu synchronizations");
SYSCTL_INT(_lwkt, OID_AUTO, ipiq_debug, CTLFLAG_RW, &ipiq_debug, 0,
    "");
#ifdef PANIC_DEBUG
SYSCTL_INT(_lwkt, OID_AUTO, panic_ipiq_cpu, CTLFLAG_RW, &panic_ipiq_cpu, 0, "");
SYSCTL_INT(_lwkt, OID_AUTO, panic_ipiq_count, CTLFLAG_RW, &panic_ipiq_count, 0, "");
#endif

#define IPIQ_STRING     "func=%p arg1=%p arg2=%d scpu=%d dcpu=%d"
#define IPIQ_ARGS       void *func, void *arg1, int arg2, int scpu, int dcpu

#if !defined(KTR_IPIQ)
#define KTR_IPIQ        KTR_ALL
#endif
KTR_INFO_MASTER(ipiq);
KTR_INFO(KTR_IPIQ, ipiq, send_norm, 0, IPIQ_STRING, IPIQ_ARGS);
KTR_INFO(KTR_IPIQ, ipiq, send_pasv, 1, IPIQ_STRING, IPIQ_ARGS);
KTR_INFO(KTR_IPIQ, ipiq, send_nbio, 2, IPIQ_STRING, IPIQ_ARGS);
KTR_INFO(KTR_IPIQ, ipiq, send_fail, 3, IPIQ_STRING, IPIQ_ARGS);
KTR_INFO(KTR_IPIQ, ipiq, receive, 4, IPIQ_STRING, IPIQ_ARGS);
KTR_INFO(KTR_IPIQ, ipiq, sync_start, 5, "cpumask=%08lx", unsigned long mask);
KTR_INFO(KTR_IPIQ, ipiq, sync_end, 6, "cpumask=%08lx", unsigned long mask);
KTR_INFO(KTR_IPIQ, ipiq, cpu_send, 7, IPIQ_STRING, IPIQ_ARGS);
KTR_INFO(KTR_IPIQ, ipiq, send_end, 8, IPIQ_STRING, IPIQ_ARGS);

#define logipiq(name, func, arg1, arg2, sgd, dgd)       \
        KTR_LOG(ipiq_ ## name, func, arg1, arg2, sgd->gd_cpuid, dgd->gd_cpuid)
#define logipiq2(name, arg)     \
        KTR_LOG(ipiq_ ## name, arg)

#endif  /* SMP */

#ifdef SMP

static int lwkt_process_ipiq_core(globaldata_t sgd, lwkt_ipiq_t ip, 
                                  struct intrframe *frame);
static void lwkt_cpusync_remote1(lwkt_cpusync_t cs);
static void lwkt_cpusync_remote2(lwkt_cpusync_t cs);

/*
 * Send a function execution request to another cpu.  The request is queued
 * on the cpu<->cpu ipiq matrix.  Each cpu owns a unique ipiq FIFO for every
 * possible target cpu.  The FIFO can be written.
 *
 * If the FIFO fills up we have to enable interrupts to avoid an APIC
 * deadlock and process pending IPIQs while waiting for it to empty.   
 * Otherwise we may soft-deadlock with another cpu whos FIFO is also full.
 *
 * We can safely bump gd_intr_nesting_level because our crit_exit() at the
 * end will take care of any pending interrupts.
 *
 * The actual hardware IPI is avoided if the target cpu is already processing
 * the queue from a prior IPI.  It is possible to pipeline IPI messages
 * very quickly between cpus due to the FIFO hysteresis.
 *
 * Need not be called from a critical section.
 */
int
lwkt_send_ipiq3(globaldata_t target, ipifunc3_t func, void *arg1, int arg2)
{
    lwkt_ipiq_t ip;
    int windex;
    struct globaldata *gd = mycpu;

    logipiq(send_norm, func, arg1, arg2, gd, target);

    if (target == gd) {
        func(arg1, arg2, NULL);
        logipiq(send_end, func, arg1, arg2, gd, target);
        return(0);
    } 
    crit_enter();
    ++gd->gd_intr_nesting_level;
#ifdef INVARIANTS
    if (gd->gd_intr_nesting_level > 20)
        panic("lwkt_send_ipiq: TOO HEAVILY NESTED!");
#endif
    KKASSERT(curthread->td_critcount);
    ++ipiq_count;
    ip = &gd->gd_ipiq[target->gd_cpuid];

    /*
     * Do not allow the FIFO to become full.  Interrupts must be physically
     * enabled while we liveloop to avoid deadlocking the APIC.
     *
     * The target ipiq may have gotten filled up due to passive IPIs and thus
     * not be aware that its queue is too full, so be sure to issue an
     * ipiq interrupt to the target cpu.
     */
    if (ip->ip_windex - ip->ip_rindex > MAXCPUFIFO / 2) {
#if defined(__i386__)
        unsigned int eflags = read_eflags();
#elif defined(__x86_64__)
        unsigned long rflags = read_rflags();
#endif

        cpu_enable_intr();
        ++ipiq_fifofull;
        DEBUG_PUSH_INFO("send_ipiq3");
        while (ip->ip_windex - ip->ip_rindex > MAXCPUFIFO / 4) {
            if (atomic_poll_acquire_int(&target->gd_npoll)) {
                logipiq(cpu_send, func, arg1, arg2, gd, target);
                cpu_send_ipiq(target->gd_cpuid);
            }
            KKASSERT(ip->ip_windex - ip->ip_rindex != MAXCPUFIFO - 1);
            lwkt_process_ipiq();
            cpu_pause();
        }
        DEBUG_POP_INFO();
#if defined(__i386__)
        write_eflags(eflags);
#elif defined(__x86_64__)
        write_rflags(rflags);
#endif
    }

    /*
     * Queue the new message
     */
    windex = ip->ip_windex & MAXCPUFIFO_MASK;
    ip->ip_info[windex].func = func;
    ip->ip_info[windex].arg1 = arg1;
    ip->ip_info[windex].arg2 = arg2;
    cpu_sfence();
    ++ip->ip_windex;
    atomic_set_cpumask(&target->gd_ipimask, gd->gd_cpumask);

    /*
     * signal the target cpu that there is work pending.
     */
    if (atomic_poll_acquire_int(&target->gd_npoll)) {
        logipiq(cpu_send, func, arg1, arg2, gd, target);
        cpu_send_ipiq(target->gd_cpuid);
    } else {
        ++ipiq_avoided;
    }
    --gd->gd_intr_nesting_level;
    crit_exit();
    logipiq(send_end, func, arg1, arg2, gd, target);

    return(ip->ip_windex);
}

/*
 * Similar to lwkt_send_ipiq() but this function does not actually initiate
 * the IPI to the target cpu unless the FIFO has become too full, so it is
 * very fast.
 *
 * This function is used for non-critical IPI messages, such as memory
 * deallocations.  The queue will typically be flushed by the target cpu at
 * the next clock interrupt.
 *
 * Need not be called from a critical section.
 */
int
lwkt_send_ipiq3_passive(globaldata_t target, ipifunc3_t func,
                        void *arg1, int arg2)
{
    lwkt_ipiq_t ip;
    int windex;
    struct globaldata *gd = mycpu;

    KKASSERT(target != gd);
    crit_enter();
    ++gd->gd_intr_nesting_level;
    logipiq(send_pasv, func, arg1, arg2, gd, target);
#ifdef INVARIANTS
    if (gd->gd_intr_nesting_level > 20)
        panic("lwkt_send_ipiq: TOO HEAVILY NESTED!");
#endif
    KKASSERT(curthread->td_critcount);
    ++ipiq_count;
    ++ipiq_passive;
    ip = &gd->gd_ipiq[target->gd_cpuid];

    /*
     * Do not allow the FIFO to become full.  Interrupts must be physically
     * enabled while we liveloop to avoid deadlocking the APIC.
     */
    if (ip->ip_windex - ip->ip_rindex > MAXCPUFIFO / 2) {
#if defined(__i386__)
        unsigned int eflags = read_eflags();
#elif defined(__x86_64__)
        unsigned long rflags = read_rflags();
#endif

        cpu_enable_intr();
        ++ipiq_fifofull;
        DEBUG_PUSH_INFO("send_ipiq3_passive");
        while (ip->ip_windex - ip->ip_rindex > MAXCPUFIFO / 4) {
            if (atomic_poll_acquire_int(&target->gd_npoll)) {
                logipiq(cpu_send, func, arg1, arg2, gd, target);
                cpu_send_ipiq(target->gd_cpuid);
            }
            KKASSERT(ip->ip_windex - ip->ip_rindex != MAXCPUFIFO - 1);
            lwkt_process_ipiq();
            cpu_pause();
        }
        DEBUG_POP_INFO();
#if defined(__i386__)
        write_eflags(eflags);
#elif defined(__x86_64__)
        write_rflags(rflags);
#endif
    }

    /*
     * Queue the new message
     */
    windex = ip->ip_windex & MAXCPUFIFO_MASK;
    ip->ip_info[windex].func = func;
    ip->ip_info[windex].arg1 = arg1;
    ip->ip_info[windex].arg2 = arg2;
    cpu_sfence();
    ++ip->ip_windex;
    atomic_set_cpumask(&target->gd_ipimask, gd->gd_cpumask);
    --gd->gd_intr_nesting_level;

    /*
     * Do not signal the target cpu, it will pick up the IPI when it next
     * polls (typically on the next tick).
     */
    crit_exit();
    logipiq(send_end, func, arg1, arg2, gd, target);

    return(ip->ip_windex);
}

/*
 * Send an IPI request without blocking, return 0 on success, ENOENT on 
 * failure.  The actual queueing of the hardware IPI may still force us
 * to spin and process incoming IPIs but that will eventually go away
 * when we've gotten rid of the other general IPIs.
 */
int
lwkt_send_ipiq3_nowait(globaldata_t target, ipifunc3_t func, 
                       void *arg1, int arg2)
{
    lwkt_ipiq_t ip;
    int windex;
    struct globaldata *gd = mycpu;

    logipiq(send_nbio, func, arg1, arg2, gd, target);
    KKASSERT(curthread->td_critcount);
    if (target == gd) {
        func(arg1, arg2, NULL);
        logipiq(send_end, func, arg1, arg2, gd, target);
        return(0);
    } 
    crit_enter();
    ++gd->gd_intr_nesting_level;
    ++ipiq_count;
    ip = &gd->gd_ipiq[target->gd_cpuid];

    if (ip->ip_windex - ip->ip_rindex >= MAXCPUFIFO * 2 / 3) {
        logipiq(send_fail, func, arg1, arg2, gd, target);
        --gd->gd_intr_nesting_level;
        crit_exit();
        return(ENOENT);
    }
    windex = ip->ip_windex & MAXCPUFIFO_MASK;
    ip->ip_info[windex].func = func;
    ip->ip_info[windex].arg1 = arg1;
    ip->ip_info[windex].arg2 = arg2;
    cpu_sfence();
    ++ip->ip_windex;
    atomic_set_cpumask(&target->gd_ipimask, gd->gd_cpumask);

    /*
     * This isn't a passive IPI, we still have to signal the target cpu.
     */
    if (atomic_poll_acquire_int(&target->gd_npoll)) {
        logipiq(cpu_send, func, arg1, arg2, gd, target);
        cpu_send_ipiq(target->gd_cpuid);
    } else {
        ++ipiq_avoided;
    }
    --gd->gd_intr_nesting_level;
    crit_exit();

    logipiq(send_end, func, arg1, arg2, gd, target);
    return(0);
}

/*
 * deprecated, used only by fast int forwarding.
 */
int
lwkt_send_ipiq3_bycpu(int dcpu, ipifunc3_t func, void *arg1, int arg2)
{
    return(lwkt_send_ipiq3(globaldata_find(dcpu), func, arg1, arg2));
}

/*
 * Send a message to several target cpus.  Typically used for scheduling.
 * The message will not be sent to stopped cpus.
 */
int
lwkt_send_ipiq3_mask(cpumask_t mask, ipifunc3_t func, void *arg1, int arg2)
{
    int cpuid;
    int count = 0;

    mask &= ~stopped_cpus;
    while (mask) {
        cpuid = BSFCPUMASK(mask);
        lwkt_send_ipiq3(globaldata_find(cpuid), func, arg1, arg2);
        mask &= ~CPUMASK(cpuid);
        ++count;
    }
    return(count);
}

/*
 * Wait for the remote cpu to finish processing a function.
 *
 * YYY we have to enable interrupts and process the IPIQ while waiting
 * for it to empty or we may deadlock with another cpu.  Create a CPU_*()
 * function to do this!  YYY we really should 'block' here.
 *
 * MUST be called from a critical section.  This routine may be called
 * from an interrupt (for example, if an interrupt wakes a foreign thread
 * up).
 */
void
lwkt_wait_ipiq(globaldata_t target, int seq)
{
    lwkt_ipiq_t ip;
    int maxc = 100000000;

    if (target != mycpu) {
        ip = &mycpu->gd_ipiq[target->gd_cpuid];
        if ((int)(ip->ip_xindex - seq) < 0) {
#if defined(__i386__)
            unsigned int eflags = read_eflags();
#elif defined(__x86_64__)
            unsigned long rflags = read_rflags();
#endif
            cpu_enable_intr();
            DEBUG_PUSH_INFO("wait_ipiq");
            while ((int)(ip->ip_xindex - seq) < 0) {
                crit_enter();
                lwkt_process_ipiq();
                crit_exit();
                if (--maxc == 0)
                        kprintf("LWKT_WAIT_IPIQ WARNING! %d wait %d (%d)\n", mycpu->gd_cpuid, target->gd_cpuid, ip->ip_xindex - seq);
                if (maxc < -1000000)
                        panic("LWKT_WAIT_IPIQ");
                /*
                 * xindex may be modified by another cpu, use a load fence
                 * to ensure that the loop does not use a speculative value
                 * (which may improve performance).
                 */
                cpu_lfence();
            }
            DEBUG_POP_INFO();
#if defined(__i386__)
            write_eflags(eflags);
#elif defined(__x86_64__)
            write_rflags(rflags);
#endif
        }
    }
}

int
lwkt_seq_ipiq(globaldata_t target)
{
    lwkt_ipiq_t ip;

    ip = &mycpu->gd_ipiq[target->gd_cpuid];
    return(ip->ip_windex);
}

/*
 * Called from IPI interrupt (like a fast interrupt), which has placed
 * us in a critical section.  The MP lock may or may not be held.
 * May also be called from doreti or splz, or be reentrantly called
 * indirectly through the ip_info[].func we run.
 *
 * There are two versions, one where no interrupt frame is available (when
 * called from the send code and from splz, and one where an interrupt
 * frame is available.
 *
 * When the current cpu is mastering a cpusync we do NOT internally loop
 * on the cpusyncq poll.  We also do not re-flag a pending ipi due to
 * the cpusyncq poll because this can cause doreti/splz to loop internally.
 * The cpusync master's own loop must be allowed to run to avoid a deadlock.
 */
void
lwkt_process_ipiq(void)
{
    globaldata_t gd = mycpu;
    globaldata_t sgd;
    lwkt_ipiq_t ip;
    cpumask_t mask;
    int n;

    ++gd->gd_processing_ipiq;
again:
    cpu_lfence();
    mask = gd->gd_ipimask;
    atomic_clear_cpumask(&gd->gd_ipimask, mask);
    while (mask) {
        n = BSFCPUMASK(mask);
        if (n != gd->gd_cpuid) {
            sgd = globaldata_find(n);
            ip = sgd->gd_ipiq;
            if (ip != NULL) {
                while (lwkt_process_ipiq_core(sgd, &ip[gd->gd_cpuid], NULL))
                    ;
            }
        }
        mask &= ~CPUMASK(n);
    }

    /*
     * Process pending cpusyncs.  If the current thread has a cpusync
     * active cpusync we only run the list once and do not re-flag
     * as the thread itself is processing its interlock.
     */
    if (lwkt_process_ipiq_core(gd, &gd->gd_cpusyncq, NULL)) {
        if (gd->gd_curthread->td_cscount == 0)
            goto again;
        /* need_ipiq(); do not reflag */
    }

    /*
     * Interlock to allow more IPI interrupts.  Recheck ipimask after
     * releasing gd_npoll.
     */
    if (gd->gd_ipimask)
        goto again;
    atomic_poll_release_int(&gd->gd_npoll);
    cpu_mfence();
    if (gd->gd_ipimask)
        goto again;
    --gd->gd_processing_ipiq;
}

void
lwkt_process_ipiq_frame(struct intrframe *frame)
{
    globaldata_t gd = mycpu;
    globaldata_t sgd;
    lwkt_ipiq_t ip;
    cpumask_t mask;
    int n;

again:
    cpu_lfence();
    mask = gd->gd_ipimask;
    atomic_clear_cpumask(&gd->gd_ipimask, mask);
    while (mask) {
        n = BSFCPUMASK(mask);
        if (n != gd->gd_cpuid) {
            sgd = globaldata_find(n);
            ip = sgd->gd_ipiq;
            if (ip != NULL) {
                while (lwkt_process_ipiq_core(sgd, &ip[gd->gd_cpuid], frame))
                    ;
            }
        }
        mask &= ~CPUMASK(n);
    }
    if (gd->gd_cpusyncq.ip_rindex != gd->gd_cpusyncq.ip_windex) {
        if (lwkt_process_ipiq_core(gd, &gd->gd_cpusyncq, frame)) {
            if (gd->gd_curthread->td_cscount == 0)
                goto again;
            /* need_ipiq(); do not reflag */
        }
    }

    /*
     * Interlock to allow more IPI interrupts.  Recheck ipimask after
     * releasing gd_npoll.
     */
    if (gd->gd_ipimask)
        goto again;
    atomic_poll_release_int(&gd->gd_npoll);
    cpu_mfence();
    if (gd->gd_ipimask)
        goto again;
}

#if 0
static int iqticks[SMP_MAXCPU];
static int iqcount[SMP_MAXCPU];
#endif
#if 0
static int iqterm[SMP_MAXCPU];
#endif

static int
lwkt_process_ipiq_core(globaldata_t sgd, lwkt_ipiq_t ip, 
                       struct intrframe *frame)
{
    globaldata_t mygd = mycpu;
    int ri;
    int wi;
    ipifunc3_t copy_func;
    void *copy_arg1;
    int copy_arg2;

#if 0
    if (iqticks[mygd->gd_cpuid] != ticks) {
            iqticks[mygd->gd_cpuid] = ticks;
            iqcount[mygd->gd_cpuid] = 0;
    }
    if (++iqcount[mygd->gd_cpuid] > 3000000) {
        kprintf("cpu %d ipiq maxed cscount %d spin %d\n",
                mygd->gd_cpuid,
                mygd->gd_curthread->td_cscount,
                mygd->gd_spinlocks);
        iqcount[mygd->gd_cpuid] = 0;
#if 0
        if (++iqterm[mygd->gd_cpuid] > 10)
                panic("cpu %d ipiq maxed", mygd->gd_cpuid);
#endif
        int i;
        for (i = 0; i < ncpus; ++i) {
                if (globaldata_find(i)->gd_infomsg)
                        kprintf(" %s", globaldata_find(i)->gd_infomsg);
        }
        kprintf("\n");
    }
#endif

    /*
     * Clear the originating core from our ipimask, we will process all
     * incoming messages.
     *
     * Obtain the current write index, which is modified by a remote cpu.
     * Issue a load fence to prevent speculative reads of e.g. data written
     * by the other cpu prior to it updating the index.
     */
    KKASSERT(curthread->td_critcount);
    wi = ip->ip_windex;
    cpu_lfence();
    ++mygd->gd_intr_nesting_level;

    /*
     * NOTE: xindex is only updated after we are sure the function has
     *       finished execution.  Beware lwkt_process_ipiq() reentrancy!
     *       The function may send an IPI which may block/drain.
     *
     * NOTE: Due to additional IPI operations that the callback function
     *       may make, it is possible for both rindex and windex to advance and
     *       thus for rindex to advance passed our cached windex.
     *
     * NOTE: A load fence is required to prevent speculative loads prior
     *       to the loading of ip_rindex.  Even though stores might be
     *       ordered, loads are probably not.  A memory fence is required
     *       to prevent reordering of the loads after the ip_rindex update.
     *
     * NOTE: Single pass only.  Returns non-zero if the queue is not empty
     *       on return.
     */
    while (wi - (ri = ip->ip_rindex) > 0) {
        ri &= MAXCPUFIFO_MASK;
        cpu_lfence();
        copy_func = ip->ip_info[ri].func;
        copy_arg1 = ip->ip_info[ri].arg1;
        copy_arg2 = ip->ip_info[ri].arg2;
        cpu_mfence();
        ++ip->ip_rindex;
        KKASSERT((ip->ip_rindex & MAXCPUFIFO_MASK) ==
                 ((ri + 1) & MAXCPUFIFO_MASK));
        logipiq(receive, copy_func, copy_arg1, copy_arg2, sgd, mycpu);
#ifdef INVARIANTS
        if (ipiq_debug && (ip->ip_rindex & 0xFFFFFF) == 0) {
                kprintf("cpu %d ipifunc %p %p %d (frame %p)\n",
                        mycpu->gd_cpuid,
                        copy_func, copy_arg1, copy_arg2,
#if defined(__i386__)
                        (frame ? (void *)frame->if_eip : NULL));
#elif defined(__amd64__)
                        (frame ? (void *)frame->if_rip : NULL));
#else
                        NULL);
#endif
        }
#endif
        copy_func(copy_arg1, copy_arg2, frame);
        cpu_sfence();
        ip->ip_xindex = ip->ip_rindex;

#ifdef PANIC_DEBUG
        /*
         * Simulate panics during the processing of an IPI
         */
        if (mycpu->gd_cpuid == panic_ipiq_cpu && panic_ipiq_count) {
                if (--panic_ipiq_count == 0) {
#ifdef DDB
                        Debugger("PANIC_DEBUG");
#else
                        panic("PANIC_DEBUG");
#endif
                }
        }
#endif
    }
    --mygd->gd_intr_nesting_level;

    /*
     * Return non-zero if there is still more in the queue.
     */
    cpu_lfence();
    return (ip->ip_rindex != ip->ip_windex);
}

static void
lwkt_sync_ipiq(void *arg)
{
    volatile cpumask_t *cpumask = arg;

    atomic_clear_cpumask(cpumask, mycpu->gd_cpumask);
    if (*cpumask == 0)
        wakeup(cpumask);
}

void
lwkt_synchronize_ipiqs(const char *wmesg)
{
    volatile cpumask_t other_cpumask;

    other_cpumask = mycpu->gd_other_cpus & smp_active_mask;
    lwkt_send_ipiq_mask(other_cpumask, lwkt_sync_ipiq,
        __DEVOLATILE(void *, &other_cpumask));

    while (other_cpumask != 0) {
        tsleep_interlock(&other_cpumask, 0);
        if (other_cpumask != 0)
            tsleep(&other_cpumask, PINTERLOCKED, wmesg, 0);
    }
}

#endif

/*
 * CPU Synchronization Support
 *
 * lwkt_cpusync_interlock()     - Place specified cpus in a quiescent state.
 *                                The current cpu is placed in a hard critical
 *                                section.
 *
 * lwkt_cpusync_deinterlock()   - Execute cs_func on specified cpus, including
 *                                current cpu if specified, then return.
 */
void
lwkt_cpusync_simple(cpumask_t mask, cpusync_func_t func, void *arg)
{
    struct lwkt_cpusync cs;

    lwkt_cpusync_init(&cs, mask, func, arg);
    lwkt_cpusync_interlock(&cs);
    lwkt_cpusync_deinterlock(&cs);
}


void
lwkt_cpusync_interlock(lwkt_cpusync_t cs)
{
#ifdef SMP
#if 0
    const char *smsg = "SMPSYNL";
#endif
    globaldata_t gd = mycpu;
    cpumask_t mask;

    /*
     * mask acknowledge (cs_mack):  0->mask for stage 1
     *
     * mack does not include the current cpu.
     */
    mask = cs->cs_mask & gd->gd_other_cpus & smp_active_mask;
    cs->cs_mack = 0;
    crit_enter_id("cpusync");
    if (mask) {
        DEBUG_PUSH_INFO("cpusync_interlock");
        ++ipiq_cscount;
        ++gd->gd_curthread->td_cscount;
        lwkt_send_ipiq_mask(mask, (ipifunc1_t)lwkt_cpusync_remote1, cs);
        logipiq2(sync_start, (long)mask);
#if 0
        if (gd->gd_curthread->td_wmesg == NULL)
                gd->gd_curthread->td_wmesg = smsg;
#endif
        while (cs->cs_mack != mask) {
            lwkt_process_ipiq();
            cpu_pause();
        }
#if 0
        if (gd->gd_curthread->td_wmesg == smsg)
                gd->gd_curthread->td_wmesg = NULL;
#endif
        DEBUG_POP_INFO();
    }
#else
    cs->cs_mack = 0;
#endif
}

/*
 * Interlocked cpus have executed remote1 and are polling in remote2.
 * To deinterlock we clear cs_mack and wait for the cpus to execute
 * the func and set their bit in cs_mack again.
 *
 */
void
lwkt_cpusync_deinterlock(lwkt_cpusync_t cs)
{
    globaldata_t gd = mycpu;
#ifdef SMP
#if 0
    const char *smsg = "SMPSYNU";
#endif
    cpumask_t mask;

    /*
     * mask acknowledge (cs_mack):  mack->0->mack for stage 2
     *
     * Clearing cpu bits for polling cpus in cs_mack will cause them to
     * execute stage 2, which executes the cs_func(cs_data) and then sets
     * their bit in cs_mack again.
     *
     * mack does not include the current cpu.
     */
    mask = cs->cs_mack;
    cpu_ccfence();
    cs->cs_mack = 0;
    if (cs->cs_func && (cs->cs_mask & gd->gd_cpumask))
            cs->cs_func(cs->cs_data);
    if (mask) {
        DEBUG_PUSH_INFO("cpusync_deinterlock");
#if 0
        if (gd->gd_curthread->td_wmesg == NULL)
                gd->gd_curthread->td_wmesg = smsg;
#endif
        while (cs->cs_mack != mask) {
            lwkt_process_ipiq();
            cpu_pause();
        }
#if 0
        if (gd->gd_curthread->td_wmesg == smsg)
                gd->gd_curthread->td_wmesg = NULL;
#endif
        DEBUG_POP_INFO();
        /*
         * cpusyncq ipis may be left queued without the RQF flag set due to
         * a non-zero td_cscount, so be sure to process any laggards after
         * decrementing td_cscount.
         */
        --gd->gd_curthread->td_cscount;
        lwkt_process_ipiq();
        logipiq2(sync_end, (long)mask);
    }
    crit_exit_id("cpusync");
#else
    if (cs->cs_func && (cs->cs_mask & gd->gd_cpumask))
        cs->cs_func(cs->cs_data);
#endif
}

#ifdef SMP

/*
 * helper IPI remote messaging function.
 * 
 * Called on remote cpu when a new cpu synchronization request has been
 * sent to us.  Execute the run function and adjust cs_count, then requeue
 * the request so we spin on it.
 */
static void
lwkt_cpusync_remote1(lwkt_cpusync_t cs)
{
    globaldata_t gd = mycpu;

    atomic_set_cpumask(&cs->cs_mack, gd->gd_cpumask);
    lwkt_cpusync_remote2(cs);
}

/*
 * helper IPI remote messaging function.
 *
 * Poll for the originator telling us to finish.  If it hasn't, requeue
 * our request so we spin on it.
 */
static void
lwkt_cpusync_remote2(lwkt_cpusync_t cs)
{
    globaldata_t gd = mycpu;

    if ((cs->cs_mack & gd->gd_cpumask) == 0) {
        if (cs->cs_func)
                cs->cs_func(cs->cs_data);
        atomic_set_cpumask(&cs->cs_mack, gd->gd_cpumask);
    } else {
        lwkt_ipiq_t ip;
        int wi;

        ip = &gd->gd_cpusyncq;
        wi = ip->ip_windex & MAXCPUFIFO_MASK;
        ip->ip_info[wi].func = (ipifunc3_t)(ipifunc1_t)lwkt_cpusync_remote2;
        ip->ip_info[wi].arg1 = cs;
        ip->ip_info[wi].arg2 = 0;
        cpu_sfence();
        KKASSERT(ip->ip_windex - ip->ip_rindex < MAXCPUFIFO);
        ++ip->ip_windex;
        if (ipiq_debug && (ip->ip_windex & 0xFFFFFF) == 0) {
                kprintf("cpu %d cm=%016jx %016jx f=%p\n",
                        gd->gd_cpuid,
                        (intmax_t)cs->cs_mask, (intmax_t)cs->cs_mack,
                        cs->cs_func);
        }
    }
}

#endif