FreeBSD/Linux Kernel Cross Reference
sys/kern/subr_smp.c

     /*-
      * Copyright (c) 2001, John Baldwin <jhb@FreeBSD.org>.
      * All rights reserved.
      *
      * 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 author nor the names of any co-contributors
     *    may be used to endorse or promote products derived from this software
     *    without specific prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR 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 AUTHOR 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 holds the global variables and machine independent functions
     * used for the kernel SMP support.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD: releng/9.2/sys/kern/subr_smp.c 248085 2013-03-09 02:36:32Z marius $");
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/ktr.h>
    #include <sys/proc.h>
    #include <sys/bus.h>
    #include <sys/lock.h>
    #include <sys/mutex.h>
    #include <sys/pcpu.h>
    #include <sys/smp.h>
    #include <sys/sysctl.h>
    
    #include <machine/cpu.h>
    #include <machine/smp.h>
    
    #include "opt_sched.h"
    
    #ifdef SMP
    volatile cpuset_t stopped_cpus;
    volatile cpuset_t started_cpus;
    cpuset_t hlt_cpus_mask;
    cpuset_t logical_cpus_mask;
    
    void (*cpustop_restartfunc)(void);
    #endif
    /* This is used in modules that need to work in both SMP and UP. */
    cpuset_t all_cpus;
    
    int mp_ncpus;
    /* export this for libkvm consumers. */
    int mp_maxcpus = MAXCPU;
    
    volatile int smp_started;
    u_int mp_maxid;
    
    static SYSCTL_NODE(_kern, OID_AUTO, smp, CTLFLAG_RD|CTLFLAG_CAPRD, NULL,
        "Kernel SMP");
    
    SYSCTL_INT(_kern_smp, OID_AUTO, maxid, CTLFLAG_RD|CTLFLAG_CAPRD, &mp_maxid, 0,
        "Max CPU ID.");
    
    SYSCTL_INT(_kern_smp, OID_AUTO, maxcpus, CTLFLAG_RD|CTLFLAG_CAPRD, &mp_maxcpus,
        0, "Max number of CPUs that the system was compiled for.");
    
    int smp_active = 0;     /* are the APs allowed to run? */
    SYSCTL_INT(_kern_smp, OID_AUTO, active, CTLFLAG_RW, &smp_active, 0,
        "Number of Auxillary Processors (APs) that were successfully started");
    
    int smp_disabled = 0;   /* has smp been disabled? */
    SYSCTL_INT(_kern_smp, OID_AUTO, disabled, CTLFLAG_RDTUN|CTLFLAG_CAPRD,
        &smp_disabled, 0, "SMP has been disabled from the loader");
    TUNABLE_INT("kern.smp.disabled", &smp_disabled);
    
    int smp_cpus = 1;       /* how many cpu's running */
    SYSCTL_INT(_kern_smp, OID_AUTO, cpus, CTLFLAG_RD|CTLFLAG_CAPRD, &smp_cpus, 0,
        "Number of CPUs online");
    
    int smp_topology = 0;   /* Which topology we're using. */
    SYSCTL_INT(_kern_smp, OID_AUTO, topology, CTLFLAG_RD, &smp_topology, 0,
        "Topology override setting; 0 is default provided by hardware.");
    TUNABLE_INT("kern.smp.topology", &smp_topology);
    
   #ifdef SMP
   /* Enable forwarding of a signal to a process running on a different CPU */
   static int forward_signal_enabled = 1;
   SYSCTL_INT(_kern_smp, OID_AUTO, forward_signal_enabled, CTLFLAG_RW,
              &forward_signal_enabled, 0,
              "Forwarding of a signal to a process on a different CPU");
   
   /* Variables needed for SMP rendezvous. */
   static volatile int smp_rv_ncpus;
   static void (*volatile smp_rv_setup_func)(void *arg);
   static void (*volatile smp_rv_action_func)(void *arg);
   static void (*volatile smp_rv_teardown_func)(void *arg);
   static void *volatile smp_rv_func_arg;
   static volatile int smp_rv_waiters[4];
   
   /* 
    * Shared mutex to restrict busywaits between smp_rendezvous() and
    * smp(_targeted)_tlb_shootdown().  A deadlock occurs if both of these
    * functions trigger at once and cause multiple CPUs to busywait with
    * interrupts disabled. 
    */
   struct mtx smp_ipi_mtx;
   
   /*
    * Let the MD SMP code initialize mp_maxid very early if it can.
    */
   static void
   mp_setmaxid(void *dummy)
   {
           cpu_mp_setmaxid();
   }
   SYSINIT(cpu_mp_setmaxid, SI_SUB_TUNABLES, SI_ORDER_FIRST, mp_setmaxid, NULL);
   
   /*
    * Call the MD SMP initialization code.
    */
   static void
   mp_start(void *dummy)
   {
   
           mtx_init(&smp_ipi_mtx, "smp rendezvous", NULL, MTX_SPIN);
   
           /* Probe for MP hardware. */
           if (smp_disabled != 0 || cpu_mp_probe() == 0) {
                   mp_ncpus = 1;
                   CPU_SETOF(PCPU_GET(cpuid), &all_cpus);
                   return;
           }
   
           cpu_mp_start();
           printf("FreeBSD/SMP: Multiprocessor System Detected: %d CPUs\n",
               mp_ncpus);
           cpu_mp_announce();
   }
   SYSINIT(cpu_mp, SI_SUB_CPU, SI_ORDER_THIRD, mp_start, NULL);
   
   void
   forward_signal(struct thread *td)
   {
           int id;
   
           /*
            * signotify() has already set TDF_ASTPENDING and TDF_NEEDSIGCHECK on
            * this thread, so all we need to do is poke it if it is currently
            * executing so that it executes ast().
            */
           THREAD_LOCK_ASSERT(td, MA_OWNED);
           KASSERT(TD_IS_RUNNING(td),
               ("forward_signal: thread is not TDS_RUNNING"));
   
           CTR1(KTR_SMP, "forward_signal(%p)", td->td_proc);
   
           if (!smp_started || cold || panicstr)
                   return;
           if (!forward_signal_enabled)
                   return;
   
           /* No need to IPI ourself. */
           if (td == curthread)
                   return;
   
           id = td->td_oncpu;
           if (id == NOCPU)
                   return;
           ipi_cpu(id, IPI_AST);
   }
   
   /*
    * When called the executing CPU will send an IPI to all other CPUs
    *  requesting that they halt execution.
    *
    * Usually (but not necessarily) called with 'other_cpus' as its arg.
    *
    *  - Signals all CPUs in map to stop.
    *  - Waits for each to stop.
    *
    * Returns:
    *  -1: error
    *   0: NA
    *   1: ok
    *
    */
   static int
   generic_stop_cpus(cpuset_t map, u_int type)
   {
   #ifdef KTR
           char cpusetbuf[CPUSETBUFSIZ];
   #endif
           static volatile u_int stopping_cpu = NOCPU;
           int i;
   
           KASSERT(
   #if defined(__amd64__) || defined(__i386__)
               type == IPI_STOP || type == IPI_STOP_HARD || type == IPI_SUSPEND,
   #else
               type == IPI_STOP || type == IPI_STOP_HARD,
   #endif
               ("%s: invalid stop type", __func__));
   
           if (!smp_started)
                   return (0);
   
           CTR2(KTR_SMP, "stop_cpus(%s) with %u type",
               cpusetobj_strprint(cpusetbuf, &map), type);
   
           if (stopping_cpu != PCPU_GET(cpuid))
                   while (atomic_cmpset_int(&stopping_cpu, NOCPU,
                       PCPU_GET(cpuid)) == 0)
                           while (stopping_cpu != NOCPU)
                                   cpu_spinwait(); /* spin */
   
           /* send the stop IPI to all CPUs in map */
           ipi_selected(map, type);
   
           i = 0;
           while (!CPU_SUBSET(&stopped_cpus, &map)) {
                   /* spin */
                   cpu_spinwait();
                   i++;
                   if (i == 100000000) {
                           printf("timeout stopping cpus\n");
                           break;
                   }
           }
   
           stopping_cpu = NOCPU;
           return (1);
   }
   
   int
   stop_cpus(cpuset_t map)
   {
   
           return (generic_stop_cpus(map, IPI_STOP));
   }
   
   int
   stop_cpus_hard(cpuset_t map)
   {
   
           return (generic_stop_cpus(map, IPI_STOP_HARD));
   }
   
   #if defined(__amd64__) || defined(__i386__)
   int
   suspend_cpus(cpuset_t map)
   {
   
           return (generic_stop_cpus(map, IPI_SUSPEND));
   }
   #endif
   
   /*
    * Called by a CPU to restart stopped CPUs. 
    *
    * Usually (but not necessarily) called with 'stopped_cpus' as its arg.
    *
    *  - Signals all CPUs in map to restart.
    *  - Waits for each to restart.
    *
    * Returns:
    *  -1: error
    *   0: NA
    *   1: ok
    */
   int
   restart_cpus(cpuset_t map)
   {
   #ifdef KTR
           char cpusetbuf[CPUSETBUFSIZ];
   #endif
   
           if (!smp_started)
                   return 0;
   
           CTR1(KTR_SMP, "restart_cpus(%s)", cpusetobj_strprint(cpusetbuf, &map));
   
           /* signal other cpus to restart */
           CPU_COPY_STORE_REL(&map, &started_cpus);
   
           /* wait for each to clear its bit */
           while (CPU_OVERLAP(&stopped_cpus, &map))
                   cpu_spinwait();
   
           return 1;
   }
   
   /*
    * All-CPU rendezvous.  CPUs are signalled, all execute the setup function 
    * (if specified), rendezvous, execute the action function (if specified),
    * rendezvous again, execute the teardown function (if specified), and then
    * resume.
    *
    * Note that the supplied external functions _must_ be reentrant and aware
    * that they are running in parallel and in an unknown lock context.
    */
   void
   smp_rendezvous_action(void)
   {
           struct thread *td;
           void *local_func_arg;
           void (*local_setup_func)(void*);
           void (*local_action_func)(void*);
           void (*local_teardown_func)(void*);
   #ifdef INVARIANTS
           int owepreempt;
   #endif
   
           /* Ensure we have up-to-date values. */
           atomic_add_acq_int(&smp_rv_waiters[0], 1);
           while (smp_rv_waiters[0] < smp_rv_ncpus)
                   cpu_spinwait();
   
           /* Fetch rendezvous parameters after acquire barrier. */
           local_func_arg = smp_rv_func_arg;
           local_setup_func = smp_rv_setup_func;
           local_action_func = smp_rv_action_func;
           local_teardown_func = smp_rv_teardown_func;
   
           /*
            * Use a nested critical section to prevent any preemptions
            * from occurring during a rendezvous action routine.
            * Specifically, if a rendezvous handler is invoked via an IPI
            * and the interrupted thread was in the critical_exit()
            * function after setting td_critnest to 0 but before
            * performing a deferred preemption, this routine can be
            * invoked with td_critnest set to 0 and td_owepreempt true.
            * In that case, a critical_exit() during the rendezvous
            * action would trigger a preemption which is not permitted in
            * a rendezvous action.  To fix this, wrap all of the
            * rendezvous action handlers in a critical section.  We
            * cannot use a regular critical section however as having
            * critical_exit() preempt from this routine would also be
            * problematic (the preemption must not occur before the IPI
            * has been acknowledged via an EOI).  Instead, we
            * intentionally ignore td_owepreempt when leaving the
            * critical section.  This should be harmless because we do
            * not permit rendezvous action routines to schedule threads,
            * and thus td_owepreempt should never transition from 0 to 1
            * during this routine.
            */
           td = curthread;
           td->td_critnest++;
   #ifdef INVARIANTS
           owepreempt = td->td_owepreempt;
   #endif
           
           /*
            * If requested, run a setup function before the main action
            * function.  Ensure all CPUs have completed the setup
            * function before moving on to the action function.
            */
           if (local_setup_func != smp_no_rendevous_barrier) {
                   if (smp_rv_setup_func != NULL)
                           smp_rv_setup_func(smp_rv_func_arg);
                   atomic_add_int(&smp_rv_waiters[1], 1);
                   while (smp_rv_waiters[1] < smp_rv_ncpus)
                           cpu_spinwait();
           }
   
           if (local_action_func != NULL)
                   local_action_func(local_func_arg);
   
           if (local_teardown_func != smp_no_rendevous_barrier) {
                   /*
                    * Signal that the main action has been completed.  If a
                    * full exit rendezvous is requested, then all CPUs will
                    * wait here until all CPUs have finished the main action.
                    */
                   atomic_add_int(&smp_rv_waiters[2], 1);
                   while (smp_rv_waiters[2] < smp_rv_ncpus)
                           cpu_spinwait();
   
                   if (local_teardown_func != NULL)
                           local_teardown_func(local_func_arg);
           }
   
           /*
            * Signal that the rendezvous is fully completed by this CPU.
            * This means that no member of smp_rv_* pseudo-structure will be
            * accessed by this target CPU after this point; in particular,
            * memory pointed by smp_rv_func_arg.
            */
           atomic_add_int(&smp_rv_waiters[3], 1);
   
           td->td_critnest--;
           KASSERT(owepreempt == td->td_owepreempt,
               ("rendezvous action changed td_owepreempt"));
   }
   
   void
   smp_rendezvous_cpus(cpuset_t map,
           void (* setup_func)(void *), 
           void (* action_func)(void *),
           void (* teardown_func)(void *),
           void *arg)
   {
           int curcpumap, i, ncpus = 0;
   
           /* Look comments in the !SMP case. */
           if (!smp_started) {
                   spinlock_enter();
                   if (setup_func != NULL)
                           setup_func(arg);
                   if (action_func != NULL)
                           action_func(arg);
                   if (teardown_func != NULL)
                           teardown_func(arg);
                   spinlock_exit();
                   return;
           }
   
           CPU_FOREACH(i) {
                   if (CPU_ISSET(i, &map))
                           ncpus++;
           }
           if (ncpus == 0)
                   panic("ncpus is 0 with non-zero map");
   
           mtx_lock_spin(&smp_ipi_mtx);
   
           /* Pass rendezvous parameters via global variables. */
           smp_rv_ncpus = ncpus;
           smp_rv_setup_func = setup_func;
           smp_rv_action_func = action_func;
           smp_rv_teardown_func = teardown_func;
           smp_rv_func_arg = arg;
           smp_rv_waiters[1] = 0;
           smp_rv_waiters[2] = 0;
           smp_rv_waiters[3] = 0;
           atomic_store_rel_int(&smp_rv_waiters[0], 0);
   
           /*
            * Signal other processors, which will enter the IPI with
            * interrupts off.
            */
           curcpumap = CPU_ISSET(curcpu, &map);
           CPU_CLR(curcpu, &map);
           ipi_selected(map, IPI_RENDEZVOUS);
   
           /* Check if the current CPU is in the map */
           if (curcpumap != 0)
                   smp_rendezvous_action();
   
           /*
            * Ensure that the master CPU waits for all the other
            * CPUs to finish the rendezvous, so that smp_rv_*
            * pseudo-structure and the arg are guaranteed to not
            * be in use.
            */
           while (atomic_load_acq_int(&smp_rv_waiters[3]) < ncpus)
                   cpu_spinwait();
   
           mtx_unlock_spin(&smp_ipi_mtx);
   }
   
   void
   smp_rendezvous(void (* setup_func)(void *), 
                  void (* action_func)(void *),
                  void (* teardown_func)(void *),
                  void *arg)
   {
           smp_rendezvous_cpus(all_cpus, setup_func, action_func, teardown_func, arg);
   }
   
   static struct cpu_group group[MAXCPU];
   
   struct cpu_group *
   smp_topo(void)
   {
           char cpusetbuf[CPUSETBUFSIZ], cpusetbuf2[CPUSETBUFSIZ];
           struct cpu_group *top;
   
           /*
            * Check for a fake topology request for debugging purposes.
            */
           switch (smp_topology) {
           case 1:
                   /* Dual core with no sharing.  */
                   top = smp_topo_1level(CG_SHARE_NONE, 2, 0);
                   break;
           case 2:
                   /* No topology, all cpus are equal. */
                   top = smp_topo_none();
                   break;
           case 3:
                   /* Dual core with shared L2.  */
                   top = smp_topo_1level(CG_SHARE_L2, 2, 0);
                   break;
           case 4:
                   /* quad core, shared l3 among each package, private l2.  */
                   top = smp_topo_1level(CG_SHARE_L3, 4, 0);
                   break;
           case 5:
                   /* quad core,  2 dualcore parts on each package share l2.  */
                   top = smp_topo_2level(CG_SHARE_NONE, 2, CG_SHARE_L2, 2, 0);
                   break;
           case 6:
                   /* Single-core 2xHTT */
                   top = smp_topo_1level(CG_SHARE_L1, 2, CG_FLAG_HTT);
                   break;
           case 7:
                   /* quad core with a shared l3, 8 threads sharing L2.  */
                   top = smp_topo_2level(CG_SHARE_L3, 4, CG_SHARE_L2, 8,
                       CG_FLAG_SMT);
                   break;
           default:
                   /* Default, ask the system what it wants. */
                   top = cpu_topo();
                   break;
           }
           /*
            * Verify the returned topology.
            */
           if (top->cg_count != mp_ncpus)
                   panic("Built bad topology at %p.  CPU count %d != %d",
                       top, top->cg_count, mp_ncpus);
           if (CPU_CMP(&top->cg_mask, &all_cpus))
                   panic("Built bad topology at %p.  CPU mask (%s) != (%s)",
                       top, cpusetobj_strprint(cpusetbuf, &top->cg_mask),
                       cpusetobj_strprint(cpusetbuf2, &all_cpus));
           return (top);
   }
   
   struct cpu_group *
   smp_topo_none(void)
   {
           struct cpu_group *top;
   
           top = &group[0];
           top->cg_parent = NULL;
           top->cg_child = NULL;
           top->cg_mask = all_cpus;
           top->cg_count = mp_ncpus;
           top->cg_children = 0;
           top->cg_level = CG_SHARE_NONE;
           top->cg_flags = 0;
           
           return (top);
   }
   
   static int
   smp_topo_addleaf(struct cpu_group *parent, struct cpu_group *child, int share,
       int count, int flags, int start)
   {
           char cpusetbuf[CPUSETBUFSIZ], cpusetbuf2[CPUSETBUFSIZ];
           cpuset_t mask;
           int i;
   
           CPU_ZERO(&mask);
           for (i = 0; i < count; i++, start++)
                   CPU_SET(start, &mask);
           child->cg_parent = parent;
           child->cg_child = NULL;
           child->cg_children = 0;
           child->cg_level = share;
           child->cg_count = count;
           child->cg_flags = flags;
           child->cg_mask = mask;
           parent->cg_children++;
           for (; parent != NULL; parent = parent->cg_parent) {
                   if (CPU_OVERLAP(&parent->cg_mask, &child->cg_mask))
                           panic("Duplicate children in %p.  mask (%s) child (%s)",
                               parent,
                               cpusetobj_strprint(cpusetbuf, &parent->cg_mask),
                               cpusetobj_strprint(cpusetbuf2, &child->cg_mask));
                   CPU_OR(&parent->cg_mask, &child->cg_mask);
                   parent->cg_count += child->cg_count;
           }
   
           return (start);
   }
   
   struct cpu_group *
   smp_topo_1level(int share, int count, int flags)
   {
           struct cpu_group *child;
           struct cpu_group *top;
           int packages;
           int cpu;
           int i;
   
           cpu = 0;
           top = &group[0];
           packages = mp_ncpus / count;
           top->cg_child = child = &group[1];
           top->cg_level = CG_SHARE_NONE;
           for (i = 0; i < packages; i++, child++)
                   cpu = smp_topo_addleaf(top, child, share, count, flags, cpu);
           return (top);
   }
   
   struct cpu_group *
   smp_topo_2level(int l2share, int l2count, int l1share, int l1count,
       int l1flags)
   {
           struct cpu_group *top;
           struct cpu_group *l1g;
           struct cpu_group *l2g;
           int cpu;
           int i;
           int j;
   
           cpu = 0;
           top = &group[0];
           l2g = &group[1];
           top->cg_child = l2g;
           top->cg_level = CG_SHARE_NONE;
           top->cg_children = mp_ncpus / (l2count * l1count);
           l1g = l2g + top->cg_children;
           for (i = 0; i < top->cg_children; i++, l2g++) {
                   l2g->cg_parent = top;
                   l2g->cg_child = l1g;
                   l2g->cg_level = l2share;
                   for (j = 0; j < l2count; j++, l1g++)
                           cpu = smp_topo_addleaf(l2g, l1g, l1share, l1count,
                               l1flags, cpu);
           }
           return (top);
   }
   
   
   struct cpu_group *
   smp_topo_find(struct cpu_group *top, int cpu)
   {
           struct cpu_group *cg;
           cpuset_t mask;
           int children;
           int i;
   
           CPU_SETOF(cpu, &mask);
           cg = top;
           for (;;) {
                   if (!CPU_OVERLAP(&cg->cg_mask, &mask))
                           return (NULL);
                   if (cg->cg_children == 0)
                           return (cg);
                   children = cg->cg_children;
                   for (i = 0, cg = cg->cg_child; i < children; cg++, i++)
                           if (CPU_OVERLAP(&cg->cg_mask, &mask))
                                   break;
           }
           return (NULL);
   }
   #else /* !SMP */
   
   void
   smp_rendezvous_cpus(cpuset_t map,
           void (*setup_func)(void *), 
           void (*action_func)(void *),
           void (*teardown_func)(void *),
           void *arg)
   {
           /*
            * In the !SMP case we just need to ensure the same initial conditions
            * as the SMP case.
            */
           spinlock_enter();
           if (setup_func != NULL)
                   setup_func(arg);
           if (action_func != NULL)
                   action_func(arg);
           if (teardown_func != NULL)
                   teardown_func(arg);
           spinlock_exit();
   }
   
   void
   smp_rendezvous(void (*setup_func)(void *), 
                  void (*action_func)(void *),
                  void (*teardown_func)(void *),
                  void *arg)
   {
   
           /* Look comments in the smp_rendezvous_cpus() case. */
           spinlock_enter();
           if (setup_func != NULL)
                   setup_func(arg);
           if (action_func != NULL)
                   action_func(arg);
           if (teardown_func != NULL)
                   teardown_func(arg);
           spinlock_exit();
   }
   
   /*
    * Provide dummy SMP support for UP kernels.  Modules that need to use SMP
    * APIs will still work using this dummy support.
    */
   static void
   mp_setvariables_for_up(void *dummy)
   {
           mp_ncpus = 1;
           mp_maxid = PCPU_GET(cpuid);
           CPU_SETOF(mp_maxid, &all_cpus);
           KASSERT(PCPU_GET(cpuid) == 0, ("UP must have a CPU ID of zero"));
   }
   SYSINIT(cpu_mp_setvariables, SI_SUB_TUNABLES, SI_ORDER_FIRST,
       mp_setvariables_for_up, NULL);
   #endif /* SMP */
   
   void
   smp_no_rendevous_barrier(void *dummy)
   {
   #ifdef SMP
           KASSERT((!smp_started),("smp_no_rendevous called and smp is started"));
   #endif
   }

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