[dragonfly.git] / sys / kern / lwkt_thread.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.
 * 
 * $DragonFly: src/sys/kern/lwkt_thread.c,v 1.120 2008/10/26 04:29:19 sephe Exp $
 */

/*
 * Each cpu in a system has its own self-contained light weight kernel
 * thread scheduler, which means that generally speaking we only need
 * to use a critical section to avoid problems.  Foreign thread 
 * scheduling is queued via (async) IPIs.
 */
#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/sysctl.h>
#include <sys/kthread.h>
#include <machine/cpu.h>
#include <sys/lock.h>
#include <sys/caps.h>
#include <sys/spinlock.h>
#include <sys/ktr.h>

#include <sys/thread2.h>
#include <sys/spinlock2.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 <machine/stdarg.h>
#include <machine/smp.h>

#ifdef DDB
#include <ddb/ddb.h>
#endif

static MALLOC_DEFINE(M_THREAD, "thread", "lwkt threads");

static int untimely_switch = 0;
#ifdef  INVARIANTS
static int panic_on_cscount = 0;
#endif
static __int64_t switch_count = 0;
static __int64_t preempt_hit = 0;
static __int64_t preempt_miss = 0;
static __int64_t preempt_weird = 0;
static __int64_t token_contention_count = 0;
static __int64_t mplock_contention_count = 0;
static int lwkt_use_spin_port;
#ifdef SMP
static int chain_mplock = 0;
#endif
static struct objcache *thread_cache;

volatile cpumask_t mp_lock_contention_mask;

extern void cpu_heavy_restore(void);
extern void cpu_lwkt_restore(void);
extern void cpu_kthread_restore(void);
extern void cpu_idle_restore(void);

int
jg_tos_ok(struct thread *td)
{
        if (td == NULL) {
                return 1;
        }
        KKASSERT(td->td_sp != NULL);
        unsigned long tos = ((unsigned long *)td->td_sp)[0];
        int tos_ok = 0;
        if ((tos == cpu_heavy_restore) || (tos == cpu_lwkt_restore)
                || (tos == cpu_kthread_restore) || (tos == cpu_idle_restore)) {
                tos_ok = 1;
        }
        return tos_ok;
}

/*
 * We can make all thread ports use the spin backend instead of the thread
 * backend.  This should only be set to debug the spin backend.
 */
TUNABLE_INT("lwkt.use_spin_port", &lwkt_use_spin_port);

SYSCTL_INT(_lwkt, OID_AUTO, untimely_switch, CTLFLAG_RW, &untimely_switch, 0, "");
#ifdef  INVARIANTS
SYSCTL_INT(_lwkt, OID_AUTO, panic_on_cscount, CTLFLAG_RW, &panic_on_cscount, 0, "");
#endif
#ifdef SMP
SYSCTL_INT(_lwkt, OID_AUTO, chain_mplock, CTLFLAG_RW, &chain_mplock, 0, "");
#endif
SYSCTL_QUAD(_lwkt, OID_AUTO, switch_count, CTLFLAG_RW, &switch_count, 0, "");
SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_hit, CTLFLAG_RW, &preempt_hit, 0, "");
SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_miss, CTLFLAG_RW, &preempt_miss, 0, "");
SYSCTL_QUAD(_lwkt, OID_AUTO, preempt_weird, CTLFLAG_RW, &preempt_weird, 0, "");
#ifdef  INVARIANTS
SYSCTL_QUAD(_lwkt, OID_AUTO, token_contention_count, CTLFLAG_RW,
        &token_contention_count, 0, "spinning due to token contention");
SYSCTL_QUAD(_lwkt, OID_AUTO, mplock_contention_count, CTLFLAG_RW,
        &mplock_contention_count, 0, "spinning due to MPLOCK contention");
#endif

/*
 * Kernel Trace
 */
#if !defined(KTR_GIANT_CONTENTION)
#define KTR_GIANT_CONTENTION    KTR_ALL
#endif

KTR_INFO_MASTER(giant);
KTR_INFO(KTR_GIANT_CONTENTION, giant, beg, 0, "thread=%p", sizeof(void *));
KTR_INFO(KTR_GIANT_CONTENTION, giant, end, 1, "thread=%p", sizeof(void *));

#define loggiant(name)  KTR_LOG(giant_ ## name, curthread)

/*
 * These helper procedures handle the runq, they can only be called from
 * within a critical section.
 *
 * WARNING!  Prior to SMP being brought up it is possible to enqueue and
 * dequeue threads belonging to other cpus, so be sure to use td->td_gd
 * instead of 'mycpu' when referencing the globaldata structure.   Once
 * SMP live enqueuing and dequeueing only occurs on the current cpu.
 */
static __inline
void
_lwkt_dequeue(thread_t td)
{
    if (td->td_flags & TDF_RUNQ) {
        int nq = td->td_pri & TDPRI_MASK;
        struct globaldata *gd = td->td_gd;

        td->td_flags &= ~TDF_RUNQ;
        TAILQ_REMOVE(&gd->gd_tdrunq[nq], td, td_threadq);
        /* runqmask is passively cleaned up by the switcher */
    }
}

static __inline
void
_lwkt_enqueue(thread_t td)
{
    if ((td->td_flags & (TDF_RUNQ|TDF_MIGRATING|TDF_TSLEEPQ|TDF_BLOCKQ)) == 0) {
        int nq = td->td_pri & TDPRI_MASK;
        struct globaldata *gd = td->td_gd;

        td->td_flags |= TDF_RUNQ;
        TAILQ_INSERT_TAIL(&gd->gd_tdrunq[nq], td, td_threadq);
        gd->gd_runqmask |= 1 << nq;
    }
}

static __boolean_t
_lwkt_thread_ctor(void *obj, void *privdata, int ocflags)
{
        struct thread *td = (struct thread *)obj;

        td->td_kstack = NULL;
        td->td_kstack_size = 0;
        td->td_flags = TDF_ALLOCATED_THREAD;
        return (1);
}

static void
_lwkt_thread_dtor(void *obj, void *privdata)
{
        struct thread *td = (struct thread *)obj;

        KASSERT(td->td_flags & TDF_ALLOCATED_THREAD,
            ("_lwkt_thread_dtor: not allocated from objcache"));
        KASSERT((td->td_flags & TDF_ALLOCATED_STACK) && td->td_kstack &&
                td->td_kstack_size > 0,
            ("_lwkt_thread_dtor: corrupted stack"));
        kmem_free(&kernel_map, (vm_offset_t)td->td_kstack, td->td_kstack_size);
}

/*
 * Initialize the lwkt s/system.
 */
void
lwkt_init(void)
{
    /* An objcache has 2 magazines per CPU so divide cache size by 2. */
    thread_cache = objcache_create_mbacked(M_THREAD, sizeof(struct thread),
                        NULL, CACHE_NTHREADS/2,
                        _lwkt_thread_ctor, _lwkt_thread_dtor, NULL);
}

/*
 * Schedule a thread to run.  As the current thread we can always safely
 * schedule ourselves, and a shortcut procedure is provided for that
 * function.
 *
 * (non-blocking, self contained on a per cpu basis)
 */
void
lwkt_schedule_self(thread_t td)
{
    crit_enter_quick(td);
    KASSERT(td != &td->td_gd->gd_idlethread, ("lwkt_schedule_self(): scheduling gd_idlethread is illegal!"));
    KKASSERT(td->td_lwp == NULL || (td->td_lwp->lwp_flag & LWP_ONRUNQ) == 0);
    _lwkt_enqueue(td);
    crit_exit_quick(td);
}

/*
 * Deschedule a thread.
 *
 * (non-blocking, self contained on a per cpu basis)
 */
void
lwkt_deschedule_self(thread_t td)
{
    crit_enter_quick(td);
    _lwkt_dequeue(td);
    crit_exit_quick(td);
}

/*
 * LWKTs operate on a per-cpu basis
 *
 * WARNING!  Called from early boot, 'mycpu' may not work yet.
 */
void
lwkt_gdinit(struct globaldata *gd)
{
    int i;

    for (i = 0; i < sizeof(gd->gd_tdrunq)/sizeof(gd->gd_tdrunq[0]); ++i)
        TAILQ_INIT(&gd->gd_tdrunq[i]);
    gd->gd_runqmask = 0;
    TAILQ_INIT(&gd->gd_tdallq);
}

/*
 * Create a new thread.  The thread must be associated with a process context
 * or LWKT start address before it can be scheduled.  If the target cpu is
 * -1 the thread will be created on the current cpu.
 *
 * If you intend to create a thread without a process context this function
 * does everything except load the startup and switcher function.
 */
thread_t
lwkt_alloc_thread(struct thread *td, int stksize, int cpu, int flags)
{
    globaldata_t gd = mycpu;
    void *stack;

    /*
     * If static thread storage is not supplied allocate a thread.  Reuse
     * a cached free thread if possible.  gd_freetd is used to keep an exiting
     * thread intact through the exit.
     */
    if (td == NULL) {
        if ((td = gd->gd_freetd) != NULL)
            gd->gd_freetd = NULL;
        else
            td = objcache_get(thread_cache, M_WAITOK);
        KASSERT((td->td_flags &
                 (TDF_ALLOCATED_THREAD|TDF_RUNNING)) == TDF_ALLOCATED_THREAD,
                ("lwkt_alloc_thread: corrupted td flags 0x%X", td->td_flags));
        flags |= td->td_flags & (TDF_ALLOCATED_THREAD|TDF_ALLOCATED_STACK);
    }

    /*
     * Try to reuse cached stack.
     */
    if ((stack = td->td_kstack) != NULL && td->td_kstack_size != stksize) {
        if (flags & TDF_ALLOCATED_STACK) {
            kmem_free(&kernel_map, (vm_offset_t)stack, td->td_kstack_size);
            stack = NULL;
        }
    }
    if (stack == NULL) {
        stack = (void *)kmem_alloc(&kernel_map, stksize);
        flags |= TDF_ALLOCATED_STACK;
    }
    if (cpu < 0)
        lwkt_init_thread(td, stack, stksize, flags, gd);
    else
        lwkt_init_thread(td, stack, stksize, flags, globaldata_find(cpu));
    return(td);
}

/*
 * Initialize a preexisting thread structure.  This function is used by
 * lwkt_alloc_thread() and also used to initialize the per-cpu idlethread.
 *
 * All threads start out in a critical section at a priority of
 * TDPRI_KERN_DAEMON.  Higher level code will modify the priority as
 * appropriate.  This function may send an IPI message when the 
 * requested cpu is not the current cpu and consequently gd_tdallq may
 * not be initialized synchronously from the point of view of the originating
 * cpu.
 *
 * NOTE! we have to be careful in regards to creating threads for other cpus
 * if SMP has not yet been activated.
 */
#ifdef SMP

static void
lwkt_init_thread_remote(void *arg)
{
    thread_t td = arg;

    /*
     * Protected by critical section held by IPI dispatch
     */
    TAILQ_INSERT_TAIL(&td->td_gd->gd_tdallq, td, td_allq);
}

#endif

void
lwkt_init_thread(thread_t td, void *stack, int stksize, int flags,
                struct globaldata *gd)
{
    globaldata_t mygd = mycpu;

    bzero(td, sizeof(struct thread));
    td->td_kstack = stack;
    td->td_kstack_size = stksize;
    td->td_flags = flags;
    td->td_gd = gd;
    td->td_pri = TDPRI_KERN_DAEMON + TDPRI_CRIT;
#ifdef SMP
    if ((flags & TDF_MPSAFE) == 0)
        td->td_mpcount = 1;
#endif
    if (lwkt_use_spin_port)
        lwkt_initport_spin(&td->td_msgport);
    else
        lwkt_initport_thread(&td->td_msgport, td);
    pmap_init_thread(td);
#ifdef SMP
    /*
     * Normally initializing a thread for a remote cpu requires sending an
     * IPI.  However, the idlethread is setup before the other cpus are
     * activated so we have to treat it as a special case.  XXX manipulation
     * of gd_tdallq requires the BGL.
     */
    if (gd == mygd || td == &gd->gd_idlethread) {
        crit_enter_gd(mygd);
        TAILQ_INSERT_TAIL(&gd->gd_tdallq, td, td_allq);
        crit_exit_gd(mygd);
    } else {
        lwkt_send_ipiq(gd, lwkt_init_thread_remote, td);
    }
#else
    crit_enter_gd(mygd);
    TAILQ_INSERT_TAIL(&gd->gd_tdallq, td, td_allq);
    crit_exit_gd(mygd);
#endif
}

void
lwkt_set_comm(thread_t td, const char *ctl, ...)
{
    __va_list va;

    __va_start(va, ctl);
    kvsnprintf(td->td_comm, sizeof(td->td_comm), ctl, va);
    __va_end(va);
}

void
lwkt_hold(thread_t td)
{
    ++td->td_refs;
}

void
lwkt_rele(thread_t td)
{
    KKASSERT(td->td_refs > 0);
    --td->td_refs;
}

void
lwkt_wait_free(thread_t td)
{
    while (td->td_refs)
        tsleep(td, 0, "tdreap", hz);
}

void
lwkt_free_thread(thread_t td)
{
    KASSERT((td->td_flags & TDF_RUNNING) == 0,
        ("lwkt_free_thread: did not exit! %p", td));

    if (td->td_flags & TDF_ALLOCATED_THREAD) {
        objcache_put(thread_cache, td);
    } else if (td->td_flags & TDF_ALLOCATED_STACK) {
        /* client-allocated struct with internally allocated stack */
        KASSERT(td->td_kstack && td->td_kstack_size > 0,
            ("lwkt_free_thread: corrupted stack"));
        kmem_free(&kernel_map, (vm_offset_t)td->td_kstack, td->td_kstack_size);
        td->td_kstack = NULL;
        td->td_kstack_size = 0;
    }
}


/*
 * Switch to the next runnable lwkt.  If no LWKTs are runnable then 
 * switch to the idlethread.  Switching must occur within a critical
 * section to avoid races with the scheduling queue.
 *
 * We always have full control over our cpu's run queue.  Other cpus
 * that wish to manipulate our queue must use the cpu_*msg() calls to
 * talk to our cpu, so a critical section is all that is needed and
 * the result is very, very fast thread switching.
 *
 * The LWKT scheduler uses a fixed priority model and round-robins at
 * each priority level.  User process scheduling is a totally
 * different beast and LWKT priorities should not be confused with
 * user process priorities.
 *
 * The MP lock may be out of sync with the thread's td_mpcount.  lwkt_switch()
 * cleans it up.  Note that the td_switch() function cannot do anything that
 * requires the MP lock since the MP lock will have already been setup for
 * the target thread (not the current thread).  It's nice to have a scheduler
 * that does not need the MP lock to work because it allows us to do some
 * really cool high-performance MP lock optimizations.
 *
 * PREEMPTION NOTE: Preemption occurs via lwkt_preempt().  lwkt_switch()
 * is not called by the current thread in the preemption case, only when
 * the preempting thread blocks (in order to return to the original thread).
 */
void
lwkt_switch(void)
{
    globaldata_t gd = mycpu;
    thread_t td = gd->gd_curthread;
    thread_t ntd;
#ifdef SMP
    int mpheld;
#endif

    /*
     * Switching from within a 'fast' (non thread switched) interrupt or IPI
     * is illegal.  However, we may have to do it anyway if we hit a fatal
     * kernel trap or we have paniced.
     *
     * If this case occurs save and restore the interrupt nesting level.
     */
    if (gd->gd_intr_nesting_level) {
        int savegdnest;
        int savegdtrap;

        if (gd->gd_trap_nesting_level == 0 && panicstr == NULL) {
            panic("lwkt_switch: cannot switch from within "
                  "a fast interrupt, yet, td %p\n", td);
        } else {
            savegdnest = gd->gd_intr_nesting_level;
            savegdtrap = gd->gd_trap_nesting_level;
            gd->gd_intr_nesting_level = 0;
            gd->gd_trap_nesting_level = 0;
            if ((td->td_flags & TDF_PANICWARN) == 0) {
                td->td_flags |= TDF_PANICWARN;
                kprintf("Warning: thread switch from interrupt or IPI, "
                        "thread %p (%s)\n", td, td->td_comm);
#ifdef DDB
                db_print_backtrace();
#endif
            }
            lwkt_switch();
            gd->gd_intr_nesting_level = savegdnest;
            gd->gd_trap_nesting_level = savegdtrap;
            return;
        }
    }

    /*
     * Passive release (used to transition from user to kernel mode
     * when we block or switch rather then when we enter the kernel).
     * This function is NOT called if we are switching into a preemption
     * or returning from a preemption.  Typically this causes us to lose
     * our current process designation (if we have one) and become a true
     * LWKT thread, and may also hand the current process designation to
     * another process and schedule thread.
     */
    if (td->td_release)
            td->td_release(td);

    crit_enter_gd(gd);
    if (td->td_toks)
            lwkt_relalltokens(td);

    /*
     * We had better not be holding any spin locks, but don't get into an
     * endless panic loop.
     */
    KASSERT(gd->gd_spinlock_rd == NULL || panicstr != NULL, 
            ("lwkt_switch: still holding a shared spinlock %p!", 
             gd->gd_spinlock_rd));
    KASSERT(gd->gd_spinlocks_wr == 0 || panicstr != NULL, 
            ("lwkt_switch: still holding %d exclusive spinlocks!",
             gd->gd_spinlocks_wr));


#ifdef SMP
    /*
     * td_mpcount cannot be used to determine if we currently hold the
     * MP lock because get_mplock() will increment it prior to attempting
     * to get the lock, and switch out if it can't.  Our ownership of 
     * the actual lock will remain stable while we are in a critical section
     * (but, of course, another cpu may own or release the lock so the
     * actual value of mp_lock is not stable).
     */
    mpheld = MP_LOCK_HELD();
#ifdef  INVARIANTS
    if (td->td_cscount) {
        kprintf("Diagnostic: attempt to switch while mastering cpusync: %p\n",
                td);
        if (panic_on_cscount)
            panic("switching while mastering cpusync");
    }
#endif
#endif
    if ((ntd = td->td_preempted) != NULL) {
        /*
         * We had preempted another thread on this cpu, resume the preempted
         * thread.  This occurs transparently, whether the preempted thread
         * was scheduled or not (it may have been preempted after descheduling
         * itself). 
         *
         * We have to setup the MP lock for the original thread after backing
         * out the adjustment that was made to curthread when the original
         * was preempted.
         */
        KKASSERT(ntd->td_flags & TDF_PREEMPT_LOCK);
#ifdef SMP
        if (ntd->td_mpcount && mpheld == 0) {
            panic("MPLOCK NOT HELD ON RETURN: %p %p %d %d",
               td, ntd, td->td_mpcount, ntd->td_mpcount);
        }
        if (ntd->td_mpcount) {
            td->td_mpcount -= ntd->td_mpcount;
            KKASSERT(td->td_mpcount >= 0);
        }
#endif
        ntd->td_flags |= TDF_PREEMPT_DONE;

        /*
         * The interrupt may have woken a thread up, we need to properly
         * set the reschedule flag if the originally interrupted thread is
         * at a lower priority.
         */
        if (gd->gd_runqmask > (2 << (ntd->td_pri & TDPRI_MASK)) - 1)
            need_lwkt_resched();
        /* YYY release mp lock on switchback if original doesn't need it */
    } else {
        /*
         * Priority queue / round-robin at each priority.  Note that user
         * processes run at a fixed, low priority and the user process
         * scheduler deals with interactions between user processes
         * by scheduling and descheduling them from the LWKT queue as
         * necessary.
         *
         * We have to adjust the MP lock for the target thread.  If we 
         * need the MP lock and cannot obtain it we try to locate a
         * thread that does not need the MP lock.  If we cannot, we spin
         * instead of HLT.
         *
         * A similar issue exists for the tokens held by the target thread.
         * If we cannot obtain ownership of the tokens we cannot immediately
         * schedule the thread.
         */

        /*
         * If an LWKT reschedule was requested, well that is what we are
         * doing now so clear it.
         */
        clear_lwkt_resched();
again:
        if (gd->gd_runqmask) {
            int nq = bsrl(gd->gd_runqmask);
            if ((ntd = TAILQ_FIRST(&gd->gd_tdrunq[nq])) == NULL) {
                gd->gd_runqmask &= ~(1 << nq);
                goto again;
            }
#ifdef SMP
            /*
             * THREAD SELECTION FOR AN SMP MACHINE BUILD
             *
             * If the target needs the MP lock and we couldn't get it,
             * or if the target is holding tokens and we could not 
             * gain ownership of the tokens, continue looking for a
             * thread to schedule and spin instead of HLT if we can't.
             *
             * NOTE: the mpheld variable invalid after this conditional, it
             * can change due to both cpu_try_mplock() returning success
             * AND interactions in lwkt_getalltokens() due to the fact that
             * we are trying to check the mpcount of a thread other then
             * the current thread.  Because of this, if the current thread
             * is not holding td_mpcount, an IPI indirectly run via
             * lwkt_getalltokens() can obtain and release the MP lock and
             * cause the core MP lock to be released. 
             */
            if ((ntd->td_mpcount && mpheld == 0 && !cpu_try_mplock()) ||
                (ntd->td_toks && lwkt_getalltokens(ntd) == 0)
            ) {
                u_int32_t rqmask = gd->gd_runqmask;

                mpheld = MP_LOCK_HELD();
                ntd = NULL;
                while (rqmask) {
                    TAILQ_FOREACH(ntd, &gd->gd_tdrunq[nq], td_threadq) {
                        if (ntd->td_mpcount && !mpheld && !cpu_try_mplock()) {
                            /* spinning due to MP lock being held */
#ifdef  INVARIANTS
                            ++mplock_contention_count;
#endif
                            /* mplock still not held, 'mpheld' still valid */
                            continue;
                        }

                        /*
                         * mpheld state invalid after getalltokens call returns
                         * failure, but the variable is only needed for
                         * the loop.
                         */
                        if (ntd->td_toks && !lwkt_getalltokens(ntd)) {
                            /* spinning due to token contention */
#ifdef  INVARIANTS
                            ++token_contention_count;
#endif
                            mpheld = MP_LOCK_HELD();
                            continue;
                        }
                        break;
                    }
                    if (ntd)
                        break;
                    rqmask &= ~(1 << nq);
                    nq = bsrl(rqmask);

                    /*
                     * We have two choices. We can either refuse to run a
                     * user thread when a kernel thread needs the MP lock
                     * but could not get it, or we can allow it to run but
                     * then expect an IPI (hopefully) later on to force a
                     * reschedule when the MP lock might become available.
                     */
                    if (nq < TDPRI_KERN_LPSCHED) {
                        if (chain_mplock == 0)
                                break;
                        atomic_set_int(&mp_lock_contention_mask,
                                       gd->gd_cpumask);
                        /* continue loop, allow user threads to be scheduled */
                    }
                }
                if (ntd == NULL) {
                    cpu_mplock_contested();
                    ntd = &gd->gd_idlethread;
                    ntd->td_flags |= TDF_IDLE_NOHLT;
                    goto using_idle_thread;
                } else {
                    ++gd->gd_cnt.v_swtch;
                    TAILQ_REMOVE(&gd->gd_tdrunq[nq], ntd, td_threadq);
                    TAILQ_INSERT_TAIL(&gd->gd_tdrunq[nq], ntd, td_threadq);
                }
            } else {
                ++gd->gd_cnt.v_swtch;
                TAILQ_REMOVE(&gd->gd_tdrunq[nq], ntd, td_threadq);
                TAILQ_INSERT_TAIL(&gd->gd_tdrunq[nq], ntd, td_threadq);
            }
#else
            /*
             * THREAD SELECTION FOR A UP MACHINE BUILD.  We don't have to
             * worry about tokens or the BGL.  However, we still have
             * to call lwkt_getalltokens() in order to properly detect
             * stale tokens.  This call cannot fail for a UP build!
             */
            lwkt_getalltokens(ntd);
            ++gd->gd_cnt.v_swtch;
            TAILQ_REMOVE(&gd->gd_tdrunq[nq], ntd, td_threadq);
            TAILQ_INSERT_TAIL(&gd->gd_tdrunq[nq], ntd, td_threadq);
#endif
        } else {
            /*
             * We have nothing to run but only let the idle loop halt
             * the cpu if there are no pending interrupts.
             */
            ntd = &gd->gd_idlethread;
            if (gd->gd_reqflags & RQF_IDLECHECK_MASK)
                ntd->td_flags |= TDF_IDLE_NOHLT;
#ifdef SMP
using_idle_thread:
            /*
             * The idle thread should not be holding the MP lock unless we
             * are trapping in the kernel or in a panic.  Since we select the
             * idle thread unconditionally when no other thread is available,
             * if the MP lock is desired during a panic or kernel trap, we
             * have to loop in the scheduler until we get it.
             */
            if (ntd->td_mpcount) {
                mpheld = MP_LOCK_HELD();
                if (gd->gd_trap_nesting_level == 0 && panicstr == NULL) {
                    panic("Idle thread %p was holding the BGL!", ntd);
                } else if (mpheld == 0) {
                    cpu_mplock_contested();
                    goto again;
                }
            }
#endif
        }
    }
    KASSERT(ntd->td_pri >= TDPRI_CRIT,
        ("priority problem in lwkt_switch %d %d", td->td_pri, ntd->td_pri));

    /*
     * Do the actual switch.  If the new target does not need the MP lock
     * and we are holding it, release the MP lock.  If the new target requires
     * the MP lock we have already acquired it for the target.
     */
#ifdef SMP
    if (ntd->td_mpcount == 0 ) {
        if (MP_LOCK_HELD())
            cpu_rel_mplock();
    } else {
        ASSERT_MP_LOCK_HELD(ntd);
    }
#endif
    if (td != ntd) {
        ++switch_count;
        KKASSERT(jg_tos_ok(ntd));
        td->td_switch(ntd);
    }
    /* NOTE: current cpu may have changed after switch */
    crit_exit_quick(td);
}

/*
 * Request that the target thread preempt the current thread.  Preemption
 * only works under a specific set of conditions:
 *
 *      - We are not preempting ourselves
 *      - The target thread is owned by the current cpu
 *      - We are not currently being preempted
 *      - The target is not currently being preempted
 *      - We are not holding any spin locks
 *      - The target thread is not holding any tokens
 *      - We are able to satisfy the target's MP lock requirements (if any).
 *
 * THE CALLER OF LWKT_PREEMPT() MUST BE IN A CRITICAL SECTION.  Typically
 * this is called via lwkt_schedule() through the td_preemptable callback.
 * critpri is the managed critical priority that we should ignore in order
 * to determine whether preemption is possible (aka usually just the crit
 * priority of lwkt_schedule() itself).
 *
 * XXX at the moment we run the target thread in a critical section during
 * the preemption in order to prevent the target from taking interrupts
 * that *WE* can't.  Preemption is strictly limited to interrupt threads
 * and interrupt-like threads, outside of a critical section, and the
 * preempted source thread will be resumed the instant the target blocks
 * whether or not the source is scheduled (i.e. preemption is supposed to
 * be as transparent as possible).
 *
 * The target thread inherits our MP count (added to its own) for the
 * duration of the preemption in order to preserve the atomicy of the
 * MP lock during the preemption.  Therefore, any preempting targets must be
 * careful in regards to MP assertions.  Note that the MP count may be
 * out of sync with the physical mp_lock, but we do not have to preserve
 * the original ownership of the lock if it was out of synch (that is, we
 * can leave it synchronized on return).
 */
void
lwkt_preempt(thread_t ntd, int critpri)
{
    struct globaldata *gd = mycpu;
    thread_t td;
#ifdef SMP
    int mpheld;
    int savecnt;
#endif

    /*
     * The caller has put us in a critical section.  We can only preempt
     * if the caller of the caller was not in a critical section (basically
     * a local interrupt), as determined by the 'critpri' parameter.  We
     * also can't preempt if the caller is holding any spinlocks (even if
     * he isn't in a critical section).  This also handles the tokens test.
     *
     * YYY The target thread must be in a critical section (else it must
     * inherit our critical section?  I dunno yet).
     *
     * Set need_lwkt_resched() unconditionally for now YYY.
     */
    KASSERT(ntd->td_pri >= TDPRI_CRIT, ("BADCRIT0 %d", ntd->td_pri));

    td = gd->gd_curthread;
    if ((ntd->td_pri & TDPRI_MASK) <= (td->td_pri & TDPRI_MASK)) {
        ++preempt_miss;
        return;
    }
    if ((td->td_pri & ~TDPRI_MASK) > critpri) {
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
#ifdef SMP
    if (ntd->td_gd != gd) {
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
#endif
    /*
     * Take the easy way out and do not preempt if we are holding
     * any spinlocks.  We could test whether the thread(s) being
     * preempted interlock against the target thread's tokens and whether
     * we can get all the target thread's tokens, but this situation 
     * should not occur very often so its easier to simply not preempt.
     * Also, plain spinlocks are impossible to figure out at this point so 
     * just don't preempt.
     *
     * Do not try to preempt if the target thread is holding any tokens.
     * We could try to acquire the tokens but this case is so rare there
     * is no need to support it.
     */
    if (gd->gd_spinlock_rd || gd->gd_spinlocks_wr) {
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
    if (ntd->td_toks) {
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
    if (td == ntd || ((td->td_flags | ntd->td_flags) & TDF_PREEMPT_LOCK)) {
        ++preempt_weird;
        need_lwkt_resched();
        return;
    }
    if (ntd->td_preempted) {
        ++preempt_hit;
        need_lwkt_resched();
        return;
    }
#ifdef SMP
    /*
     * note: an interrupt might have occured just as we were transitioning
     * to or from the MP lock.  In this case td_mpcount will be pre-disposed
     * (non-zero) but not actually synchronized with the actual state of the
     * lock.  We can use it to imply an MP lock requirement for the
     * preemption but we cannot use it to test whether we hold the MP lock
     * or not.
     */
    savecnt = td->td_mpcount;
    mpheld = MP_LOCK_HELD();
    ntd->td_mpcount += td->td_mpcount;
    if (mpheld == 0 && ntd->td_mpcount && !cpu_try_mplock()) {
        ntd->td_mpcount -= td->td_mpcount;
        ++preempt_miss;
        need_lwkt_resched();
        return;
    }
#endif

    /*
     * Since we are able to preempt the current thread, there is no need to
     * call need_lwkt_resched().
     */
    ++preempt_hit;
    ntd->td_preempted = td;
    td->td_flags |= TDF_PREEMPT_LOCK;
    td->td_switch(ntd);

    KKASSERT(ntd->td_preempted && (td->td_flags & TDF_PREEMPT_DONE));
#ifdef SMP
    KKASSERT(savecnt == td->td_mpcount);
    mpheld = MP_LOCK_HELD();
    if (mpheld && td->td_mpcount == 0)
        cpu_rel_mplock();
    else if (mpheld == 0 && td->td_mpcount)
        panic("lwkt_preempt(): MP lock was not held through");
#endif
    ntd->td_preempted = NULL;
    td->td_flags &= ~(TDF_PREEMPT_LOCK|TDF_PREEMPT_DONE);
}

/*
 * Yield our thread while higher priority threads are pending.  This is
 * typically called when we leave a critical section but it can be safely
 * called while we are in a critical section.
 *
 * This function will not generally yield to equal priority threads but it
 * can occur as a side effect.  Note that lwkt_switch() is called from
 * inside the critical section to prevent its own crit_exit() from reentering
 * lwkt_yield_quick().
 *
 * gd_reqflags indicates that *something* changed, e.g. an interrupt or softint
 * came along but was blocked and made pending.
 *
 * (self contained on a per cpu basis)
 */
void
lwkt_yield_quick(void)
{
    globaldata_t gd = mycpu;
    thread_t td = gd->gd_curthread;

    /*
     * gd_reqflags is cleared in splz if the cpl is 0.  If we were to clear
     * it with a non-zero cpl then we might not wind up calling splz after
     * a task switch when the critical section is exited even though the
     * new task could accept the interrupt.
     *
     * XXX from crit_exit() only called after last crit section is released.
     * If called directly will run splz() even if in a critical section.
     *
     * td_nest_count prevent deep nesting via splz() or doreti().  Note that
     * except for this special case, we MUST call splz() here to handle any
     * pending ints, particularly after we switch, or we might accidently
     * halt the cpu with interrupts pending.
     */
    if (gd->gd_reqflags && td->td_nest_count < 2)
        splz();

    /*
     * YYY enabling will cause wakeup() to task-switch, which really
     * confused the old 4.x code.  This is a good way to simulate
     * preemption and MP without actually doing preemption or MP, because a
     * lot of code assumes that wakeup() does not block.
     */
    if (untimely_switch && td->td_nest_count == 0 &&
        gd->gd_intr_nesting_level == 0
    ) {
        crit_enter_quick(td);
        /*
         * YYY temporary hacks until we disassociate the userland scheduler
         * from the LWKT scheduler.
         */
        if (td->td_flags & TDF_RUNQ) {
            lwkt_switch();              /* will not reenter yield function */
        } else {
            lwkt_schedule_self(td);     /* make sure we are scheduled */
            lwkt_switch();              /* will not reenter yield function */
            lwkt_deschedule_self(td);   /* make sure we are descheduled */
        }
        crit_exit_noyield(td);
    }
}

/*
 * This implements a normal yield which, unlike _quick, will yield to equal
 * priority threads as well.  Note that gd_reqflags tests will be handled by
 * the crit_exit() call in lwkt_switch().
 *
 * (self contained on a per cpu basis)
 */
void
lwkt_yield(void)
{
    lwkt_schedule_self(curthread);
    lwkt_switch();
}

/*
 * Return 0 if no runnable threads are pending at the same or higher
 * priority as the passed thread.
 *
 * Return 1 if runnable threads are pending at the same priority.
 *
 * Return 2 if runnable threads are pending at a higher priority.
 */
int
lwkt_check_resched(thread_t td)
{
        int pri = td->td_pri & TDPRI_MASK;

        if (td->td_gd->gd_runqmask > (2 << pri) - 1)
                return(2);
        if (TAILQ_NEXT(td, td_threadq))
                return(1);
        return(0);
}

/*
 * Generic schedule.  Possibly schedule threads belonging to other cpus and
 * deal with threads that might be blocked on a wait queue.
 *
 * We have a little helper inline function which does additional work after
 * the thread has been enqueued, including dealing with preemption and
 * setting need_lwkt_resched() (which prevents the kernel from returning
 * to userland until it has processed higher priority threads).
 *
 * It is possible for this routine to be called after a failed _enqueue
 * (due to the target thread migrating, sleeping, or otherwise blocked).
 * We have to check that the thread is actually on the run queue!
 *
 * reschedok is an optimized constant propagated from lwkt_schedule() or
 * lwkt_schedule_noresched().  By default it is non-zero, causing a
 * reschedule to be requested if the target thread has a higher priority.
 * The port messaging code will set MSG_NORESCHED and cause reschedok to
 * be 0, prevented undesired reschedules.
 */
static __inline
void
_lwkt_schedule_post(globaldata_t gd, thread_t ntd, int cpri, int reschedok)
{
    thread_t otd;

    if (ntd->td_flags & TDF_RUNQ) {
        if (ntd->td_preemptable && reschedok) {
            ntd->td_preemptable(ntd, cpri);     /* YYY +token */
        } else if (reschedok) {
            otd = curthread;
            if ((ntd->td_pri & TDPRI_MASK) > (otd->td_pri & TDPRI_MASK))
                need_lwkt_resched();
        }
    }
}

static __inline
void
_lwkt_schedule(thread_t td, int reschedok)
{
    globaldata_t mygd = mycpu;

    KASSERT(td != &td->td_gd->gd_idlethread, ("lwkt_schedule(): scheduling gd_idlethread is illegal!"));
    crit_enter_gd(mygd);
    KKASSERT(td->td_lwp == NULL || (td->td_lwp->lwp_flag & LWP_ONRUNQ) == 0);
    if (td == mygd->gd_curthread) {
        _lwkt_enqueue(td);
    } else {
        /*
         * If we own the thread, there is no race (since we are in a
         * critical section).  If we do not own the thread there might
         * be a race but the target cpu will deal with it.
         */
#ifdef SMP
        if (td->td_gd == mygd) {
            _lwkt_enqueue(td);
            _lwkt_schedule_post(mygd, td, TDPRI_CRIT, reschedok);
        } else {
            lwkt_send_ipiq(td->td_gd, (ipifunc1_t)lwkt_schedule, td);
        }
#else
        _lwkt_enqueue(td);
        _lwkt_schedule_post(mygd, td, TDPRI_CRIT, reschedok);
#endif
    }
    crit_exit_gd(mygd);
}

void
lwkt_schedule(thread_t td)
{
    _lwkt_schedule(td, 1);
}

void
lwkt_schedule_noresched(thread_t td)
{
    _lwkt_schedule(td, 0);
}

#ifdef SMP

/*
 * Thread migration using a 'Pull' method.  The thread may or may not be
 * the current thread.  It MUST be descheduled and in a stable state.
 * lwkt_giveaway() must be called on the cpu owning the thread.
 *
 * At any point after lwkt_giveaway() is called, the target cpu may
 * 'pull' the thread by calling lwkt_acquire().
 *
 * MPSAFE - must be called under very specific conditions.
 */
void
lwkt_giveaway(thread_t td)
{
        globaldata_t gd = mycpu;

        crit_enter_gd(gd);
        KKASSERT(td->td_gd == gd);
        TAILQ_REMOVE(&gd->gd_tdallq, td, td_allq);
        td->td_flags |= TDF_MIGRATING;
        crit_exit_gd(gd);
}

void
lwkt_acquire(thread_t td)
{
    globaldata_t gd;
    globaldata_t mygd;

    KKASSERT(td->td_flags & TDF_MIGRATING);
    gd = td->td_gd;
    mygd = mycpu;
    if (gd != mycpu) {
        cpu_lfence();
        KKASSERT((td->td_flags & TDF_RUNQ) == 0);
        crit_enter_gd(mygd);
        while (td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) {
#ifdef SMP
            lwkt_process_ipiq();
#endif
            cpu_lfence();
        }
        td->td_gd = mygd;
        TAILQ_INSERT_TAIL(&mygd->gd_tdallq, td, td_allq);
        td->td_flags &= ~TDF_MIGRATING;
        crit_exit_gd(mygd);
    } else {
        crit_enter_gd(mygd);
        TAILQ_INSERT_TAIL(&mygd->gd_tdallq, td, td_allq);
        td->td_flags &= ~TDF_MIGRATING;
        crit_exit_gd(mygd);
    }
}

#endif

/*
 * Generic deschedule.  Descheduling threads other then your own should be
 * done only in carefully controlled circumstances.  Descheduling is 
 * asynchronous.  
 *
 * This function may block if the cpu has run out of messages.
 */
void
lwkt_deschedule(thread_t td)
{
    crit_enter();
#ifdef SMP
    if (td == curthread) {
        _lwkt_dequeue(td);
    } else {
        if (td->td_gd == mycpu) {
            _lwkt_dequeue(td);
        } else {
            lwkt_send_ipiq(td->td_gd, (ipifunc1_t)lwkt_deschedule, td);
        }
    }
#else
    _lwkt_dequeue(td);
#endif
    crit_exit();
}

/*
 * Set the target thread's priority.  This routine does not automatically
 * switch to a higher priority thread, LWKT threads are not designed for
 * continuous priority changes.  Yield if you want to switch.
 *
 * We have to retain the critical section count which uses the high bits
 * of the td_pri field.  The specified priority may also indicate zero or
 * more critical sections by adding TDPRI_CRIT*N.
 *
 * Note that we requeue the thread whether it winds up on a different runq
 * or not.  uio_yield() depends on this and the routine is not normally
 * called with the same priority otherwise.
 */
void
lwkt_setpri(thread_t td, int pri)
{
    KKASSERT(pri >= 0);
    KKASSERT(td->td_gd == mycpu);
    crit_enter();
    if (td->td_flags & TDF_RUNQ) {
        _lwkt_dequeue(td);
        td->td_pri = (td->td_pri & ~TDPRI_MASK) + pri;
        _lwkt_enqueue(td);
    } else {
        td->td_pri = (td->td_pri & ~TDPRI_MASK) + pri;
    }
    crit_exit();
}

void
lwkt_setpri_self(int pri)
{
    thread_t td = curthread;

    KKASSERT(pri >= 0 && pri <= TDPRI_MAX);
    crit_enter();
    if (td->td_flags & TDF_RUNQ) {
        _lwkt_dequeue(td);
        td->td_pri = (td->td_pri & ~TDPRI_MASK) + pri;
        _lwkt_enqueue(td);
    } else {
        td->td_pri = (td->td_pri & ~TDPRI_MASK) + pri;
    }
    crit_exit();
}

/*
 * Migrate the current thread to the specified cpu. 
 *
 * This is accomplished by descheduling ourselves from the current cpu,
 * moving our thread to the tdallq of the target cpu, IPI messaging the
 * target cpu, and switching out.  TDF_MIGRATING prevents scheduling
 * races while the thread is being migrated.
 */
#ifdef SMP
static void lwkt_setcpu_remote(void *arg);
#endif

void
lwkt_setcpu_self(globaldata_t rgd)
{
#ifdef SMP
    thread_t td = curthread;

    if (td->td_gd != rgd) {
        crit_enter_quick(td);
        td->td_flags |= TDF_MIGRATING;
        lwkt_deschedule_self(td);
        TAILQ_REMOVE(&td->td_gd->gd_tdallq, td, td_allq);
        lwkt_send_ipiq(rgd, (ipifunc1_t)lwkt_setcpu_remote, td);
        lwkt_switch();
        /* we are now on the target cpu */
        TAILQ_INSERT_TAIL(&rgd->gd_tdallq, td, td_allq);
        crit_exit_quick(td);
    }
#endif
}

void
lwkt_migratecpu(int cpuid)
{
#ifdef SMP
        globaldata_t rgd;

        rgd = globaldata_find(cpuid);
        lwkt_setcpu_self(rgd);
#endif
}

/*
 * Remote IPI for cpu migration (called while in a critical section so we
 * do not have to enter another one).  The thread has already been moved to
 * our cpu's allq, but we must wait for the thread to be completely switched
 * out on the originating cpu before we schedule it on ours or the stack
 * state may be corrupt.  We clear TDF_MIGRATING after flushing the GD
 * change to main memory.
 *
 * XXX The use of TDF_MIGRATING might not be sufficient to avoid races
 * against wakeups.  It is best if this interface is used only when there
 * are no pending events that might try to schedule the thread.
 */
#ifdef SMP
static void
lwkt_setcpu_remote(void *arg)
{
    thread_t td = arg;
    globaldata_t gd = mycpu;

    while (td->td_flags & (TDF_RUNNING|TDF_PREEMPT_LOCK)) {
#ifdef SMP
        lwkt_process_ipiq();
#endif
        cpu_lfence();
    }
    td->td_gd = gd;
    cpu_sfence();
    td->td_flags &= ~TDF_MIGRATING;
    KKASSERT(td->td_lwp == NULL || (td->td_lwp->lwp_flag & LWP_ONRUNQ) == 0);
    _lwkt_enqueue(td);
}
#endif

struct lwp *
lwkt_preempted_proc(void)
{
    thread_t td = curthread;
    while (td->td_preempted)
        td = td->td_preempted;
    return(td->td_lwp);
}

/*
 * Create a kernel process/thread/whatever.  It shares it's address space
 * with proc0 - ie: kernel only.
 *
 * NOTE!  By default new threads are created with the MP lock held.  A 
 * thread which does not require the MP lock should release it by calling
 * rel_mplock() at the start of the new thread.
 */
int
lwkt_create(void (*func)(void *), void *arg,
    struct thread **tdp, thread_t template, int tdflags, int cpu,
    const char *fmt, ...)
{
    thread_t td;
    __va_list ap;

    td = lwkt_alloc_thread(template, LWKT_THREAD_STACK, cpu,
                           tdflags);
    if (tdp)
        *tdp = td;
    cpu_set_thread_handler(td, lwkt_exit, func, arg);

    /*
     * Set up arg0 for 'ps' etc
     */
    __va_start(ap, fmt);
    kvsnprintf(td->td_comm, sizeof(td->td_comm), fmt, ap);
    __va_end(ap);

    /*
     * Schedule the thread to run
     */
    if ((td->td_flags & TDF_STOPREQ) == 0)
        lwkt_schedule(td);
    else
        td->td_flags &= ~TDF_STOPREQ;
    return 0;
}

/*
 * Destroy an LWKT thread.   Warning!  This function is not called when
 * a process exits, cpu_proc_exit() directly calls cpu_thread_exit() and
 * uses a different reaping mechanism.
 */
void
lwkt_exit(void)
{
    thread_t td = curthread;
    thread_t std;
    globaldata_t gd;

    if (td->td_flags & TDF_VERBOSE)
        kprintf("kthread %p %s has exited\n", td, td->td_comm);
    caps_exit(td);

    /*
     * Get us into a critical section to interlock gd_freetd and loop
     * until we can get it freed.
     *
     * We have to cache the current td in gd_freetd because objcache_put()ing
     * it would rip it out from under us while our thread is still active.
     */
    gd = mycpu;
    crit_enter_quick(td);
    while ((std = gd->gd_freetd) != NULL) {
        gd->gd_freetd = NULL;
        objcache_put(thread_cache, std);
    }
    lwkt_deschedule_self(td);
    lwkt_remove_tdallq(td);
    if (td->td_flags & TDF_ALLOCATED_THREAD)
        gd->gd_freetd = td;
    cpu_thread_exit();
}

void
lwkt_remove_tdallq(thread_t td)
{
    KKASSERT(td->td_gd == mycpu);
    TAILQ_REMOVE(&td->td_gd->gd_tdallq, td, td_allq);
}

void
crit_panic(void)
{
    thread_t td = curthread;
    int lpri = td->td_pri;

    td->td_pri = 0;
    panic("td_pri is/would-go negative! %p %d", td, lpri);
}

#ifdef SMP

/*
 * Called from debugger/panic on cpus which have been stopped.  We must still
 * process the IPIQ while stopped, even if we were stopped while in a critical
 * section (XXX).
 *
 * If we are dumping also try to process any pending interrupts.  This may
 * or may not work depending on the state of the cpu at the point it was
 * stopped.
 */
void
lwkt_smp_stopped(void)
{
    globaldata_t gd = mycpu;

    crit_enter_gd(gd);
    if (dumping) {
        lwkt_process_ipiq();
        splz();
    } else {
        lwkt_process_ipiq();
    }
    crit_exit_gd(gd);
}

/*
 * get_mplock() calls this routine if it is unable to obtain the MP lock.
 * get_mplock() has already incremented td_mpcount.  We must block and
 * not return until giant is held.
 *
 * All we have to do is lwkt_switch() away.  The LWKT scheduler will not
 * reschedule the thread until it can obtain the giant lock for it.
 */
void
lwkt_mp_lock_contested(void)
{
    loggiant(beg);
    lwkt_switch();
    loggiant(end);
}

/*
 * The rel_mplock() code will call this function after releasing the
 * last reference on the MP lock if mp_lock_contention_mask is non-zero.
 *
 * We then chain an IPI to a single other cpu potentially needing the
 * lock.  This is a bit heuristical and we can wind up with IPIs flying
 * all over the place.
 */
static void lwkt_mp_lock_uncontested_remote(void *arg __unused);

void
lwkt_mp_lock_uncontested(void)
{
    globaldata_t gd;
    globaldata_t dgd;
    cpumask_t mask;
    cpumask_t tmpmask;
    int cpuid;

    if (chain_mplock) {
        gd = mycpu;
        atomic_clear_int(&mp_lock_contention_mask, gd->gd_cpumask);
        mask = mp_lock_contention_mask;
        tmpmask = ~((1 << gd->gd_cpuid) - 1);

        if (mask) {
            if (mask & tmpmask)
                    cpuid = bsfl(mask & tmpmask);
            else
                    cpuid = bsfl(mask);
            atomic_clear_int(&mp_lock_contention_mask, 1 << cpuid);
            dgd = globaldata_find(cpuid);
            lwkt_send_ipiq(dgd, lwkt_mp_lock_uncontested_remote, NULL);
        }
    }
}

/*
 * The idea is for this IPI to interrupt a potentially lower priority
 * thread, such as a user thread, to allow the scheduler to reschedule
 * a higher priority kernel thread that needs the MP lock.
 *
 * For now we set the LWKT reschedule flag which generates an AST in
 * doreti, though theoretically it is also possible to possibly preempt
 * here if the underlying thread was operating in user mode.  Nah.
 */
static void
lwkt_mp_lock_uncontested_remote(void *arg __unused)
{
        need_lwkt_resched();
}

#endif