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[netbsd-mini2440.git] / sys / kern / kern_lwp.c

/*      $NetBSD: kern_lwp.c,v 1.136 2009/10/27 02:58:28 rmind Exp $     */

/*-
 * Copyright (c) 2001, 2006, 2007, 2008, 2009 The NetBSD Foundation, Inc.
 * All rights reserved.
 *
 * This code is derived from software contributed to The NetBSD Foundation
 * by Nathan J. Williams, and Andrew Doran.
 *
 * 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.
 *
 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. 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 FOUNDATION 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.
 */

/*
 * Overview
 *
 *      Lightweight processes (LWPs) are the basic unit or thread of
 *      execution within the kernel.  The core state of an LWP is described
 *      by "struct lwp", also known as lwp_t.
 *
 *      Each LWP is contained within a process (described by "struct proc"),
 *      Every process contains at least one LWP, but may contain more.  The
 *      process describes attributes shared among all of its LWPs such as a
 *      private address space, global execution state (stopped, active,
 *      zombie, ...), signal disposition and so on.  On a multiprocessor
 *      machine, multiple LWPs be executing concurrently in the kernel.
 *
 * Execution states
 *
 *      At any given time, an LWP has overall state that is described by
 *      lwp::l_stat.  The states are broken into two sets below.  The first
 *      set is guaranteed to represent the absolute, current state of the
 *      LWP:
 *
 *      LSONPROC
 *
 *              On processor: the LWP is executing on a CPU, either in the
 *              kernel or in user space.
 *
 *      LSRUN
 *
 *              Runnable: the LWP is parked on a run queue, and may soon be
 *              chosen to run by an idle processor, or by a processor that
 *              has been asked to preempt a currently runnning but lower
 *              priority LWP.
 *
 *      LSIDL
 *
 *              Idle: the LWP has been created but has not yet executed,
 *              or it has ceased executing a unit of work and is waiting
 *              to be started again.
 *
 *      LSSUSPENDED:
 *
 *              Suspended: the LWP has had its execution suspended by
 *              another LWP in the same process using the _lwp_suspend()
 *              system call.  User-level LWPs also enter the suspended
 *              state when the system is shutting down.
 *
 *      The second set represent a "statement of intent" on behalf of the
 *      LWP.  The LWP may in fact be executing on a processor, may be
 *      sleeping or idle. It is expected to take the necessary action to
 *      stop executing or become "running" again within a short timeframe.
 *      The LP_RUNNING flag in lwp::l_pflag indicates that an LWP is running.
 *      Importantly, it indicates that its state is tied to a CPU.
 *
 *      LSZOMB:
 *
 *              Dead or dying: the LWP has released most of its resources
 *              and is about to switch away into oblivion, or has already
 *              switched away.  When it switches away, its few remaining
 *              resources can be collected.
 *
 *      LSSLEEP:
 *
 *              Sleeping: the LWP has entered itself onto a sleep queue, and
 *              has switched away or will switch away shortly to allow other
 *              LWPs to run on the CPU.
 *
 *      LSSTOP:
 *
 *              Stopped: the LWP has been stopped as a result of a job
 *              control signal, or as a result of the ptrace() interface. 
 *
 *              Stopped LWPs may run briefly within the kernel to handle
 *              signals that they receive, but will not return to user space
 *              until their process' state is changed away from stopped. 
 *
 *              Single LWPs within a process can not be set stopped
 *              selectively: all actions that can stop or continue LWPs
 *              occur at the process level.
 *
 * State transitions
 *
 *      Note that the LSSTOP state may only be set when returning to
 *      user space in userret(), or when sleeping interruptably.  The
 *      LSSUSPENDED state may only be set in userret().  Before setting
 *      those states, we try to ensure that the LWPs will release all
 *      locks that they hold, and at a minimum try to ensure that the
 *      LWP can be set runnable again by a signal.
 *
 *      LWPs may transition states in the following ways:
 *
 *       RUN -------> ONPROC            ONPROC -----> RUN
 *                                                  > SLEEP
 *                                                  > STOPPED
 *                                                  > SUSPENDED
 *                                                  > ZOMB
 *                                                  > IDL (special cases)
 *
 *       STOPPED ---> RUN               SUSPENDED --> RUN
 *                  > SLEEP
 *
 *       SLEEP -----> ONPROC            IDL --------> RUN
 *                  > RUN                           > SUSPENDED
 *                  > STOPPED                       > STOPPED
 *                                                  > ONPROC (special cases)
 *
 *      Some state transitions are only possible with kernel threads (eg
 *      ONPROC -> IDL) and happen under tightly controlled circumstances
 *      free of unwanted side effects.
 *
 * Migration
 *
 *      Migration of threads from one CPU to another could be performed
 *      internally by the scheduler via sched_takecpu() or sched_catchlwp()
 *      functions.  The universal lwp_migrate() function should be used for
 *      any other cases.  Subsystems in the kernel must be aware that CPU
 *      of LWP may change, while it is not locked.
 *
 * Locking
 *
 *      The majority of fields in 'struct lwp' are covered by a single,
 *      general spin lock pointed to by lwp::l_mutex.  The locks covering
 *      each field are documented in sys/lwp.h.
 *
 *      State transitions must be made with the LWP's general lock held,
 *      and may cause the LWP's lock pointer to change. Manipulation of
 *      the general lock is not performed directly, but through calls to
 *      lwp_lock(), lwp_relock() and similar.
 *
 *      States and their associated locks:
 *
 *      LSONPROC, LSZOMB:
 *
 *              Always covered by spc_lwplock, which protects running LWPs.
 *              This is a per-CPU lock and matches lwp::l_cpu.
 *
 *      LSIDL, LSRUN:
 *
 *              Always covered by spc_mutex, which protects the run queues.
 *              This is a per-CPU lock and matches lwp::l_cpu.
 *
 *      LSSLEEP:
 *
 *              Covered by a lock associated with the sleep queue that the
 *              LWP resides on.  Matches lwp::l_sleepq::sq_mutex.
 *
 *      LSSTOP, LSSUSPENDED:
 *
 *              If the LWP was previously sleeping (l_wchan != NULL), then
 *              l_mutex references the sleep queue lock.  If the LWP was
 *              runnable or on the CPU when halted, or has been removed from
 *              the sleep queue since halted, then the lock is spc_lwplock.
 *
 *      The lock order is as follows:
 *
 *              spc::spc_lwplock ->
 *                  sleeptab::st_mutex ->
 *                      tschain_t::tc_mutex ->
 *                          spc::spc_mutex
 *
 *      Each process has an scheduler state lock (proc::p_lock), and a
 *      number of counters on LWPs and their states: p_nzlwps, p_nrlwps, and
 *      so on.  When an LWP is to be entered into or removed from one of the
 *      following states, p_lock must be held and the process wide counters
 *      adjusted:
 *
 *              LSIDL, LSZOMB, LSSTOP, LSSUSPENDED
 *
 *      (But not always for kernel threads.  There are some special cases
 *      as mentioned above.  See kern_softint.c.)
 *
 *      Note that an LWP is considered running or likely to run soon if in
 *      one of the following states.  This affects the value of p_nrlwps:
 *
 *              LSRUN, LSONPROC, LSSLEEP
 *
 *      p_lock does not need to be held when transitioning among these
 *      three states, hence p_lock is rarely taken for state transitions.
 */

#include <sys/cdefs.h>
__KERNEL_RCSID(0, "$NetBSD: kern_lwp.c,v 1.136 2009/10/27 02:58:28 rmind Exp $");

#include "opt_ddb.h"
#include "opt_lockdebug.h"
#include "opt_sa.h"

#define _LWP_API_PRIVATE

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/cpu.h>
#include <sys/pool.h>
#include <sys/proc.h>
#include <sys/sa.h>
#include <sys/savar.h>
#include <sys/syscallargs.h>
#include <sys/syscall_stats.h>
#include <sys/kauth.h>
#include <sys/sleepq.h>
#include <sys/lockdebug.h>
#include <sys/kmem.h>
#include <sys/pset.h>
#include <sys/intr.h>
#include <sys/lwpctl.h>
#include <sys/atomic.h>
#include <sys/filedesc.h>

#include <uvm/uvm_extern.h>
#include <uvm/uvm_object.h>

struct lwplist  alllwp = LIST_HEAD_INITIALIZER(alllwp);

struct pool lwp_uc_pool;

static pool_cache_t lwp_cache;
static specificdata_domain_t lwp_specificdata_domain;

void
lwpinit(void)
{

        pool_init(&lwp_uc_pool, sizeof(ucontext_t), 0, 0, 0, "lwpucpl",
            &pool_allocator_nointr, IPL_NONE);
        lwp_specificdata_domain = specificdata_domain_create();
        KASSERT(lwp_specificdata_domain != NULL);
        lwp_sys_init();
        lwp_cache = pool_cache_init(sizeof(lwp_t), MIN_LWP_ALIGNMENT, 0, 0,
            "lwppl", NULL, IPL_NONE, NULL, NULL, NULL);
}

/*
 * Set an suspended.
 *
 * Must be called with p_lock held, and the LWP locked.  Will unlock the
 * LWP before return.
 */
int
lwp_suspend(struct lwp *curl, struct lwp *t)
{
        int error;

        KASSERT(mutex_owned(t->l_proc->p_lock));
        KASSERT(lwp_locked(t, NULL));

        KASSERT(curl != t || curl->l_stat == LSONPROC);

        /*
         * If the current LWP has been told to exit, we must not suspend anyone
         * else or deadlock could occur.  We won't return to userspace.
         */
        if ((curl->l_flag & (LW_WEXIT | LW_WCORE)) != 0) {
                lwp_unlock(t);
                return (EDEADLK);
        }

        error = 0;

        switch (t->l_stat) {
        case LSRUN:
        case LSONPROC:
                t->l_flag |= LW_WSUSPEND;
                lwp_need_userret(t);
                lwp_unlock(t);
                break;

        case LSSLEEP:
                t->l_flag |= LW_WSUSPEND;

                /*
                 * Kick the LWP and try to get it to the kernel boundary
                 * so that it will release any locks that it holds.
                 * setrunnable() will release the lock.
                 */
                if ((t->l_flag & LW_SINTR) != 0)
                        setrunnable(t);
                else
                        lwp_unlock(t);
                break;

        case LSSUSPENDED:
                lwp_unlock(t);
                break;

        case LSSTOP:
                t->l_flag |= LW_WSUSPEND;
                setrunnable(t);
                break;

        case LSIDL:
        case LSZOMB:
                error = EINTR; /* It's what Solaris does..... */
                lwp_unlock(t);
                break;
        }

        return (error);
}

/*
 * Restart a suspended LWP.
 *
 * Must be called with p_lock held, and the LWP locked.  Will unlock the
 * LWP before return.
 */
void
lwp_continue(struct lwp *l)
{

        KASSERT(mutex_owned(l->l_proc->p_lock));
        KASSERT(lwp_locked(l, NULL));

        /* If rebooting or not suspended, then just bail out. */
        if ((l->l_flag & LW_WREBOOT) != 0) {
                lwp_unlock(l);
                return;
        }

        l->l_flag &= ~LW_WSUSPEND;

        if (l->l_stat != LSSUSPENDED) {
                lwp_unlock(l);
                return;
        }

        /* setrunnable() will release the lock. */
        setrunnable(l);
}

/*
 * Wait for an LWP within the current process to exit.  If 'lid' is
 * non-zero, we are waiting for a specific LWP.
 *
 * Must be called with p->p_lock held.
 */
int
lwp_wait1(struct lwp *l, lwpid_t lid, lwpid_t *departed, int flags)
{
        struct proc *p = l->l_proc;
        struct lwp *l2;
        int nfound, error;
        lwpid_t curlid;
        bool exiting;

        KASSERT(mutex_owned(p->p_lock));

        p->p_nlwpwait++;
        l->l_waitingfor = lid;
        curlid = l->l_lid;
        exiting = ((flags & LWPWAIT_EXITCONTROL) != 0);

        for (;;) {
                /*
                 * Avoid a race between exit1() and sigexit(): if the
                 * process is dumping core, then we need to bail out: call
                 * into lwp_userret() where we will be suspended until the
                 * deed is done.
                 */
                if ((p->p_sflag & PS_WCORE) != 0) {
                        mutex_exit(p->p_lock);
                        lwp_userret(l);
#ifdef DIAGNOSTIC
                        panic("lwp_wait1");
#endif
                        /* NOTREACHED */
                }

                /*
                 * First off, drain any detached LWP that is waiting to be
                 * reaped.
                 */
                while ((l2 = p->p_zomblwp) != NULL) {
                        p->p_zomblwp = NULL;
                        lwp_free(l2, false, false);/* releases proc mutex */
                        mutex_enter(p->p_lock);
                }

                /*
                 * Now look for an LWP to collect.  If the whole process is
                 * exiting, count detached LWPs as eligible to be collected,
                 * but don't drain them here.
                 */
                nfound = 0;
                error = 0;
                LIST_FOREACH(l2, &p->p_lwps, l_sibling) {
                        /*
                         * If a specific wait and the target is waiting on
                         * us, then avoid deadlock.  This also traps LWPs
                         * that try to wait on themselves.
                         *
                         * Note that this does not handle more complicated
                         * cycles, like: t1 -> t2 -> t3 -> t1.  The process
                         * can still be killed so it is not a major problem.
                         */
                        if (l2->l_lid == lid && l2->l_waitingfor == curlid) {
                                error = EDEADLK;
                                break;
                        }
                        if (l2 == l)
                                continue;
                        if ((l2->l_prflag & LPR_DETACHED) != 0) {
                                nfound += exiting;
                                continue;
                        }
                        if (lid != 0) {
                                if (l2->l_lid != lid)
                                        continue;
                                /*
                                 * Mark this LWP as the first waiter, if there
                                 * is no other.
                                 */
                                if (l2->l_waiter == 0)
                                        l2->l_waiter = curlid;
                        } else if (l2->l_waiter != 0) {
                                /*
                                 * It already has a waiter - so don't
                                 * collect it.  If the waiter doesn't
                                 * grab it we'll get another chance
                                 * later.
                                 */
                                nfound++;
                                continue;
                        }
                        nfound++;

                        /* No need to lock the LWP in order to see LSZOMB. */
                        if (l2->l_stat != LSZOMB)
                                continue;

                        /*
                         * We're no longer waiting.  Reset the "first waiter"
                         * pointer on the target, in case it was us.
                         */
                        l->l_waitingfor = 0;
                        l2->l_waiter = 0;
                        p->p_nlwpwait--;
                        if (departed)
                                *departed = l2->l_lid;
                        sched_lwp_collect(l2);

                        /* lwp_free() releases the proc lock. */
                        lwp_free(l2, false, false);
                        mutex_enter(p->p_lock);
                        return 0;
                }

                if (error != 0)
                        break;
                if (nfound == 0) {
                        error = ESRCH;
                        break;
                }

                /*
                 * The kernel is careful to ensure that it can not deadlock
                 * when exiting - just keep waiting.
                 */
                if (exiting) {
                        KASSERT(p->p_nlwps > 1);
                        cv_wait(&p->p_lwpcv, p->p_lock);
                        continue;
                }

                /*
                 * If all other LWPs are waiting for exits or suspends
                 * and the supply of zombies and potential zombies is
                 * exhausted, then we are about to deadlock.
                 *
                 * If the process is exiting (and this LWP is not the one
                 * that is coordinating the exit) then bail out now.
                 */
                if ((p->p_sflag & PS_WEXIT) != 0 ||
                    p->p_nrlwps + p->p_nzlwps - p->p_ndlwps <= p->p_nlwpwait) {
                        error = EDEADLK;
                        break;
                }

                /*
                 * Sit around and wait for something to happen.  We'll be 
                 * awoken if any of the conditions examined change: if an
                 * LWP exits, is collected, or is detached.
                 */
                if ((error = cv_wait_sig(&p->p_lwpcv, p->p_lock)) != 0)
                        break;
        }

        /*
         * We didn't find any LWPs to collect, we may have received a 
         * signal, or some other condition has caused us to bail out.
         *
         * If waiting on a specific LWP, clear the waiters marker: some
         * other LWP may want it.  Then, kick all the remaining waiters
         * so that they can re-check for zombies and for deadlock.
         */
        if (lid != 0) {
                LIST_FOREACH(l2, &p->p_lwps, l_sibling) {
                        if (l2->l_lid == lid) {
                                if (l2->l_waiter == curlid)
                                        l2->l_waiter = 0;
                                break;
                        }
                }
        }
        p->p_nlwpwait--;
        l->l_waitingfor = 0;
        cv_broadcast(&p->p_lwpcv);

        return error;
}

/*
 * Create a new LWP within process 'p2', using LWP 'l1' as a template.
 * The new LWP is created in state LSIDL and must be set running,
 * suspended, or stopped by the caller.
 */
int
lwp_create(lwp_t *l1, proc_t *p2, vaddr_t uaddr, int flags,
           void *stack, size_t stacksize, void (*func)(void *), void *arg,
           lwp_t **rnewlwpp, int sclass)
{
        struct lwp *l2, *isfree;
        turnstile_t *ts;

        KASSERT(l1 == curlwp || l1->l_proc == &proc0);

        /*
         * First off, reap any detached LWP waiting to be collected.
         * We can re-use its LWP structure and turnstile.
         */
        isfree = NULL;
        if (p2->p_zomblwp != NULL) {
                mutex_enter(p2->p_lock);
                if ((isfree = p2->p_zomblwp) != NULL) {
                        p2->p_zomblwp = NULL;
                        lwp_free(isfree, true, false);/* releases proc mutex */
                } else
                        mutex_exit(p2->p_lock);
        }
        if (isfree == NULL) {
                l2 = pool_cache_get(lwp_cache, PR_WAITOK);
                memset(l2, 0, sizeof(*l2));
                l2->l_ts = pool_cache_get(turnstile_cache, PR_WAITOK);
                SLIST_INIT(&l2->l_pi_lenders);
        } else {
                l2 = isfree;
                ts = l2->l_ts;
                KASSERT(l2->l_inheritedprio == -1);
                KASSERT(SLIST_EMPTY(&l2->l_pi_lenders));
                memset(l2, 0, sizeof(*l2));
                l2->l_ts = ts;
        }

        l2->l_stat = LSIDL;
        l2->l_proc = p2;
        l2->l_refcnt = 1;
        l2->l_class = sclass;

        /*
         * If vfork(), we want the LWP to run fast and on the same CPU
         * as its parent, so that it can reuse the VM context and cache
         * footprint on the local CPU.
         */
        l2->l_kpriority = ((flags & LWP_VFORK) ? true : false);
        l2->l_kpribase = PRI_KERNEL;
        l2->l_priority = l1->l_priority;
        l2->l_inheritedprio = -1;
        l2->l_flag = 0;
        l2->l_pflag = LP_MPSAFE;
        TAILQ_INIT(&l2->l_ld_locks);

        /*
         * If not the first LWP in the process, grab a reference to the
         * descriptor table.
         */
        l2->l_fd = p2->p_fd;
        if (p2->p_nlwps != 0) {
                KASSERT(l1->l_proc == p2);
                fd_hold(l2);
        } else {
                KASSERT(l1->l_proc != p2);
        }

        if (p2->p_flag & PK_SYSTEM) {
                /* Mark it as a system LWP. */
                l2->l_flag |= LW_SYSTEM;
        }

        kpreempt_disable();
        l2->l_mutex = l1->l_cpu->ci_schedstate.spc_mutex;
        l2->l_cpu = l1->l_cpu;
        kpreempt_enable();

        lwp_initspecific(l2);
        sched_lwp_fork(l1, l2);
        lwp_update_creds(l2);
        callout_init(&l2->l_timeout_ch, CALLOUT_MPSAFE);
        callout_setfunc(&l2->l_timeout_ch, sleepq_timeout, l2);
        cv_init(&l2->l_sigcv, "sigwait");
        l2->l_syncobj = &sched_syncobj;

        if (rnewlwpp != NULL)
                *rnewlwpp = l2;

        uvm_lwp_setuarea(l2, uaddr);
        uvm_lwp_fork(l1, l2, stack, stacksize, func,
            (arg != NULL) ? arg : l2);

        mutex_enter(p2->p_lock);

        if ((flags & LWP_DETACHED) != 0) {
                l2->l_prflag = LPR_DETACHED;
                p2->p_ndlwps++;
        } else
                l2->l_prflag = 0;

        l2->l_sigmask = l1->l_sigmask;
        CIRCLEQ_INIT(&l2->l_sigpend.sp_info);
        sigemptyset(&l2->l_sigpend.sp_set);

        p2->p_nlwpid++;
        if (p2->p_nlwpid == 0)
                p2->p_nlwpid++;
        l2->l_lid = p2->p_nlwpid;
        LIST_INSERT_HEAD(&p2->p_lwps, l2, l_sibling);
        p2->p_nlwps++;

        if ((p2->p_flag & PK_SYSTEM) == 0) {
                /* Inherit an affinity */
                if (l1->l_flag & LW_AFFINITY) {
                        /*
                         * Note that we hold the state lock while inheriting
                         * the affinity to avoid race with sched_setaffinity().
                         */
                        lwp_lock(l1);
                        if (l1->l_flag & LW_AFFINITY) {
                                kcpuset_use(l1->l_affinity);
                                l2->l_affinity = l1->l_affinity;
                                l2->l_flag |= LW_AFFINITY;
                        }
                        lwp_unlock(l1);
                }
                lwp_lock(l2);
                /* Inherit a processor-set */
                l2->l_psid = l1->l_psid;
                /* Look for a CPU to start */
                l2->l_cpu = sched_takecpu(l2);
                lwp_unlock_to(l2, l2->l_cpu->ci_schedstate.spc_mutex);
        }
        mutex_exit(p2->p_lock);

        mutex_enter(proc_lock);
        LIST_INSERT_HEAD(&alllwp, l2, l_list);
        mutex_exit(proc_lock);

        SYSCALL_TIME_LWP_INIT(l2);

        if (p2->p_emul->e_lwp_fork)
                (*p2->p_emul->e_lwp_fork)(l1, l2);

        return (0);
}

/*
 * Called by MD code when a new LWP begins execution.  Must be called
 * with the previous LWP locked (so at splsched), or if there is no
 * previous LWP, at splsched.
 */
void
lwp_startup(struct lwp *prev, struct lwp *new)
{

        KASSERT(kpreempt_disabled());
        if (prev != NULL) {
                /*
                 * Normalize the count of the spin-mutexes, it was
                 * increased in mi_switch().  Unmark the state of
                 * context switch - it is finished for previous LWP.
                 */
                curcpu()->ci_mtx_count++;
                membar_exit();
                prev->l_ctxswtch = 0;
        }
        KPREEMPT_DISABLE(new);
        spl0();
        pmap_activate(new);
        LOCKDEBUG_BARRIER(NULL, 0);
        KPREEMPT_ENABLE(new);
        if ((new->l_pflag & LP_MPSAFE) == 0) {
                KERNEL_LOCK(1, new);
        }
}

/*
 * Exit an LWP.
 */
void
lwp_exit(struct lwp *l)
{
        struct proc *p = l->l_proc;
        struct lwp *l2;
        bool current;

        current = (l == curlwp);

        KASSERT(current || (l->l_stat == LSIDL && l->l_target_cpu == NULL));
        KASSERT(p == curproc);

        /*
         * Verify that we hold no locks other than the kernel lock.
         */
        LOCKDEBUG_BARRIER(&kernel_lock, 0);

        /*
         * If we are the last live LWP in a process, we need to exit the
         * entire process.  We do so with an exit status of zero, because
         * it's a "controlled" exit, and because that's what Solaris does.
         *
         * We are not quite a zombie yet, but for accounting purposes we
         * must increment the count of zombies here.
         *
         * Note: the last LWP's specificdata will be deleted here.
         */
        mutex_enter(p->p_lock);
        if (p->p_nlwps - p->p_nzlwps == 1) {
                KASSERT(current == true);
                /* XXXSMP kernel_lock not held */
                exit1(l, 0);
                /* NOTREACHED */
        }
        p->p_nzlwps++;
        mutex_exit(p->p_lock);

        if (p->p_emul->e_lwp_exit)
                (*p->p_emul->e_lwp_exit)(l);

        /* Drop filedesc reference. */
        fd_free();

        /* Delete the specificdata while it's still safe to sleep. */
        specificdata_fini(lwp_specificdata_domain, &l->l_specdataref);

        /*
         * Release our cached credentials.
         */
        kauth_cred_free(l->l_cred);
        callout_destroy(&l->l_timeout_ch);

        /*
         * Remove the LWP from the global list.
         */
        mutex_enter(proc_lock);
        LIST_REMOVE(l, l_list);
        mutex_exit(proc_lock);

        /*
         * Get rid of all references to the LWP that others (e.g. procfs)
         * may have, and mark the LWP as a zombie.  If the LWP is detached,
         * mark it waiting for collection in the proc structure.  Note that
         * before we can do that, we need to free any other dead, deatched
         * LWP waiting to meet its maker.
         */
        mutex_enter(p->p_lock);
        lwp_drainrefs(l);

        if ((l->l_prflag & LPR_DETACHED) != 0) {
                while ((l2 = p->p_zomblwp) != NULL) {
                        p->p_zomblwp = NULL;
                        lwp_free(l2, false, false);/* releases proc mutex */
                        mutex_enter(p->p_lock);
                        l->l_refcnt++;
                        lwp_drainrefs(l);
                }
                p->p_zomblwp = l;
        }

        /*
         * If we find a pending signal for the process and we have been
         * asked to check for signals, then we loose: arrange to have
         * all other LWPs in the process check for signals.
         */
        if ((l->l_flag & LW_PENDSIG) != 0 &&
            firstsig(&p->p_sigpend.sp_set) != 0) {
                LIST_FOREACH(l2, &p->p_lwps, l_sibling) {
                        lwp_lock(l2);
                        l2->l_flag |= LW_PENDSIG;
                        lwp_unlock(l2);
                }
        }

        lwp_lock(l);
        l->l_stat = LSZOMB;
        if (l->l_name != NULL)
                strcpy(l->l_name, "(zombie)");
        if (l->l_flag & LW_AFFINITY) {
                l->l_flag &= ~LW_AFFINITY;
        } else {
                KASSERT(l->l_affinity == NULL);
        }
        lwp_unlock(l);
        p->p_nrlwps--;
        cv_broadcast(&p->p_lwpcv);
        if (l->l_lwpctl != NULL)
                l->l_lwpctl->lc_curcpu = LWPCTL_CPU_EXITED;
        mutex_exit(p->p_lock);

        /* Safe without lock since LWP is in zombie state */
        if (l->l_affinity) {
                kcpuset_unuse(l->l_affinity, NULL);
                l->l_affinity = NULL;
        }

        /*
         * We can no longer block.  At this point, lwp_free() may already
         * be gunning for us.  On a multi-CPU system, we may be off p_lwps.
         *
         * Free MD LWP resources.
         */
        cpu_lwp_free(l, 0);

        if (current) {
                pmap_deactivate(l);

                /*
                 * Release the kernel lock, and switch away into
                 * oblivion.
                 */
#ifdef notyet
                /* XXXSMP hold in lwp_userret() */
                KERNEL_UNLOCK_LAST(l);
#else
                KERNEL_UNLOCK_ALL(l, NULL);
#endif
                lwp_exit_switchaway(l);
        }
}

/*
 * Free a dead LWP's remaining resources.
 *
 * XXXLWP limits.
 */
void
lwp_free(struct lwp *l, bool recycle, bool last)
{
        struct proc *p = l->l_proc;
        struct rusage *ru;
        ksiginfoq_t kq;

        KASSERT(l != curlwp);

        /*
         * If this was not the last LWP in the process, then adjust
         * counters and unlock.
         */
        if (!last) {
                /*
                 * Add the LWP's run time to the process' base value.
                 * This needs to co-incide with coming off p_lwps.
                 */
                bintime_add(&p->p_rtime, &l->l_rtime);
                p->p_pctcpu += l->l_pctcpu;
                ru = &p->p_stats->p_ru;
                ruadd(ru, &l->l_ru);
                ru->ru_nvcsw += (l->l_ncsw - l->l_nivcsw);
                ru->ru_nivcsw += l->l_nivcsw;
                LIST_REMOVE(l, l_sibling);
                p->p_nlwps--;
                p->p_nzlwps--;
                if ((l->l_prflag & LPR_DETACHED) != 0)
                        p->p_ndlwps--;

                /*
                 * Have any LWPs sleeping in lwp_wait() recheck for
                 * deadlock.
                 */
                cv_broadcast(&p->p_lwpcv);
                mutex_exit(p->p_lock);
        }

#ifdef MULTIPROCESSOR
        /*
         * In the unlikely event that the LWP is still on the CPU,
         * then spin until it has switched away.  We need to release
         * all locks to avoid deadlock against interrupt handlers on
         * the target CPU.
         */
        if ((l->l_pflag & LP_RUNNING) != 0 || l->l_cpu->ci_curlwp == l) {
                int count;
                (void)count; /* XXXgcc */
                KERNEL_UNLOCK_ALL(curlwp, &count);
                while ((l->l_pflag & LP_RUNNING) != 0 ||
                    l->l_cpu->ci_curlwp == l)
                        SPINLOCK_BACKOFF_HOOK;
                KERNEL_LOCK(count, curlwp);
        }
#endif

        /*
         * Destroy the LWP's remaining signal information.
         */
        ksiginfo_queue_init(&kq);
        sigclear(&l->l_sigpend, NULL, &kq);
        ksiginfo_queue_drain(&kq);
        cv_destroy(&l->l_sigcv);

        /*
         * Free the LWP's turnstile and the LWP structure itself unless the
         * caller wants to recycle them.  Also, free the scheduler specific
         * data.
         *
         * We can't return turnstile0 to the pool (it didn't come from it),
         * so if it comes up just drop it quietly and move on.
         *
         * We don't recycle the VM resources at this time.
         */
        if (l->l_lwpctl != NULL)
                lwp_ctl_free(l);

        if (!recycle && l->l_ts != &turnstile0)
                pool_cache_put(turnstile_cache, l->l_ts);
        if (l->l_name != NULL)
                kmem_free(l->l_name, MAXCOMLEN);

        cpu_lwp_free2(l);
        uvm_lwp_exit(l);

        KASSERT(SLIST_EMPTY(&l->l_pi_lenders));
        KASSERT(l->l_inheritedprio == -1);
        if (!recycle)
                pool_cache_put(lwp_cache, l);
}

/*
 * Migrate the LWP to the another CPU.  Unlocks the LWP.
 */
void
lwp_migrate(lwp_t *l, struct cpu_info *tci)
{
        struct schedstate_percpu *tspc;
        int lstat = l->l_stat;

        KASSERT(lwp_locked(l, NULL));
        KASSERT(tci != NULL);

        /* If LWP is still on the CPU, it must be handled like LSONPROC */
        if ((l->l_pflag & LP_RUNNING) != 0) {
                lstat = LSONPROC;
        }

        /*
         * The destination CPU could be changed while previous migration
         * was not finished.
         */
        if (l->l_target_cpu != NULL) {
                l->l_target_cpu = tci;
                lwp_unlock(l);
                return;
        }

        /* Nothing to do if trying to migrate to the same CPU */
        if (l->l_cpu == tci) {
                lwp_unlock(l);
                return;
        }

        KASSERT(l->l_target_cpu == NULL);
        tspc = &tci->ci_schedstate;
        switch (lstat) {
        case LSRUN:
                l->l_target_cpu = tci;
                break;
        case LSIDL:
                l->l_cpu = tci;
                lwp_unlock_to(l, tspc->spc_mutex);
                return;
        case LSSLEEP:
                l->l_cpu = tci;
                break;
        case LSSTOP:
        case LSSUSPENDED:
                l->l_cpu = tci;
                if (l->l_wchan == NULL) {
                        lwp_unlock_to(l, tspc->spc_lwplock);
                        return;
                }
                break;
        case LSONPROC:
                l->l_target_cpu = tci;
                spc_lock(l->l_cpu);
                cpu_need_resched(l->l_cpu, RESCHED_KPREEMPT);
                spc_unlock(l->l_cpu);
                break;
        }
        lwp_unlock(l);
}

/*
 * Find the LWP in the process.  Arguments may be zero, in such case,
 * the calling process and first LWP in the list will be used.
 * On success - returns proc locked.
 */
struct lwp *
lwp_find2(pid_t pid, lwpid_t lid)
{
        proc_t *p;
        lwp_t *l;

        /* Find the process */
        p = (pid == 0) ? curlwp->l_proc : p_find(pid, PFIND_UNLOCK_FAIL);
        if (p == NULL)
                return NULL;
        mutex_enter(p->p_lock);
        if (pid != 0) {
                /* Case of p_find */
                mutex_exit(proc_lock);
        }

        /* Find the thread */
        l = (lid == 0) ? LIST_FIRST(&p->p_lwps) : lwp_find(p, lid);
        if (l == NULL) {
                mutex_exit(p->p_lock);
        }

        return l;
}

/*
 * Look up a live LWP within the speicifed process, and return it locked.
 *
 * Must be called with p->p_lock held.
 */
struct lwp *
lwp_find(struct proc *p, int id)
{
        struct lwp *l;

        KASSERT(mutex_owned(p->p_lock));

        LIST_FOREACH(l, &p->p_lwps, l_sibling) {
                if (l->l_lid == id)
                        break;
        }

        /*
         * No need to lock - all of these conditions will
         * be visible with the process level mutex held.
         */
        if (l != NULL && (l->l_stat == LSIDL || l->l_stat == LSZOMB))
                l = NULL;

        return l;
}

/*
 * Update an LWP's cached credentials to mirror the process' master copy.
 *
 * This happens early in the syscall path, on user trap, and on LWP
 * creation.  A long-running LWP can also voluntarily choose to update
 * it's credentials by calling this routine.  This may be called from
 * LWP_CACHE_CREDS(), which checks l->l_cred != p->p_cred beforehand.
 */
void
lwp_update_creds(struct lwp *l)
{
        kauth_cred_t oc;
        struct proc *p;

        p = l->l_proc;
        oc = l->l_cred;

        mutex_enter(p->p_lock);
        kauth_cred_hold(p->p_cred);
        l->l_cred = p->p_cred;
        l->l_prflag &= ~LPR_CRMOD;
        mutex_exit(p->p_lock);
        if (oc != NULL)
                kauth_cred_free(oc);
}

/*
 * Verify that an LWP is locked, and optionally verify that the lock matches
 * one we specify.
 */
int
lwp_locked(struct lwp *l, kmutex_t *mtx)
{
        kmutex_t *cur = l->l_mutex;

        return mutex_owned(cur) && (mtx == cur || mtx == NULL);
}

/*
 * Lock an LWP.
 */
kmutex_t *
lwp_lock_retry(struct lwp *l, kmutex_t *old)
{

        /*
         * XXXgcc ignoring kmutex_t * volatile on i386
         *
         * gcc version 4.1.2 20061021 prerelease (NetBSD nb1 20061021)
         */
#if 1
        while (l->l_mutex != old) {
#else
        for (;;) {
#endif
                mutex_spin_exit(old);
                old = l->l_mutex;
                mutex_spin_enter(old);

                /*
                 * mutex_enter() will have posted a read barrier.  Re-test
                 * l->l_mutex.  If it has changed, we need to try again.
                 */
#if 1
        }
#else
        } while (__predict_false(l->l_mutex != old));
#endif

        return old;
}

/*
 * Lend a new mutex to an LWP.  The old mutex must be held.
 */
void
lwp_setlock(struct lwp *l, kmutex_t *new)
{

        KASSERT(mutex_owned(l->l_mutex));

        membar_exit();
        l->l_mutex = new;
}

/*
 * Lend a new mutex to an LWP, and release the old mutex.  The old mutex
 * must be held.
 */
void
lwp_unlock_to(struct lwp *l, kmutex_t *new)
{
        kmutex_t *old;

        KASSERT(mutex_owned(l->l_mutex));

        old = l->l_mutex;
        membar_exit();
        l->l_mutex = new;
        mutex_spin_exit(old);
}

/*
 * Acquire a new mutex, and donate it to an LWP.  The LWP must already be
 * locked.
 */
void
lwp_relock(struct lwp *l, kmutex_t *new)
{
        kmutex_t *old;

        KASSERT(mutex_owned(l->l_mutex));

        old = l->l_mutex;
        if (old != new) {
                mutex_spin_enter(new);
                l->l_mutex = new;
                mutex_spin_exit(old);
        }
}

int
lwp_trylock(struct lwp *l)
{
        kmutex_t *old;

        for (;;) {
                if (!mutex_tryenter(old = l->l_mutex))
                        return 0;
                if (__predict_true(l->l_mutex == old))
                        return 1;
                mutex_spin_exit(old);
        }
}

void
lwp_unsleep(lwp_t *l, bool cleanup)
{

        KASSERT(mutex_owned(l->l_mutex));
        (*l->l_syncobj->sobj_unsleep)(l, cleanup);
}


/*
 * Handle exceptions for mi_userret().  Called if a member of LW_USERRET is
 * set.
 */
void
lwp_userret(struct lwp *l)
{
        struct proc *p;
        void (*hook)(void);
        int sig;

        KASSERT(l == curlwp);
        KASSERT(l->l_stat == LSONPROC);
        p = l->l_proc;

#ifndef __HAVE_FAST_SOFTINTS
        /* Run pending soft interrupts. */
        if (l->l_cpu->ci_data.cpu_softints != 0)
                softint_overlay();
#endif

#ifdef KERN_SA
        /* Generate UNBLOCKED upcall if needed */
        if (l->l_flag & LW_SA_BLOCKING) {
                sa_unblock_userret(l);
                /* NOTREACHED */
        }
#endif

        /*
         * It should be safe to do this read unlocked on a multiprocessor
         * system..
         *
         * LW_SA_UPCALL will be handled after the while() loop, so don't
         * consider it now.
         */
        while ((l->l_flag & (LW_USERRET & ~(LW_SA_UPCALL))) != 0) {
                /*
                 * Process pending signals first, unless the process
                 * is dumping core or exiting, where we will instead
                 * enter the LW_WSUSPEND case below.
                 */
                if ((l->l_flag & (LW_PENDSIG | LW_WCORE | LW_WEXIT)) ==
                    LW_PENDSIG) {
                        mutex_enter(p->p_lock);
                        while ((sig = issignal(l)) != 0)
                                postsig(sig);
                        mutex_exit(p->p_lock);
                }

                /*
                 * Core-dump or suspend pending.
                 *
                 * In case of core dump, suspend ourselves, so that the
                 * kernel stack and therefore the userland registers saved
                 * in the trapframe are around for coredump() to write them
                 * out.  We issue a wakeup on p->p_lwpcv so that sigexit()
                 * will write the core file out once all other LWPs are
                 * suspended.
                 */
                if ((l->l_flag & LW_WSUSPEND) != 0) {
                        mutex_enter(p->p_lock);
                        p->p_nrlwps--;
                        cv_broadcast(&p->p_lwpcv);
                        lwp_lock(l);
                        l->l_stat = LSSUSPENDED;
                        lwp_unlock(l);
                        mutex_exit(p->p_lock);
                        lwp_lock(l);
                        mi_switch(l);
                }

                /* Process is exiting. */
                if ((l->l_flag & LW_WEXIT) != 0) {
                        lwp_exit(l);
                        KASSERT(0);
                        /* NOTREACHED */
                }

                /* Call userret hook; used by Linux emulation. */
                if ((l->l_flag & LW_WUSERRET) != 0) {
                        lwp_lock(l);
                        l->l_flag &= ~LW_WUSERRET;
                        lwp_unlock(l);
                        hook = p->p_userret;
                        p->p_userret = NULL;
                        (*hook)();
                }
        }

#ifdef KERN_SA
        /*
         * Timer events are handled specially.  We only try once to deliver
         * pending timer upcalls; if if fails, we can try again on the next
         * loop around.  If we need to re-enter lwp_userret(), MD code will
         * bounce us back here through the trap path after we return.
         */
        if (p->p_timerpend)
                timerupcall(l);
        if (l->l_flag & LW_SA_UPCALL)
                sa_upcall_userret(l);
#endif /* KERN_SA */
}

/*
 * Force an LWP to enter the kernel, to take a trip through lwp_userret().
 */
void
lwp_need_userret(struct lwp *l)
{
        KASSERT(lwp_locked(l, NULL));

        /*
         * Since the tests in lwp_userret() are done unlocked, make sure
         * that the condition will be seen before forcing the LWP to enter
         * kernel mode.
         */
        membar_producer();
        cpu_signotify(l);
}

/*
 * Add one reference to an LWP.  This will prevent the LWP from
 * exiting, thus keep the lwp structure and PCB around to inspect.
 */
void
lwp_addref(struct lwp *l)
{

        KASSERT(mutex_owned(l->l_proc->p_lock));
        KASSERT(l->l_stat != LSZOMB);
        KASSERT(l->l_refcnt != 0);

        l->l_refcnt++;
}

/*
 * Remove one reference to an LWP.  If this is the last reference,
 * then we must finalize the LWP's death.
 */
void
lwp_delref(struct lwp *l)
{
        struct proc *p = l->l_proc;

        mutex_enter(p->p_lock);
        KASSERT(l->l_stat != LSZOMB);
        KASSERT(l->l_refcnt > 0);
        if (--l->l_refcnt == 0)
                cv_broadcast(&p->p_lwpcv);
        mutex_exit(p->p_lock);
}

/*
 * Drain all references to the current LWP.
 */
void
lwp_drainrefs(struct lwp *l)
{
        struct proc *p = l->l_proc;

        KASSERT(mutex_owned(p->p_lock));
        KASSERT(l->l_refcnt != 0);

        l->l_refcnt--;
        while (l->l_refcnt != 0)
                cv_wait(&p->p_lwpcv, p->p_lock);
}

/*
 * Return true if the specified LWP is 'alive'.  Only p->p_lock need
 * be held.
 */
bool
lwp_alive(lwp_t *l)
{

        KASSERT(mutex_owned(l->l_proc->p_lock));

        switch (l->l_stat) {
        case LSSLEEP:
        case LSRUN:
        case LSONPROC:
        case LSSTOP:
        case LSSUSPENDED:
                return true;
        default:
                return false;
        }
}

/*
 * Return first live LWP in the process.
 */
lwp_t *
lwp_find_first(proc_t *p)
{
        lwp_t *l;

        KASSERT(mutex_owned(p->p_lock));

        LIST_FOREACH(l, &p->p_lwps, l_sibling) {
                if (lwp_alive(l)) {
                        return l;
                }
        }

        return NULL;
}

/*
 * lwp_specific_key_create --
 *      Create a key for subsystem lwp-specific data.
 */
int
lwp_specific_key_create(specificdata_key_t *keyp, specificdata_dtor_t dtor)
{

        return (specificdata_key_create(lwp_specificdata_domain, keyp, dtor));
}

/*
 * lwp_specific_key_delete --
 *      Delete a key for subsystem lwp-specific data.
 */
void
lwp_specific_key_delete(specificdata_key_t key)
{

        specificdata_key_delete(lwp_specificdata_domain, key);
}

/*
 * lwp_initspecific --
 *      Initialize an LWP's specificdata container.
 */
void
lwp_initspecific(struct lwp *l)
{
        int error;

        error = specificdata_init(lwp_specificdata_domain, &l->l_specdataref);
        KASSERT(error == 0);
}

/*
 * lwp_finispecific --
 *      Finalize an LWP's specificdata container.
 */
void
lwp_finispecific(struct lwp *l)
{

        specificdata_fini(lwp_specificdata_domain, &l->l_specdataref);
}

/*
 * lwp_getspecific --
 *      Return lwp-specific data corresponding to the specified key.
 *
 *      Note: LWP specific data is NOT INTERLOCKED.  An LWP should access
 *      only its OWN SPECIFIC DATA.  If it is necessary to access another
 *      LWP's specifc data, care must be taken to ensure that doing so
 *      would not cause internal data structure inconsistency (i.e. caller
 *      can guarantee that the target LWP is not inside an lwp_getspecific()
 *      or lwp_setspecific() call).
 */
void *
lwp_getspecific(specificdata_key_t key)
{

        return (specificdata_getspecific_unlocked(lwp_specificdata_domain,
                                                  &curlwp->l_specdataref, key));
}

void *
_lwp_getspecific_by_lwp(struct lwp *l, specificdata_key_t key)
{

        return (specificdata_getspecific_unlocked(lwp_specificdata_domain,
                                                  &l->l_specdataref, key));
}

/*
 * lwp_setspecific --
 *      Set lwp-specific data corresponding to the specified key.
 */
void
lwp_setspecific(specificdata_key_t key, void *data)
{

        specificdata_setspecific(lwp_specificdata_domain,
                                 &curlwp->l_specdataref, key, data);
}

/*
 * Allocate a new lwpctl structure for a user LWP.
 */
int
lwp_ctl_alloc(vaddr_t *uaddr)
{
        lcproc_t *lp;
        u_int bit, i, offset;
        struct uvm_object *uao;
        int error;
        lcpage_t *lcp;
        proc_t *p;
        lwp_t *l;

        l = curlwp;
        p = l->l_proc;

        if (l->l_lcpage != NULL) {
                lcp = l->l_lcpage;
                *uaddr = lcp->lcp_uaddr + (vaddr_t)l->l_lwpctl - lcp->lcp_kaddr;
                return (EINVAL);
        }

        /* First time around, allocate header structure for the process. */
        if ((lp = p->p_lwpctl) == NULL) {
                lp = kmem_alloc(sizeof(*lp), KM_SLEEP);
                mutex_init(&lp->lp_lock, MUTEX_DEFAULT, IPL_NONE);
                lp->lp_uao = NULL;
                TAILQ_INIT(&lp->lp_pages);
                mutex_enter(p->p_lock);
                if (p->p_lwpctl == NULL) {
                        p->p_lwpctl = lp;
                        mutex_exit(p->p_lock);
                } else {
                        mutex_exit(p->p_lock);
                        mutex_destroy(&lp->lp_lock);
                        kmem_free(lp, sizeof(*lp));
                        lp = p->p_lwpctl;
                }
        }

        /*
         * Set up an anonymous memory region to hold the shared pages.
         * Map them into the process' address space.  The user vmspace
         * gets the first reference on the UAO.
         */
        mutex_enter(&lp->lp_lock);
        if (lp->lp_uao == NULL) {
                lp->lp_uao = uao_create(LWPCTL_UAREA_SZ, 0);
                lp->lp_cur = 0;
                lp->lp_max = LWPCTL_UAREA_SZ;
                lp->lp_uva = p->p_emul->e_vm_default_addr(p,
                     (vaddr_t)p->p_vmspace->vm_daddr, LWPCTL_UAREA_SZ);
                error = uvm_map(&p->p_vmspace->vm_map, &lp->lp_uva,
                    LWPCTL_UAREA_SZ, lp->lp_uao, 0, 0, UVM_MAPFLAG(UVM_PROT_RW,
                    UVM_PROT_RW, UVM_INH_NONE, UVM_ADV_NORMAL, 0));
                if (error != 0) {
                        uao_detach(lp->lp_uao);
                        lp->lp_uao = NULL;
                        mutex_exit(&lp->lp_lock);
                        return error;
                }
        }

        /* Get a free block and allocate for this LWP. */
        TAILQ_FOREACH(lcp, &lp->lp_pages, lcp_chain) {
                if (lcp->lcp_nfree != 0)
                        break;
        }
        if (lcp == NULL) {
                /* Nothing available - try to set up a free page. */
                if (lp->lp_cur == lp->lp_max) {
                        mutex_exit(&lp->lp_lock);
                        return ENOMEM;
                }
                lcp = kmem_alloc(LWPCTL_LCPAGE_SZ, KM_SLEEP);
                if (lcp == NULL) {
                        mutex_exit(&lp->lp_lock);
                        return ENOMEM;
                }
                /*
                 * Wire the next page down in kernel space.  Since this
                 * is a new mapping, we must add a reference.
                 */
                uao = lp->lp_uao;
                (*uao->pgops->pgo_reference)(uao);
                lcp->lcp_kaddr = vm_map_min(kernel_map);
                error = uvm_map(kernel_map, &lcp->lcp_kaddr, PAGE_SIZE,
                    uao, lp->lp_cur, PAGE_SIZE,
                    UVM_MAPFLAG(UVM_PROT_RW, UVM_PROT_RW,
                    UVM_INH_NONE, UVM_ADV_RANDOM, 0));
                if (error != 0) {
                        mutex_exit(&lp->lp_lock);
                        kmem_free(lcp, LWPCTL_LCPAGE_SZ);
                        (*uao->pgops->pgo_detach)(uao);
                        return error;
                }
                error = uvm_map_pageable(kernel_map, lcp->lcp_kaddr,
                    lcp->lcp_kaddr + PAGE_SIZE, FALSE, 0);
                if (error != 0) {
                        mutex_exit(&lp->lp_lock);
                        uvm_unmap(kernel_map, lcp->lcp_kaddr,
                            lcp->lcp_kaddr + PAGE_SIZE);
                        kmem_free(lcp, LWPCTL_LCPAGE_SZ);
                        return error;
                }
                /* Prepare the page descriptor and link into the list. */
                lcp->lcp_uaddr = lp->lp_uva + lp->lp_cur;
                lp->lp_cur += PAGE_SIZE;
                lcp->lcp_nfree = LWPCTL_PER_PAGE;
                lcp->lcp_rotor = 0;
                memset(lcp->lcp_bitmap, 0xff, LWPCTL_BITMAP_SZ);
                TAILQ_INSERT_HEAD(&lp->lp_pages, lcp, lcp_chain);
        }
        for (i = lcp->lcp_rotor; lcp->lcp_bitmap[i] == 0;) {
                if (++i >= LWPCTL_BITMAP_ENTRIES)
                        i = 0;
        }
        bit = ffs(lcp->lcp_bitmap[i]) - 1;
        lcp->lcp_bitmap[i] ^= (1 << bit);
        lcp->lcp_rotor = i;
        lcp->lcp_nfree--;
        l->l_lcpage = lcp;
        offset = (i << 5) + bit;
        l->l_lwpctl = (lwpctl_t *)lcp->lcp_kaddr + offset;
        *uaddr = lcp->lcp_uaddr + offset * sizeof(lwpctl_t);
        mutex_exit(&lp->lp_lock);

        KPREEMPT_DISABLE(l);
        l->l_lwpctl->lc_curcpu = (int)curcpu()->ci_data.cpu_index;
        KPREEMPT_ENABLE(l);

        return 0;
}

/*
 * Free an lwpctl structure back to the per-process list.
 */
void
lwp_ctl_free(lwp_t *l)
{
        lcproc_t *lp;
        lcpage_t *lcp;
        u_int map, offset;

        lp = l->l_proc->p_lwpctl;
        KASSERT(lp != NULL);

        lcp = l->l_lcpage;
        offset = (u_int)((lwpctl_t *)l->l_lwpctl - (lwpctl_t *)lcp->lcp_kaddr);
        KASSERT(offset < LWPCTL_PER_PAGE);

        mutex_enter(&lp->lp_lock);
        lcp->lcp_nfree++;
        map = offset >> 5;
        lcp->lcp_bitmap[map] |= (1 << (offset & 31));
        if (lcp->lcp_bitmap[lcp->lcp_rotor] == 0)
                lcp->lcp_rotor = map;
        if (TAILQ_FIRST(&lp->lp_pages)->lcp_nfree == 0) {
                TAILQ_REMOVE(&lp->lp_pages, lcp, lcp_chain);
                TAILQ_INSERT_HEAD(&lp->lp_pages, lcp, lcp_chain);
        }
        mutex_exit(&lp->lp_lock);
}

/*
 * Process is exiting; tear down lwpctl state.  This can only be safely
 * called by the last LWP in the process.
 */
void
lwp_ctl_exit(void)
{
        lcpage_t *lcp, *next;
        lcproc_t *lp;
        proc_t *p;
        lwp_t *l;

        l = curlwp;
        l->l_lwpctl = NULL;
        l->l_lcpage = NULL;
        p = l->l_proc;
        lp = p->p_lwpctl;

        KASSERT(lp != NULL);
        KASSERT(p->p_nlwps == 1);

        for (lcp = TAILQ_FIRST(&lp->lp_pages); lcp != NULL; lcp = next) {
                next = TAILQ_NEXT(lcp, lcp_chain);
                uvm_unmap(kernel_map, lcp->lcp_kaddr,
                    lcp->lcp_kaddr + PAGE_SIZE);
                kmem_free(lcp, LWPCTL_LCPAGE_SZ);
        }

        if (lp->lp_uao != NULL) {
                uvm_unmap(&p->p_vmspace->vm_map, lp->lp_uva,
                    lp->lp_uva + LWPCTL_UAREA_SZ);
        }

        mutex_destroy(&lp->lp_lock);
        kmem_free(lp, sizeof(*lp));
        p->p_lwpctl = NULL;
}

/*
 * Return the current LWP's "preemption counter".  Used to detect
 * preemption across operations that can tolerate preemption without
 * crashing, but which may generate incorrect results if preempted.
 */
uint64_t
lwp_pctr(void)
{

        return curlwp->l_ncsw;
}

#if defined(DDB)
void
lwp_whatis(uintptr_t addr, void (*pr)(const char *, ...))
{
        lwp_t *l;

        LIST_FOREACH(l, &alllwp, l_list) {
                uintptr_t stack = (uintptr_t)KSTACK_LOWEST_ADDR(l);

                if (addr < stack || stack + KSTACK_SIZE <= addr) {
                        continue;
                }
                (*pr)("%p is %p+%zu, LWP %p's stack\n",
                    (void *)addr, (void *)stack,
                    (size_t)(addr - stack), l);
        }
}
#endif /* defined(DDB) */