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

/*
 * Copyright (c) 1982, 1986, 1989, 1991, 1993
 *      The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
 * Co. or Unix System Laboratories, Inc. and are reproduced herein with
 * the permission of UNIX System Laboratories, Inc.
 *
 * 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. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *      This product includes software developed by the University of
 *      California, Berkeley and its contributors.
 * 4. Neither the name of the University 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 REGENTS 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 REGENTS 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.
 *
 *      @(#)kern_fork.c 8.6 (Berkeley) 4/8/94
 * $FreeBSD: src/sys/kern/kern_fork.c,v 1.72.2.14 2003/06/26 04:15:10 silby Exp $
 * $DragonFly: src/sys/kern/kern_fork.c,v 1.77 2008/05/18 20:02:02 nth Exp $
 */

#include "opt_ktrace.h"

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/sysproto.h>
#include <sys/filedesc.h>
#include <sys/kernel.h>
#include <sys/sysctl.h>
#include <sys/malloc.h>
#include <sys/proc.h>
#include <sys/resourcevar.h>
#include <sys/vnode.h>
#include <sys/acct.h>
#include <sys/ktrace.h>
#include <sys/unistd.h>
#include <sys/jail.h>
#include <sys/caps.h>

#include <vm/vm.h>
#include <sys/lock.h>
#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_extern.h>

#include <sys/vmmeter.h>
#include <sys/refcount.h>
#include <sys/thread2.h>
#include <sys/signal2.h>
#include <sys/spinlock2.h>

#include <sys/dsched.h>

static MALLOC_DEFINE(M_ATFORK, "atfork", "atfork callback");

/*
 * These are the stuctures used to create a callout list for things to do
 * when forking a process
 */
struct forklist {
        forklist_fn function;
        TAILQ_ENTRY(forklist) next;
};

TAILQ_HEAD(forklist_head, forklist);
static struct forklist_head fork_list = TAILQ_HEAD_INITIALIZER(fork_list);

static struct lwp *lwp_fork(struct lwp *, struct proc *, int flags);

int forksleep; /* Place for fork1() to sleep on. */

/*
 * Red-Black tree support for LWPs
 */

static int
rb_lwp_compare(struct lwp *lp1, struct lwp *lp2)
{
        if (lp1->lwp_tid < lp2->lwp_tid)
                return(-1);
        if (lp1->lwp_tid > lp2->lwp_tid)
                return(1);
        return(0);
}

RB_GENERATE2(lwp_rb_tree, lwp, u.lwp_rbnode, rb_lwp_compare, lwpid_t, lwp_tid);

/*
 * Fork system call
 *
 * MPALMOSTSAFE
 */
int
sys_fork(struct fork_args *uap)
{
        struct lwp *lp = curthread->td_lwp;
        struct proc *p2;
        int error;

        error = fork1(lp, RFFDG | RFPROC | RFPGLOCK, &p2);
        if (error == 0) {
                start_forked_proc(lp, p2);
                uap->sysmsg_fds[0] = p2->p_pid;
                uap->sysmsg_fds[1] = 0;
        }
        return error;
}

/*
 * MPALMOSTSAFE
 */
int
sys_vfork(struct vfork_args *uap)
{
        struct lwp *lp = curthread->td_lwp;
        struct proc *p2;
        int error;

        error = fork1(lp, RFFDG | RFPROC | RFPPWAIT | RFMEM | RFPGLOCK, &p2);
        if (error == 0) {
                start_forked_proc(lp, p2);
                uap->sysmsg_fds[0] = p2->p_pid;
                uap->sysmsg_fds[1] = 0;
        }
        return error;
}

/*
 * Handle rforks.  An rfork may (1) operate on the current process without
 * creating a new, (2) create a new process that shared the current process's
 * vmspace, signals, and/or descriptors, or (3) create a new process that does
 * not share these things (normal fork).
 *
 * Note that we only call start_forked_proc() if a new process is actually
 * created.
 *
 * rfork { int flags }
 *
 * MPALMOSTSAFE
 */
int
sys_rfork(struct rfork_args *uap)
{
        struct lwp *lp = curthread->td_lwp;
        struct proc *p2;
        int error;

        if ((uap->flags & RFKERNELONLY) != 0)
                return (EINVAL);

        error = fork1(lp, uap->flags | RFPGLOCK, &p2);
        if (error == 0) {
                if (p2)
                        start_forked_proc(lp, p2);
                uap->sysmsg_fds[0] = p2 ? p2->p_pid : 0;
                uap->sysmsg_fds[1] = 0;
        }
        return error;
}

/*
 * MPALMOSTSAFE
 */
int
sys_lwp_create(struct lwp_create_args *uap)
{
        struct proc *p = curproc;
        struct lwp *lp;
        struct lwp_params params;
        int error;

        error = copyin(uap->params, &params, sizeof(params));
        if (error)
                goto fail2;

        lwkt_gettoken(&p->p_token);
        plimit_lwp_fork(p);     /* force exclusive access */
        lp = lwp_fork(curthread->td_lwp, p, RFPROC);
        error = cpu_prepare_lwp(lp, &params);
        if (error)
                goto fail;
        if (params.tid1 != NULL &&
            (error = copyout(&lp->lwp_tid, params.tid1, sizeof(lp->lwp_tid))))
                goto fail;
        if (params.tid2 != NULL &&
            (error = copyout(&lp->lwp_tid, params.tid2, sizeof(lp->lwp_tid))))
                goto fail;

        /*
         * Now schedule the new lwp. 
         */
        p->p_usched->resetpriority(lp);
        crit_enter();
        lp->lwp_stat = LSRUN;
        p->p_usched->setrunqueue(lp);
        crit_exit();
        lwkt_reltoken(&p->p_token);

        return (0);

fail:
        lwp_rb_tree_RB_REMOVE(&p->p_lwp_tree, lp);
        --p->p_nthreads;
        /* lwp_dispose expects an exited lwp, and a held proc */
        lp->lwp_flag |= LWP_WEXIT;
        lp->lwp_thread->td_flags |= TDF_EXITING;
        lwkt_remove_tdallq(lp->lwp_thread);
        PHOLD(p);
        lwp_dispose(lp);
        lwkt_reltoken(&p->p_token);
fail2:
        return (error);
}

int     nprocs = 1;             /* process 0 */

int
fork1(struct lwp *lp1, int flags, struct proc **procp)
{
        struct proc *p1 = lp1->lwp_proc;
        struct proc *p2, *pptr;
        struct pgrp *p1grp;
        struct pgrp *plkgrp;
        uid_t uid;
        int ok, error;
        static int curfail = 0;
        static struct timeval lastfail;
        struct forklist *ep;
        struct filedesc_to_leader *fdtol;

        if ((flags & (RFFDG|RFCFDG)) == (RFFDG|RFCFDG))
                return (EINVAL);

        lwkt_gettoken(&p1->p_token);
        plkgrp = NULL;

        /*
         * Here we don't create a new process, but we divorce
         * certain parts of a process from itself.
         */
        if ((flags & RFPROC) == 0) {
                /*
                 * This kind of stunt does not work anymore if
                 * there are native threads (lwps) running
                 */
                if (p1->p_nthreads != 1) {
                        error = EINVAL;
                        goto done;
                }

                vm_fork(p1, 0, flags);

                /*
                 * Close all file descriptors.
                 */
                if (flags & RFCFDG) {
                        struct filedesc *fdtmp;
                        fdtmp = fdinit(p1);
                        fdfree(p1, fdtmp);
                }

                /*
                 * Unshare file descriptors (from parent.)
                 */
                if (flags & RFFDG) {
                        if (p1->p_fd->fd_refcnt > 1) {
                                struct filedesc *newfd;
                                error = fdcopy(p1, &newfd);
                                if (error != 0) {
                                        error = ENOMEM;
                                        goto done;
                                }
                                fdfree(p1, newfd);
                        }
                }
                *procp = NULL;
                error = 0;
                goto done;
        }

        /*
         * Interlock against process group signal delivery.  If signals
         * are pending after the interlock is obtained we have to restart
         * the system call to process the signals.  If we don't the child
         * can miss a pgsignal (such as ^C) sent during the fork.
         *
         * We can't use CURSIG() here because it will process any STOPs
         * and cause the process group lock to be held indefinitely.  If
         * a STOP occurs, the fork will be restarted after the CONT.
         */
        p1grp = p1->p_pgrp;
        if ((flags & RFPGLOCK) && (plkgrp = p1->p_pgrp) != NULL) {
                pgref(plkgrp);
                lockmgr(&plkgrp->pg_lock, LK_SHARED);
                if (CURSIG_NOBLOCK(lp1)) {
                        error = ERESTART;
                        goto done;
                }
        }

        /*
         * Although process entries are dynamically created, we still keep
         * a global limit on the maximum number we will create.  Don't allow
         * a nonprivileged user to use the last ten processes; don't let root
         * exceed the limit. The variable nprocs is the current number of
         * processes, maxproc is the limit.
         */
        uid = lp1->lwp_thread->td_ucred->cr_ruid;
        if ((nprocs >= maxproc - 10 && uid != 0) || nprocs >= maxproc) {
                if (ppsratecheck(&lastfail, &curfail, 1))
                        kprintf("maxproc limit exceeded by uid %d, please "
                               "see tuning(7) and login.conf(5).\n", uid);
                tsleep(&forksleep, 0, "fork", hz / 2);
                error = EAGAIN;
                goto done;
        }

        /*
         * Increment the nprocs resource before blocking can occur.  There
         * are hard-limits as to the number of processes that can run.
         */
        atomic_add_int(&nprocs, 1);

        /*
         * Increment the count of procs running with this uid. Don't allow
         * a nonprivileged user to exceed their current limit.
         */
        ok = chgproccnt(lp1->lwp_thread->td_ucred->cr_ruidinfo, 1,
                (uid != 0) ? p1->p_rlimit[RLIMIT_NPROC].rlim_cur : 0);
        if (!ok) {
                /*
                 * Back out the process count
                 */
                atomic_add_int(&nprocs, -1);
                if (ppsratecheck(&lastfail, &curfail, 1))
                        kprintf("maxproc limit exceeded by uid %d, please "
                               "see tuning(7) and login.conf(5).\n", uid);
                tsleep(&forksleep, 0, "fork", hz / 2);
                error = EAGAIN;
                goto done;
        }

        /* Allocate new proc. */
        p2 = kmalloc(sizeof(struct proc), M_PROC, M_WAITOK|M_ZERO);

        /*
         * Setup linkage for kernel based threading XXX lwp
         */
        if (flags & RFTHREAD) {
                p2->p_peers = p1->p_peers;
                p1->p_peers = p2;
                p2->p_leader = p1->p_leader;
        } else {
                p2->p_leader = p2;
        }

        RB_INIT(&p2->p_lwp_tree);
        spin_init(&p2->p_spin);
        lwkt_token_init(&p2->p_token, "proc");
        p2->p_lasttid = -1;     /* first tid will be 0 */

        /*
         * Setting the state to SIDL protects the partially initialized
         * process once it starts getting hooked into the rest of the system.
         */
        p2->p_stat = SIDL;
        proc_add_allproc(p2);

        /*
         * Make a proc table entry for the new process.
         * The whole structure was zeroed above, so copy the section that is
         * copied directly from the parent.
         */
        bcopy(&p1->p_startcopy, &p2->p_startcopy,
            (unsigned) ((caddr_t)&p2->p_endcopy - (caddr_t)&p2->p_startcopy));

        /*
         * Duplicate sub-structures as needed.  Increase reference counts
         * on shared objects.
         *
         * NOTE: because we are now on the allproc list it is possible for
         *       other consumers to gain temporary references to p2
         *       (p2->p_lock can change).
         */
        if (p1->p_flag & P_PROFIL)
                startprofclock(p2);
        p2->p_ucred = crhold(lp1->lwp_thread->td_ucred);

        if (jailed(p2->p_ucred))
                p2->p_flag |= P_JAILED;

        if (p2->p_args)
                refcount_acquire(&p2->p_args->ar_ref);

        p2->p_usched = p1->p_usched;
        /* XXX: verify copy of the secondary iosched stuff */
        dsched_new_proc(p2);

        if (flags & RFSIGSHARE) {
                p2->p_sigacts = p1->p_sigacts;
                refcount_acquire(&p2->p_sigacts->ps_refcnt);
        } else {
                p2->p_sigacts = kmalloc(sizeof(*p2->p_sigacts),
                                        M_SUBPROC, M_WAITOK);
                bcopy(p1->p_sigacts, p2->p_sigacts, sizeof(*p2->p_sigacts));
                refcount_init(&p2->p_sigacts->ps_refcnt, 1);
        }
        if (flags & RFLINUXTHPN) 
                p2->p_sigparent = SIGUSR1;
        else
                p2->p_sigparent = SIGCHLD;

        /* bump references to the text vnode (for procfs) */
        p2->p_textvp = p1->p_textvp;
        if (p2->p_textvp)
                vref(p2->p_textvp);

        /* copy namecache handle to the text file */
        if (p1->p_textnch.mount)
                cache_copy(&p1->p_textnch, &p2->p_textnch);

        /*
         * Handle file descriptors
         */
        if (flags & RFCFDG) {
                p2->p_fd = fdinit(p1);
                fdtol = NULL;
        } else if (flags & RFFDG) {
                error = fdcopy(p1, &p2->p_fd);
                if (error != 0) {
                        error = ENOMEM;
                        goto done;
                }
                fdtol = NULL;
        } else {
                p2->p_fd = fdshare(p1);
                if (p1->p_fdtol == NULL) {
                        lwkt_gettoken(&p1->p_token);
                        p1->p_fdtol =
                                filedesc_to_leader_alloc(NULL,
                                                         p1->p_leader);
                        lwkt_reltoken(&p1->p_token);
                }
                if ((flags & RFTHREAD) != 0) {
                        /*
                         * Shared file descriptor table and
                         * shared process leaders.
                         */
                        fdtol = p1->p_fdtol;
                        fdtol->fdl_refcount++;
                } else {
                        /* 
                         * Shared file descriptor table, and
                         * different process leaders 
                         */
                        fdtol = filedesc_to_leader_alloc(p1->p_fdtol, p2);
                }
        }
        p2->p_fdtol = fdtol;
        p2->p_limit = plimit_fork(p1);

        /*
         * Preserve some more flags in subprocess.  P_PROFIL has already
         * been preserved.
         */
        p2->p_flag |= p1->p_flag & P_SUGID;
        if (p1->p_session->s_ttyvp != NULL && p1->p_flag & P_CONTROLT)
                p2->p_flag |= P_CONTROLT;
        if (flags & RFPPWAIT)
                p2->p_flag |= P_PPWAIT;

        /*
         * Inherit the virtual kernel structure (allows a virtual kernel
         * to fork to simulate multiple cpus).
         */
        if (p1->p_vkernel)
                vkernel_inherit(p1, p2);

        /*
         * Once we are on a pglist we may receive signals.  XXX we might
         * race a ^C being sent to the process group by not receiving it
         * at all prior to this line.
         */
        pgref(p1grp);
        lwkt_gettoken(&p1grp->pg_token);
        LIST_INSERT_AFTER(p1, p2, p_pglist);
        lwkt_reltoken(&p1grp->pg_token);

        /*
         * Attach the new process to its parent.
         *
         * If RFNOWAIT is set, the newly created process becomes a child
         * of init.  This effectively disassociates the child from the
         * parent.
         */
        if (flags & RFNOWAIT)
                pptr = initproc;
        else
                pptr = p1;
        p2->p_pptr = pptr;
        LIST_INIT(&p2->p_children);

        lwkt_gettoken(&pptr->p_token);
        LIST_INSERT_HEAD(&pptr->p_children, p2, p_sibling);
        lwkt_reltoken(&pptr->p_token);

        varsymset_init(&p2->p_varsymset, &p1->p_varsymset);
        callout_init_mp(&p2->p_ithandle);

#ifdef KTRACE
        /*
         * Copy traceflag and tracefile if enabled.  If not inherited,
         * these were zeroed above but we still could have a trace race
         * so make sure p2's p_tracenode is NULL.
         */
        if ((p1->p_traceflag & KTRFAC_INHERIT) && p2->p_tracenode == NULL) {
                p2->p_traceflag = p1->p_traceflag;
                p2->p_tracenode = ktrinherit(p1->p_tracenode);
        }
#endif

        /*
         * This begins the section where we must prevent the parent
         * from being swapped.
         *
         * Gets PRELE'd in the caller in start_forked_proc().
         */
        PHOLD(p1);

        vm_fork(p1, p2, flags);

        /*
         * Create the first lwp associated with the new proc.
         * It will return via a different execution path later, directly
         * into userland, after it was put on the runq by
         * start_forked_proc().
         */
        lwp_fork(lp1, p2, flags);

        if (flags == (RFFDG | RFPROC | RFPGLOCK)) {
                mycpu->gd_cnt.v_forks++;
                mycpu->gd_cnt.v_forkpages += p2->p_vmspace->vm_dsize + p2->p_vmspace->vm_ssize;
        } else if (flags == (RFFDG | RFPROC | RFPPWAIT | RFMEM | RFPGLOCK)) {
                mycpu->gd_cnt.v_vforks++;
                mycpu->gd_cnt.v_vforkpages += p2->p_vmspace->vm_dsize + p2->p_vmspace->vm_ssize;
        } else if (p1 == &proc0) {
                mycpu->gd_cnt.v_kthreads++;
                mycpu->gd_cnt.v_kthreadpages += p2->p_vmspace->vm_dsize + p2->p_vmspace->vm_ssize;
        } else {
                mycpu->gd_cnt.v_rforks++;
                mycpu->gd_cnt.v_rforkpages += p2->p_vmspace->vm_dsize + p2->p_vmspace->vm_ssize;
        }

        /*
         * Both processes are set up, now check if any loadable modules want
         * to adjust anything.
         *   What if they have an error? XXX
         */
        TAILQ_FOREACH(ep, &fork_list, next) {
                (*ep->function)(p1, p2, flags);
        }

        /*
         * Set the start time.  Note that the process is not runnable.  The
         * caller is responsible for making it runnable.
         */
        microtime(&p2->p_start);
        p2->p_acflag = AFORK;

        /*
         * tell any interested parties about the new process
         */
        KNOTE(&p1->p_klist, NOTE_FORK | p2->p_pid);

        /*
         * Return child proc pointer to parent.
         */
        *procp = p2;
        error = 0;
done:
        lwkt_reltoken(&p1->p_token);
        if (plkgrp) {
                lockmgr(&plkgrp->pg_lock, LK_RELEASE);
                pgrel(plkgrp);
        }
        return (error);
}

static struct lwp *
lwp_fork(struct lwp *origlp, struct proc *destproc, int flags)
{
        globaldata_t gd = mycpu;
        struct lwp *lp;
        struct thread *td;

        lp = kmalloc(sizeof(struct lwp), M_LWP, M_WAITOK|M_ZERO);

        lp->lwp_proc = destproc;
        lp->lwp_vmspace = destproc->p_vmspace;
        lp->lwp_stat = LSRUN;
        bcopy(&origlp->lwp_startcopy, &lp->lwp_startcopy,
            (unsigned) ((caddr_t)&lp->lwp_endcopy -
                        (caddr_t)&lp->lwp_startcopy));
        lp->lwp_flag |= origlp->lwp_flag & LWP_ALTSTACK;
        /*
         * Set cpbase to the last timeout that occured (not the upcoming
         * timeout).
         *
         * A critical section is required since a timer IPI can update
         * scheduler specific data.
         */
        crit_enter();
        lp->lwp_cpbase = gd->gd_schedclock.time - gd->gd_schedclock.periodic;
        destproc->p_usched->heuristic_forking(origlp, lp);
        crit_exit();
        lp->lwp_cpumask &= usched_mastermask;
        lwkt_token_init(&lp->lwp_token, "lwp_token");
        spin_init(&lp->lwp_spin);

        /*
         * Assign the thread to the current cpu to begin with so we
         * can manipulate it.
         */
        td = lwkt_alloc_thread(NULL, LWKT_THREAD_STACK, gd->gd_cpuid, 0);
        lp->lwp_thread = td;
        td->td_proc = destproc;
        td->td_lwp = lp;
        td->td_switch = cpu_heavy_switch;
        lwkt_setpri(td, TDPRI_KERN_USER);
        lwkt_set_comm(td, "%s", destproc->p_comm);

        /*
         * cpu_fork will copy and update the pcb, set up the kernel stack,
         * and make the child ready to run.
         */
        cpu_fork(origlp, lp, flags);
        caps_fork(origlp->lwp_thread, lp->lwp_thread);
        kqueue_init(&lp->lwp_kqueue, destproc->p_fd);

        /*
         * Assign a TID to the lp.  Loop until the insert succeeds (returns
         * NULL).
         */
        lp->lwp_tid = destproc->p_lasttid;
        do {
                if (++lp->lwp_tid < 0)
                        lp->lwp_tid = 1;
        } while (lwp_rb_tree_RB_INSERT(&destproc->p_lwp_tree, lp) != NULL);
        destproc->p_lasttid = lp->lwp_tid;
        destproc->p_nthreads++;

        return (lp);
}

/*
 * The next two functionms are general routines to handle adding/deleting
 * items on the fork callout list.
 *
 * at_fork():
 * Take the arguments given and put them onto the fork callout list,
 * However first make sure that it's not already there.
 * Returns 0 on success or a standard error number.
 */
int
at_fork(forklist_fn function)
{
        struct forklist *ep;

#ifdef INVARIANTS
        /* let the programmer know if he's been stupid */
        if (rm_at_fork(function)) {
                kprintf("WARNING: fork callout entry (%p) already present\n",
                    function);
        }
#endif
        ep = kmalloc(sizeof(*ep), M_ATFORK, M_WAITOK|M_ZERO);
        ep->function = function;
        TAILQ_INSERT_TAIL(&fork_list, ep, next);
        return (0);
}

/*
 * Scan the exit callout list for the given item and remove it..
 * Returns the number of items removed (0 or 1)
 */
int
rm_at_fork(forklist_fn function)
{
        struct forklist *ep;

        TAILQ_FOREACH(ep, &fork_list, next) {
                if (ep->function == function) {
                        TAILQ_REMOVE(&fork_list, ep, next);
                        kfree(ep, M_ATFORK);
                        return(1);
                }
        }       
        return (0);
}

/*
 * Add a forked process to the run queue after any remaining setup, such
 * as setting the fork handler, has been completed.
 */
void
start_forked_proc(struct lwp *lp1, struct proc *p2)
{
        struct lwp *lp2 = ONLY_LWP_IN_PROC(p2);

        /*
         * Move from SIDL to RUN queue, and activate the process's thread.
         * Activation of the thread effectively makes the process "a"
         * current process, so we do not setrunqueue().
         *
         * YYY setrunqueue works here but we should clean up the trampoline
         * code so we just schedule the LWKT thread and let the trampoline
         * deal with the userland scheduler on return to userland.
         */
        KASSERT(p2->p_stat == SIDL,
            ("cannot start forked process, bad status: %p", p2));
        p2->p_usched->resetpriority(lp2);
        crit_enter();
        p2->p_stat = SACTIVE;
        lp2->lwp_stat = LSRUN;
        p2->p_usched->setrunqueue(lp2);
        crit_exit();

        /*
         * Now can be swapped.
         */
        PRELE(lp1->lwp_proc);

        /*
         * Preserve synchronization semantics of vfork.  If waiting for
         * child to exec or exit, set P_PPWAIT on child, and sleep on our
         * proc (in case of exec or exit).
         *
         * We must hold our p_token to interlock the flag/tsleep
         */
        lwkt_gettoken(&p2->p_token);
        while (p2->p_flag & P_PPWAIT)
                tsleep(lp1->lwp_proc, 0, "ppwait", 0);
        lwkt_reltoken(&p2->p_token);
}