FreeBSD/Linux Kernel Cross Reference
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. 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 $
     */
    
    #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 <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
    */
   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) {
                   PHOLD(p2);
                   start_forked_proc(lp, p2);
                   uap->sysmsg_fds[0] = p2->p_pid;
                   uap->sysmsg_fds[1] = 0;
                   PRELE(p2);
           }
           return error;
   }
   
   /*
    * vfork() system call
    */
   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) {
                   PHOLD(p2);
                   start_forked_proc(lp, p2);
                   uap->sysmsg_fds[0] = p2->p_pid;
                   uap->sysmsg_fds[1] = 0;
                   PRELE(p2);
           }
           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 }
    */
   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) {
                           PHOLD(p2);
                           start_forked_proc(lp, p2);
                           uap->sysmsg_fds[0] = p2->p_pid;
                           uap->sysmsg_fds[1] = 0;
                           PRELE(p2);
                   } else {
                           uap->sysmsg_fds[0] = 0;
                           uap->sysmsg_fds[1] = 0;
                   }
           }
           return error;
   }
   
   /*
    * Low level thread create used by pthreads.
    */
   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 */
           atomic_set_int(&lp->lwp_mpflags, LWP_MP_WEXIT);
           lp->lwp_thread->td_flags |= TDF_EXITING;
           lwkt_remove_tdallq(lp->lwp_thread);
           PHOLD(p);
           biosched_done(lp->lwp_thread);
           dsched_exit_thread(lp->lwp_thread);
           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;
           struct proc *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;
           p2 = 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 a new process, don't get fancy: zero the structure.
            */
           p2 = kmalloc(sizeof(struct proc), M_PROC, M_WAITOK|M_ZERO);
   
           /*
            * Core initialization.  SIDL is a safety state that protects the
            * partially initialized process once it starts getting hooked
            * into system structures and becomes addressable.
            *
            * We must be sure to acquire p2->p_token as well, we must hold it
            * once the process is on the allproc list to avoid things such
            * as competing modifications to p_flags.
            */
           p2->p_lasttid = -1;     /* first tid will be 0 */
           p2->p_stat = SIDL;
   
           RB_INIT(&p2->p_lwp_tree);
           spin_init(&p2->p_spin);
           lwkt_token_init(&p2->p_token, "proc");
           lwkt_gettoken(&p2->p_token);
   
           /*
            * Setup linkage for kernel based threading XXX lwp.  Also add the
            * process to the allproclist.
            *
            * The process structure is addressable after this point.
            */
           if (flags & RFTHREAD) {
                   p2->p_peers = p1->p_peers;
                   p1->p_peers = p2;
                   p2->p_leader = p1->p_leader;
           } else {
                   p2->p_leader = p2;
           }
           proc_add_allproc(p2);
   
           /*
            * Initialize the section which is copied verbatim from the parent.
            */
           bcopy(&p1->p_startcopy, &p2->p_startcopy,
                 ((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_flags & P_PROFIL)
                   startprofclock(p2);
           p2->p_ucred = crhold(lp1->lwp_thread->td_ucred);
   
           if (jailed(p2->p_ucred))
                   p2->p_flags |= 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) {
                           p1->p_fdtol = filedesc_to_leader_alloc(NULL,
                                                                  p1->p_leader);
                   }
                   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_flags |= p1->p_flags & P_SUGID;
           if (p1->p_session->s_ttyvp != NULL && (p1->p_flags & P_CONTROLT))
                   p2->p_flags |= P_CONTROLT;
           if (flags & RFPPWAIT)
                   p2->p_flags |= 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:
           if (p2)
                   lwkt_reltoken(&p2->p_token);
           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_flags |= origlp->lwp_flags & 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_ucred = crhold(destproc->p_ucred);
           td->td_proc = destproc;
           td->td_lwp = lp;
           td->td_switch = cpu_heavy_switch;
   #ifdef NO_LWKT_SPLIT_USERPRI
           lwkt_setpri(td, TDPRI_USER_NORM);
   #else
           lwkt_setpri(td, TDPRI_KERN_USER);
   #endif
           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);
           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++;
   
           /*
            * This flag is set and never cleared.  It means that the process
            * was threaded at some point.  Used to improve exit performance.
            */
           destproc->p_flags |= P_MAYBETHREADED;
   
           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.
    *
    * p2 is held by the caller.
    */
   void
   start_forked_proc(struct lwp *lp1, struct proc *p2)
   {
           struct lwp *lp2 = ONLY_LWP_IN_PROC(p2);
           int pflags;
   
           /*
            * 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.  P_PPWAIT is set in
            * the child until it has retired the parent's resources.  The parent
            * must wait for the flag to be cleared by the child.
            *
            * Interlock the flag/tsleep with atomic ops to avoid unnecessary
            * p_token conflicts.
            *
            * XXX Is this use of an atomic op on a field that is not normally
            *     manipulated with atomic ops ok?
            */
           while ((pflags = p2->p_flags) & P_PPWAIT) {
                   cpu_ccfence();
                   tsleep_interlock(lp1->lwp_proc, 0);
                   if (atomic_cmpset_int(&p2->p_flags, pflags, pflags))
                           tsleep(lp1->lwp_proc, PINTERLOCKED, "ppwait", 0);
           }
   }

Cache object: 11e4c21fe8a614e32e1c99f78ef30c59