xref: /src/sys/kern/sysv_sem.c

/*
 * Implementation of SVID semaphores
 *
 * Author:  Daniel Boulet
 *
 * This software is provided ``AS IS'' without any warranties of any kind.
 *
 *	$Id: sysv_sem.c,v 1.5 1994/02/13 10:20:02 mycroft Exp $
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/kernel.h>
#include <sys/proc.h>
#include <sys/sem.h>
#include <sys/malloc.h>

static int	semctl(), semget(), semop(), semconfig();
int	(*semcalls[])() = { semctl, semget, semop, semconfig };
int	semtot = 0;

static struct proc *semlock_holder = NULL;

int
seminit()
{
	register int i;
	vm_offset_t whocares1, whocares2;

	if (sema == NULL)
		panic("sema is NULL");
	if (semu == NULL)
		panic("semu is NULL");

	for (i = 0; i < seminfo.semmni; i++) {
		sema[i].sem_base = 0;
		sema[i].sem_perm.mode = 0;
	}
	for (i = 0; i < seminfo.semmnu; i++) {
		register struct sem_undo *suptr = SEMU(i);
		suptr->un_proc = NULL;
	}
	semu_list = NULL;
}

/*
 * Entry point for all SEM calls
 */

struct semsys_args {
	u_int	which;
};

int
semsys(p, uap, retval)
	struct proc *p;
	struct semsys_args *uap;
	int *retval;
{

	while (semlock_holder != NULL && semlock_holder != p)
		sleep((caddr_t)&semlock_holder, (PZERO - 4));

	if (uap->which >= sizeof(semcalls)/sizeof(semcalls[0]))
		return (EINVAL);
	return ((*semcalls[uap->which])(p, &uap[1], retval));
}

/*
 * Lock or unlock the entire semaphore facility.
 *
 * This will probably eventually evolve into a general purpose semaphore
 * facility status enquiry mechanism (I don't like the "read /dev/kmem"
 * approach currently taken by ipcs and the amount of info that we want
 * to be able to extract for ipcs is probably beyond what the capability
 * of the getkerninfo facility.
 *
 * At the time that the current version of semconfig was written, ipcs is
 * the only user of the semconfig facility.  It uses it to ensure that the
 * semaphore facility data structures remain static while it fishes around
 * in /dev/kmem.
 */

struct semconfig_args {
	semconfig_ctl_t	flag;
};

int
semconfig(p, uap, retval)
	struct proc *p;
	struct semconfig_args *uap;
	int *retval;
{
	int eval = 0;

	switch (uap->flag) {
	case SEM_CONFIG_FREEZE:
		semlock_holder = p;
		break;
	case SEM_CONFIG_THAW:
		semlock_holder = NULL;
		wakeup((caddr_t)&semlock_holder);
		break;
	default:
		printf("semconfig: unknown flag parameter value (%d) - ignored\n",
		    uap->flag);
		eval = EINVAL;
		break;
	}

	*retval = 0;
	return(eval);
}

/*
 * Allocate a new sem_undo structure for a process
 * (returns ptr to structure or NULL if no more room)
 */

struct sem_undo *
semu_alloc(p)
	struct proc *p;
{
	register int i;
	register struct sem_undo *suptr;
	register struct sem_undo **supptr;
	int attempt;

	/*
	 * Try twice to allocate something.
	 * (we'll purge any empty structures after the first pass so
	 * two passes are always enough)
	 */

	for (attempt = 0; attempt < 2; attempt++) {
		/*
		 * Look for a free structure.
		 * Fill it in and return it if we find one.
		 */

		for (i = 0; i < seminfo.semmnu; i++) {
			suptr = SEMU(i);
			if (suptr->un_proc == NULL) {
				suptr->un_next = semu_list;
				semu_list = suptr;
				suptr->un_cnt = 0;
				suptr->un_proc = p;
				return(suptr);
			}
		}

		/*
		 * We didn't find a free one, if this is the first attempt
		 * then try to free some structures.
		 */

		if (attempt == 0) {
			/* All the structures are in use - try to free some */
			int did_something = 0;

			supptr = &semu_list;
			while ((suptr = *supptr) != NULL) {
				if (suptr->un_cnt == 0)  {
					suptr->un_proc = NULL;
					*supptr = suptr->un_next;
					did_something = 1;
				} else
					supptr = &(suptr->un_next);
			}

			/* If we didn't free anything then just give-up */
			if (!did_something)
				return(NULL);
		} else {
			/*
			 * The second pass failed even though we freed
			 * something after the first pass!
			 * This is IMPOSSIBLE!
			 */
			panic("semu_alloc - second attempt failed");
		}
	}
}

/*
 * Adjust a particular entry for a particular proc
 */

int
semundo_adjust(p, supptr, semid, semnum, adjval)
	register struct proc *p;
	struct sem_undo **supptr;
	int semid, semnum;
	int adjval;
{
	register struct sem_undo *suptr;
	register struct undo *sunptr;
	int i;

	/* Look for and remember the sem_undo if the caller doesn't provide
	   it */

	suptr = *supptr;
	if (suptr == NULL) {
		for (suptr = semu_list; suptr != NULL;
		    suptr = suptr->un_next) {
			if (suptr->un_proc == p) {
				*supptr = suptr;
				break;
			}
		}
		if (suptr == NULL) {
			if (adjval == 0)
				return(0);
			suptr = semu_alloc(p);
			if (suptr == NULL)
				return(ENOSPC);
			*supptr = suptr;
		}
	}

	/* Look for the requested entry and adjust it (delete if adjval becomes
	   0) */
	sunptr = &(suptr->un_ent[0]);
	for (i = 0; i < suptr->un_cnt; i++, sunptr++) {
		if (sunptr->un_id == semid && sunptr->un_num == semnum) {
			/* Found the right entry - adjust it */
			if (adjval == 0)
				sunptr->un_adjval = 0;
			else
				sunptr->un_adjval += adjval;
			if (sunptr->un_adjval == 0) {
				suptr->un_cnt--;
				if (i < suptr->un_cnt)
					suptr->un_ent[i] =
					    suptr->un_ent[suptr->un_cnt];
			}
			return(0);
		}
	}

	/* Didn't find the right entry - create it */
	if (adjval == 0)
		return(0);
	if (suptr->un_cnt != SEMUME) {
		sunptr = &(suptr->un_ent[suptr->un_cnt]);
		suptr->un_cnt++;
		sunptr->un_adjval = adjval;
		sunptr->un_id = semid; sunptr->un_num = semnum;
	} else
		return(EINVAL);
	return(0);
}

void
semundo_clear(semid, semnum)
	int semid, semnum;
{
    register struct sem_undo *suptr;

    for (suptr = semu_list; suptr != NULL; suptr = suptr->un_next) {
	register struct undo *sunptr = &(suptr->un_ent[0]);
	register int i = 0;

	while (i < suptr->un_cnt) {
	    int advance = 1;

	    if (sunptr->un_id == semid) {
		if (semnum == -1 || sunptr->un_num == semnum) {
		    /* printf("clear:  %08x %d:%d(%d)\n",suptr->un_proc,semid,sunptr->un_num,sunptr->un_adjval); */
		    suptr->un_cnt--;
		    if (i < suptr->un_cnt) {
			suptr->un_ent[i] = suptr->un_ent[suptr->un_cnt];
			advance = 0;
		    }
		}
		if (semnum != -1)
		    break;
	    }

	    if (advance) {
		i++;
		sunptr++;
	    }
	}
    }
}

struct semctl_args {
	int	semid;
	int	semnum;
	int	cmd;
	union	semun *arg;
};

int
semctl(p, uap, retval)
	struct proc *p;
	register struct semctl_args *uap;
	int *retval;
{
    int semid = uap->semid;
    int semnum = uap->semnum;
    int cmd = uap->cmd;
    union semun *arg = uap->arg;
    union semun real_arg;
    struct ucred *cred = p->p_ucred;
    int i, rval, eval;
    struct semid_ds sbuf;
    register struct semid_ds *semaptr;

#ifdef SEM_DEBUG
    printf("call to semctl(%d,%d,%d,0x%x)\n",semid,semnum,cmd,arg);
#endif

    semid = IPCID_TO_IX(semid);

    if (semid < 0 || semid >= seminfo.semmsl)
	return(EINVAL);

    semaptr = &sema[semid];

    if (semaptr->sem_perm.seq != IPCID_TO_SEQ(uap->semid))
	return(EINVAL);

    if ((semaptr->sem_perm.mode & SEM_ALLOC) == 0)
	return(EINVAL);

    eval = 0;
    rval = 0;

    switch (cmd) {

    case IPC_RMID:
	if (cred->cr_uid != 0 && semaptr->sem_perm.cuid != cred->cr_uid && semaptr->sem_perm.uid != cred->cr_uid)
	    return(EPERM);
	semaptr->sem_perm.cuid = cred->cr_uid;
	semaptr->sem_perm.uid = cred->cr_uid;
	semtot -= semaptr->sem_nsems;
	for (i = semaptr->sem_base - sem; i < semtot; i++)
	    sem[i] = sem[i + semaptr->sem_nsems];
	/* printf("\n"); */
	for (i = 0; i < seminfo.semmni; i++) {
	    if ((sema[i].sem_perm.mode & SEM_ALLOC) && sema[i].sem_base > semaptr->sem_base)
		sema[i].sem_base -= semaptr->sem_nsems;
	}
	semaptr->sem_perm.mode = 0;

	/* Delete any undo entries for this semid */
	semundo_clear(semid,-1);

	/* Make sure that anybody who is waiting notices the deletion */
	wakeup((caddr_t)semaptr);

	break;

    case IPC_SET:
	/* printf("IPC_SET\n"); */
	if (cred->cr_uid != 0 && semaptr->sem_perm.cuid != cred->cr_uid && semaptr->sem_perm.uid != cred->cr_uid)
	    return(EPERM);
	if ((eval = copyin(arg, &real_arg, sizeof(real_arg))) != 0)
	    return(eval);
	if ((eval = copyin(real_arg.buf, (caddr_t)&sbuf, sizeof(sbuf))) != 0)
	    return(eval);
	semaptr->sem_perm.uid = sbuf.sem_perm.uid;
	semaptr->sem_perm.gid = sbuf.sem_perm.gid;
	semaptr->sem_perm.mode = (semaptr->sem_perm.mode & ~0777) | (sbuf.sem_perm.mode & 0777);
	semaptr->sem_ctime = time.tv_sec;
	break;

    case IPC_STAT:
	/* printf("IPC_STAT\n"); */
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_R, cred)))
	    return(eval);
	rval = 0;
	if ((eval = copyin(arg, &real_arg, sizeof(real_arg))) != 0)
	    return(eval);
	eval = copyout((caddr_t)semaptr, real_arg.buf, sizeof(struct semid_ds)) ;
	break;

    case GETNCNT:
	/* printf("GETNCNT(%d)\n",semnum); */
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_R, cred)))
	    return(eval);
	if (semnum < 0 || semnum >= semaptr->sem_nsems) return(EINVAL);
	rval = semaptr->sem_base[semnum].semncnt;
	break;

    case GETPID:
	/* printf("GETPID(%d)\n",semnum); */
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_R, cred)))
	    return(eval);
	if (semnum < 0 || semnum >= semaptr->sem_nsems)
	    return(EINVAL);
	rval = semaptr->sem_base[semnum].sempid;
	break;

    case GETVAL:
	/* printf("GETVAL(%d)\n",semnum); */
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_R, cred)))
	    return(eval);
	if (semnum < 0 || semnum >= semaptr->sem_nsems)
	    return(EINVAL);
	rval = semaptr->sem_base[semnum].semval;
	break;

    case GETALL:
	/* printf("GETALL\n"); */
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_R, cred)))
	    return(eval);
	rval = 0;
	if ((eval = copyin(arg, &real_arg, sizeof(real_arg))) != 0)
	    return(eval);
	/* printf("%d semaphores\n",semaptr->sem_nsems); */
	for (i = 0; i < semaptr->sem_nsems; i++) {
	    /* printf("copyout to 0x%x\n",&real_arg.array[i]); */
	    eval =
		copyout((caddr_t)&semaptr->sem_base[i].semval,
		&real_arg.array[i],
		sizeof(real_arg.array[0]));
	    if (eval != 0) {
		/* printf("copyout to 0x%x failed\n",&real_arg.array[i]); */
		break;
	    }
	}
	break;

    case GETZCNT:
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_R, cred)))
	    return(eval);
	/* printf("GETZCNT(%d)\n",semnum); */
	if (semnum < 0 || semnum >= semaptr->sem_nsems) return(EINVAL);
	rval = semaptr->sem_base[semnum].semzcnt;
	break;

    case SETVAL:
#ifdef SEM_DEBUG
	printf("SETVAL(%d)\n",semnum);
#endif
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_W, cred)))
	    return(eval);
	if (semnum < 0 || semnum >= semaptr->sem_nsems) return(EINVAL);
	rval = 0;
	if ((eval = copyin(arg, &real_arg, sizeof(real_arg))) != 0)
	    return(eval);
#ifdef SEM_DEBUG
	printf("semaptr=%x, sem_base=%x, semptr=%x, oldval=%d, ",
	semaptr,semaptr->sem_base,&semaptr->sem_base[semnum],semaptr->sem_base[semnum].semval);
#endif
	semaptr->sem_base[semnum].semval = real_arg.val;
#ifdef SEM_DEBUG
	printf(" newval=%d\n", semaptr->sem_base[semnum].semval);
#endif
	semundo_clear(semid,semnum);
	wakeup((caddr_t)semaptr);	/* somebody else might care */
	break;

    case SETALL:
	/* printf("SETALL\n"); */
	if ((eval = ipcaccess(&semaptr->sem_perm, IPC_W, cred)))
	    return(eval);
	rval = 0;
	if ((eval = copyin(arg, &real_arg, sizeof(real_arg))) != 0)
	    return(eval);
	for (i = 0; i < semaptr->sem_nsems; i++) {
	    eval =
		copyin(&real_arg.array[i],
		(caddr_t)&semaptr->sem_base[i].semval,
		sizeof(real_arg.array[0]));
	    if (eval != 0)
		break;
	}
	semundo_clear(semid,-1);
	wakeup((caddr_t)semaptr);	/* somebody else might care */
	break;
    default:
	/* printf("invalid command %d\n",cmd); */
	return(EINVAL);
    }

    if (eval == 0)
	*retval = rval;

    return(eval);
}

struct semget_args {
	key_t	key;
	int	nsems;
	int	semflg;
};

int
semget(p, uap, retval)
	struct proc *p;
	register struct semget_args *uap;
	int *retval;
{
    int semid, eval;
    int key = uap->key;
    int nsems = uap->nsems;
    int semflg = uap->semflg;
    struct ucred *cred = p->p_ucred;

#ifdef SEM_DEBUG
    printf("semget(0x%x,%d,0%o)\n",key,nsems,semflg);
#endif

    if (key == IPC_PRIVATE) {
#ifdef SEM_DEBUG
	printf("private key\n");
#endif
	semid = seminfo.semmni;
    } else {
	for (semid = 0; semid < seminfo.semmni; semid++) {
	    if ((sema[semid].sem_perm.mode & SEM_ALLOC) && sema[semid].sem_perm.key == key)
		break;
	}
	if (semid < seminfo.semmni) {
#ifdef SEM_DEBUG
	    printf("found public key\n");
#endif
	    if ((eval = ipcaccess(&sema[semid].sem_perm, semflg & 0700, cred))) {
		return(eval);
	    }
	    if (nsems > 0 && sema[semid].sem_nsems < nsems) {
#ifdef SEM_DEBUG
		printf("too small\n");
#endif
		return(EINVAL);
	    }
	    if ((semflg & IPC_CREAT) && (semflg & IPC_EXCL)) {
#ifdef SEM_DEBUG
		printf("not exclusive\n");
#endif
		return(EEXIST);
	    }
	} else {
#ifdef SEM_DEBUG
	    printf("didn't find public key\n");
#endif
	}
    }

    if (semid == seminfo.semmni) {
#ifdef SEM_DEBUG
	printf("need to allocate the semid_ds\n");
#endif
	if (key == IPC_PRIVATE || (semflg & IPC_CREAT)) {
	    if (nsems <= 0 || nsems > seminfo.semmsl) {
#ifdef SEM_DEBUG
		printf("nsems out of range (0<%d<=%d)\n",nsems,seminfo.semmsl);
#endif
		return(EINVAL);
	    }
	    if (nsems > seminfo.semmns - semtot) {
#ifdef SEM_DEBUG
		printf("not enough semaphores left (need %d, got %d)\n",
		nsems,seminfo.semmns - semtot);
#endif
		return(ENOSPC);
	    }
	    for (semid = 0; semid < seminfo.semmni; semid++) {
		if ((sema[semid].sem_perm.mode & SEM_ALLOC) == 0)
		    break;
	    }
	    if (semid == seminfo.semmni) {
#ifdef SEM_DEBUG
		printf("no more semid_ds's available\n");
#endif
		return(ENOSPC);
	    }
#ifdef SEM_DEBUG
	    printf("semid %d is available\n",semid);
#endif
	    sema[semid].sem_perm.key = key;
	    sema[semid].sem_perm.cuid = cred->cr_uid;
	    sema[semid].sem_perm.uid = cred->cr_uid;
	    sema[semid].sem_perm.cgid = cred->cr_gid;
	    sema[semid].sem_perm.gid = cred->cr_gid;
	    sema[semid].sem_perm.mode = (semflg & 0777) | SEM_ALLOC;
	    sema[semid].sem_perm.seq = (sema[semid].sem_perm.seq + 1) & 0x7fff;	/* avoid semid overflows */
	    sema[semid].sem_nsems = nsems;
	    sema[semid].sem_otime = 0;
	    sema[semid].sem_ctime = time.tv_sec;
	    sema[semid].sem_base = &sem[semtot];
	    semtot += nsems;
	    bzero(sema[semid].sem_base,sizeof(sema[semid].sem_base[0])*nsems);
#ifdef SEM_DEBUG
	    printf("sembase = 0x%x, next = 0x%x\n",sema[semid].sem_base,&sem[semtot]);
#endif
	} else {
#ifdef SEM_DEBUG
	    printf("didn't find it and wasn't asked to create it\n");
#endif
	    return(ENOENT);
	}
    }

    *retval = IXSEQ_TO_IPCID(semid,sema[semid].sem_perm);	/* Convert to one origin */
    return(0);
}

struct semop_args {
	int	semid;
	struct	sembuf *sops;
	int	nsops;
};

int
semop(p, uap, retval)
	struct proc *p;
	register struct semop_args *uap;
	int *retval;
{
    int semid = uap->semid;
    int nsops = uap->nsops;
    struct sembuf sops[MAX_SOPS];
    register struct semid_ds *semaptr;
    register struct sembuf *sopptr;
    register struct sem *semptr;
    struct sem_undo *suptr = NULL;
    struct ucred *cred = p->p_ucred;
    int i, j, eval;
    int all_ok, do_wakeup, do_undos;

#ifdef SEM_DEBUG
    printf("call to semop(%d,0x%x,%d)\n",semid,sops,nsops);
#endif

    semid = IPCID_TO_IX(semid);	/* Convert back to zero origin */

    if (semid < 0 || semid >= seminfo.semmsl) {
	/* printf("semid out of range (0<=%d<%d)\n",semid,seminfo.semmsl); */
	return(EINVAL);
    }

    semaptr = &sema[semid];
    if ((semaptr->sem_perm.mode & SEM_ALLOC) == 0) {
	/* printf("no such semaphore id\n"); */
	return(EINVAL);
    }

    if (semaptr->sem_perm.seq != IPCID_TO_SEQ(uap->semid)) {
	/* printf("invalid sequence number\n"); */
	return(EINVAL);
    }

    if ((eval = ipcaccess(&semaptr->sem_perm, IPC_W, cred))) {
#ifdef SEM_DEBUG
	printf("eval = %d from ipaccess\n",eval);
#endif
	return(eval);
    }

    if (nsops > MAX_SOPS) {
#ifdef SEM_DEBUG
	printf("too many sops (max=%d, nsops=%d)\n",MAX_SOPS,nsops);
#endif
	return(E2BIG);
    }

    if ((eval = copyin(uap->sops, &sops, nsops * sizeof(sops[0]))) != 0) {
#ifdef SEM_DEBUG
	printf("eval = %d from copyin(%08x, %08x, %d)\n",eval,uap->sops,&sops,nsops * sizeof(sops[0]));
#endif
	return(eval);
    }

    /*
     * Loop trying to satisfy the vector of requests.
     * If we reach a point where we must wait, any requests already
     * performed are rolled back and we go to sleep until some other
     * process wakes us up.  At this point, we start all over again.
     *
     * This ensures that from the perspective of other tasks, a set
     * of requests is atomic (never partially satisfied).
     */

    do_undos = 0;

    while (1) {
	do_wakeup = 0;

	for (i = 0; i < nsops; i++) {
	    sopptr = &sops[i];

	    if (sopptr->sem_num >= semaptr->sem_nsems)
		return(EFBIG);

	    semptr = &semaptr->sem_base[sopptr->sem_num];

#ifdef SEM_DEBUG
	    printf("semop:  semaptr=%x, sem_base=%x, semptr=%x, sem[%d]=%d : op=%d, flag=%s\n",
	    semaptr,semaptr->sem_base,semptr,
	    sopptr->sem_num,semptr->semval,sopptr->sem_op,
	    (sopptr->sem_flg & IPC_NOWAIT) ? "nowait" : "wait");
#endif

	    if (sopptr->sem_op < 0) {
		if (semptr->semval + sopptr->sem_op < 0) {
#ifdef SEM_DEBUG
		    printf("semop:  can't do it now\n");
#endif
		    break;
		} else {
		    semptr->semval += sopptr->sem_op;
		    if (semptr->semval == 0 && semptr->semzcnt > 0)
			do_wakeup = 1;
		}
		if (sopptr->sem_flg & SEM_UNDO)
		    do_undos = 1;
	    } else if (sopptr->sem_op == 0) {
		if (semptr->semval > 0) {
#ifdef SEM_DEBUG
		    printf("semop:  not zero now\n");
#endif
		    break;
		}
	    } else {
		if (semptr->semncnt > 0)
		    do_wakeup = 1;
		semptr->semval += sopptr->sem_op;
		if (sopptr->sem_flg & SEM_UNDO)
		    do_undos = 1;
	    }
	}

	/*
	 * Did we get through the entire vector?
	 */

	if (i < nsops) {
	    /*
	     * No ... rollback anything that we've already done
	     */

#ifdef SEM_DEBUG
	    printf("semop:  rollback 0 through %d\n",i-1);
#endif
	    for (j = 0; j < i; j++)
		semaptr->sem_base[sops[j].sem_num].semval -= sops[j].sem_op;

	    /*
	     * If the request that we couldn't satisfy has the NOWAIT
	     * flag set then return with EAGAIN.
	     */

	    if (sopptr->sem_flg & IPC_NOWAIT)
		return(EAGAIN);

	    if (sopptr->sem_op == 0)
		semptr->semzcnt++;
	    else
		semptr->semncnt++;

#ifdef SEM_DEBUG
	    printf("semop:  good night!\n");
#endif
	    eval = tsleep((caddr_t)semaptr, (PZERO - 4) | PCATCH, "sem wait", 0);
#ifdef SEM_DEBUG
	    printf("semop:  good morning (eval=%d)!\n",eval);
#endif

	    suptr = NULL;	/* The sem_undo may have been reallocated */

	    if (eval != 0) {
		/* printf("semop:  interrupted system call\n"); */
		return(EINTR);
	    }
#ifdef SEM_DEBUG
	    printf("semop:  good morning!\n");
#endif

	    /*
	     * Make sure that the semaphore still exists
	     */

	    if ((semaptr->sem_perm.mode & SEM_ALLOC) == 0 || semaptr->sem_perm.seq != IPCID_TO_SEQ(uap->semid)) {

		/* printf("semaphore id deleted\n"); */
		/* The man page says to return EIDRM. */
		/* Unfortunately, BSD doesn't define that code! */
#ifdef EIDRM
		return(EIDRM);
#else
		return(EINVAL);
#endif
	    }

	    /*
	     * The semaphore is still alive.  Readjust the count of
	     * waiting processes.
	     */

	    if (sopptr->sem_op == 0)
		semptr->semzcnt--;
	    else
		semptr->semncnt--;
	} else {
	    /*
	     * Yes ... we're done.
	     * Process any SEM_UNDO requests.
	     */

	    if (do_undos) {
		for (i = 0; i < nsops; i++) {
		    /* We only need to deal with SEM_UNDO's for non-zero op's */
		    int adjval;

		    if ((sops[i].sem_flg & SEM_UNDO) != 0 && (adjval = sops[i].sem_op) != 0) {

			eval = semundo_adjust(p,&suptr,semid,sops[i].sem_num,-adjval);
			if (eval != 0) {
			    /*
			     * Oh-Oh!  We ran out of either sem_undo's or undo's.
			     * Rollback the adjustments to this point and then
			     * rollback the semaphore ups and down so we can
			     * return with an error with all structures restored.
			     * We rollback the undo's in the exact reverse order that
			     * we applied them.  This guarantees that we won't run
			     * out of space as we roll things back out.
			     */

			    for (j = i - 1; j >= 0; j--) {
				if ((sops[i].sem_flg & SEM_UNDO) != 0 && (adjval = sops[i].sem_op) != 0) {
				    if (semundo_adjust(p,&suptr,semid,sops[j].sem_num,adjval) != 0) {
					/* This is impossible!  */
					panic("semop - can't undo undos");
				    }
				}
			    } /* loop backwards through sops */

			    for (j = 0; j < nsops; j++)
				semaptr->sem_base[sops[j].sem_num].semval -= sops[j].sem_op;

#ifdef SEM_DEBUG
			    printf("eval = %d from semundo_adjust\n",eval);
#endif
			    return(eval);
			} /* semundo_adjust failed */
		    } /* if (SEM_UNDO && adjval != 0) */
		} /* loop through the sops */
	    } /* if (do_undos) */

	    /* We're definitely done - set the sempid's */

	    for (i = 0; i < nsops; i++) {
		sopptr = &sops[i];
		semptr = &semaptr->sem_base[sopptr->sem_num];
		semptr->sempid = p->p_pid;
	    }

	    /* Do a wakeup if any semaphore was up'd. */

	    if (do_wakeup) {
#ifdef SEM_DEBUG
		printf("semop:  doing wakeup\n");
#ifdef SEM_WAKEUP
		sem_wakeup((caddr_t)semaptr);
#else
		wakeup((caddr_t)semaptr);
#endif
		printf("semop:  back from wakeup\n");
#else
		wakeup((caddr_t)semaptr);
#endif
	    }
#ifdef SEM_DEBUG
	    printf("semop:  done\n");
#endif
	    *retval = 0;
	    return(0);
	}
    }

    panic("semop: how did we get here???");
}

/*
 * Go through the undo structures for this process and apply the
 * adjustments to semaphores.
 */

semexit(p)
    struct proc *p;
{
    register struct sem_undo *suptr;
    register struct sem_undo **supptr;
    int did_something;

    /*
     * If somebody else is holding the global semaphore facility lock
     * then sleep until it is released.
     */

    while (semlock_holder != NULL && semlock_holder != p) {
#ifdef SEM_DEBUG
	printf("semaphore facility locked - sleeping ...\n");
#endif
	sleep((caddr_t)&semlock_holder, (PZERO - 4));
    }

    did_something = 0;

    /*
     * Go through the chain of undo vectors looking for one
     * associated with this process.
     */

    for (supptr = &semu_list; (suptr = *supptr) != NULL; supptr = &(suptr->un_next)) {
	if (suptr->un_proc != p)
	    continue;

#ifdef SEM_DEBUG
	printf("proc @%08x has undo structure with %d entries\n",p,suptr->un_cnt);
#endif

	/*
	 * If there are any active undo elements then process them.
	 */

	if (suptr->un_cnt > 0) {
	    int ix;

	    for (ix = 0; ix < suptr->un_cnt; ix++) {
		int semid = suptr->un_ent[ix].un_id;
		int semnum = suptr->un_ent[ix].un_num;
		int adjval = suptr->un_ent[ix].un_adjval;
		struct semid_ds *semaptr;

		semaptr = &sema[semid];
		if ((semaptr->sem_perm.mode & SEM_ALLOC) == 0)
		    panic("semexit - semid not allocated");
		if (semnum >= semaptr->sem_nsems)
		    panic("semexit - semnum out of range");

#ifdef SEM_DEBUG
		printf("semexit:  %08x id=%d num=%d(adj=%d) ; sem=%d\n",suptr->un_proc,
		suptr->un_ent[ix].un_id,suptr->un_ent[ix].un_num,suptr->un_ent[ix].un_adjval,
		semaptr->sem_base[semnum].semval);
#endif

		if (adjval < 0) {
		    if (semaptr->sem_base[semnum].semval < -adjval)
			semaptr->sem_base[semnum].semval = 0;
		    else
			semaptr->sem_base[semnum].semval += adjval;
		} else
		    semaptr->sem_base[semnum].semval += adjval;

		/* printf("semval now %d\n",semaptr->sem_base[semnum].semval); */

#ifdef SEM_WAKEUP
		sem_wakeup((caddr_t)semaptr);	/* A little sloppy (we should KNOW if anybody is waiting). */
#else
		wakeup((caddr_t)semaptr);		/* A little sloppy (we should KNOW if anybody is waiting). */
#endif
#ifdef SEM_DEBUG
		printf("semexit:  back from wakeup\n");
#endif
	    }
	}

	/*
	 * Deallocate the undo vector.
	 */

#ifdef SEM_DEBUG
	printf("removing vector\n");
#endif
	suptr->un_proc = NULL;
	*supptr = suptr->un_next;

	/* Done. */
	break;
    }

    /*
     * If the exiting process is holding the global semaphore facility
     * lock then release it.
     */

    if (semlock_holder == p) {
	semlock_holder = NULL;
	wakeup((caddr_t)&semlock_holder);
    }
}