/*
 * linux/ipc/sem.c
 * Copyright (C) 1992 Krishna Balasubramanian
 * Copyright (C) 1995 Eric Schenk, Bruno Haible
 *
 * IMPLEMENTATION NOTES ON CODE REWRITE (Eric Schenk, January 1995):
 * This code underwent a massive rewrite in order to solve some problems
 * with the original code. In particular the original code failed to
 * wake up processes that were waiting for semval to go to 0 if the
 * value went to 0 and was then incremented rapidly enough. In solving
 * this problem I have also modified the implementation so that it
 * processes pending operations in a FIFO manner, thus give a guarantee
 * that processes waiting for a lock on the semaphore won't starve
 * unless another locking process fails to unlock.
 * In addition the following two changes in behavior have been introduced:
 * - The original implementation of semop returned the value
 *   last semaphore element examined on success. This does not
 *   match the manual page specifications, and effectively
 *   allows the user to read the semaphore even if they do not
 *   have read permissions. The implementation now returns 0
 *   on success as stated in the manual page.
 * - There is some confusion over whether the set of undo adjustments
 *   to be performed at exit should be done in an atomic manner.
 *   That is, if we are attempting to decrement the semval should we queue
 *   up and wait until we can do so legally?
 *   The original implementation attempted to do this.
 *   The current implementation does not do so. This is because I don't
 *   think it is the right thing (TM) to do, and because I couldn't
 *   see a clean way to get the old behavior with the new design.
 *   The POSIX standard and SVID should be consulted to determine
 *   what behavior is mandated.
 *
 * Further notes on refinement (Christoph Rohland, December 1998):
 * - The POSIX standard says, that the undo adjustments simply should
 *   redo. So the current implementation is o.K.
 * - The previous code had two flaws:
 *   1) It actively gave the semaphore to the next waiting process
 *      sleeping on the semaphore. Since this process did not have the
 *      cpu this led to many unnecessary context switches and bad
 *      performance. Now we only check which process should be able to
 *      get the semaphore and if this process wants to reduce some
 *      semaphore value we simply wake it up without doing the
 *      operation. So it has to try to get it later. Thus e.g. the
 *      running process may reaquire the semaphore during the current
 *      time slice. If it only waits for zero or increases the semaphore,
 *      we do the operation in advance and wake it up.
 *   2) It did not wake up all zero waiting processes. We try to do
 *      better but only get the semops right which only wait for zero or
 *      increase. If there are decrement operations in the operations
 *      array we do the same as before.
 */

#include <linux/malloc.h>
#include <linux/smp_lock.h>
#include <linux/init.h>

#include <asm/uaccess.h>

extern int ipcperms (struct ipc_perm *ipcp, short semflg);
static int newary (key_t, int, int);
static int findkey (key_t key);
static void freeary (int id);

static struct semid_ds *semary[SEMMNI];
static int used_sems = 0, used_semids = 0;
static struct wait_queue *sem_lock = NULL;
static int max_semid = 0;

static unsigned short sem_seq = 0;

void __init sem_init (void)
{
        int i;

        sem_lock = NULL;
        used_sems = used_semids = max_semid = sem_seq = 0;
        for (i = 0; i < SEMMNI; i++)
                semary[i] = (struct semid_ds *) IPC_UNUSED;
        return;
}

static int findkey (key_t key)
{
        int id;
        struct semid_ds *sma;

        for (id = 0; id <= max_semid; id++) {
                while ((sma = semary[id]) == IPC_NOID)
                        interruptible_sleep_on (&sem_lock);
                if (sma == IPC_UNUSED)
                        continue;
                if (key == sma->sem_perm.key)
                        return id;
        }
        return -1;
}

static int newary (key_t key, int nsems, int semflg)
{
        int id;
        struct semid_ds *sma;
        struct ipc_perm *ipcp;
        int size;

        if (!nsems)
                return -EINVAL;
        if (used_sems + nsems > SEMMNS)
                return -ENOSPC;
        for (id = 0; id < SEMMNI; id++)
                if (semary[id] == IPC_UNUSED) {
                        semary[id] = (struct semid_ds *) IPC_NOID;
                        goto found;
                }
        return -ENOSPC;
found:
        size = sizeof (*sma) + nsems * sizeof (struct sem);
        used_sems += nsems;
        sma = (struct semid_ds *) kmalloc (size, GFP_KERNEL);
        if (!sma) {
                semary[id] = (struct semid_ds *) IPC_UNUSED;
                used_sems -= nsems;
                wake_up (&sem_lock);
                return -ENOMEM;
        }
        memset (sma, 0, size);
        sma->sem_base = (struct sem *) &sma[1];
        ipcp = &sma->sem_perm;
        ipcp->mode = (semflg & S_IRWXUGO);
        ipcp->key = key;
        ipcp->cuid = ipcp->uid = current->euid;
        ipcp->gid = ipcp->cgid = current->egid;
        sma->sem_perm.seq = sem_seq;
        /* sma->sem_pending = NULL; */
        sma->sem_pending_last = &sma->sem_pending;
        /* sma->undo = NULL; */
        sma->sem_nsems = nsems;
        sma->sem_ctime = CURRENT_TIME;
        if (id > max_semid)
                max_semid = id;
        used_semids++;
        semary[id] = sma;
        wake_up (&sem_lock);
        return (unsigned int) sma->sem_perm.seq * SEMMNI + id;
}

asmlinkage int sys_semget (key_t key, int nsems, int semflg)
{
        int id, err = -EINVAL;
        struct semid_ds *sma;

        lock_kernel();
        if (nsems < 0 || nsems > SEMMSL)
                goto out;
        if (key == IPC_PRIVATE) {
                err = newary(key, nsems, semflg);
        } else if ((id = findkey (key)) == -1) {  /* key not used */
                if (!(semflg & IPC_CREAT))
                        err = -ENOENT;
                else
                        err = newary(key, nsems, semflg);
        } else if (semflg & IPC_CREAT && semflg & IPC_EXCL) {
                err = -EEXIST;
        } else {
                sma = semary[id];
                if (nsems > sma->sem_nsems)
                        err = -EINVAL;
                else if (ipcperms(&sma->sem_perm, semflg))
                        err = -EACCES;
                else
                        err = (int) sma->sem_perm.seq * SEMMNI + id;
        }
out:
        unlock_kernel();
        return err;
}

/* Manage the doubly linked list sma->sem_pending as a FIFO:
 * insert new queue elements at the tail sma->sem_pending_last.
 */
static inline void append_to_queue (struct semid_ds * sma,
                                    struct sem_queue * q)
{
        *(q->prev = sma->sem_pending_last) = q;
        *(sma->sem_pending_last = &q->next) = NULL;
}

static inline void prepend_to_queue (struct semid_ds * sma,
                                     struct sem_queue * q)
{
        q->next = sma->sem_pending;
        *(q->prev = &sma->sem_pending) = q;
        if (q->next)
                q->next->prev = &q->next;
        else /* sma->sem_pending_last == &sma->sem_pending */
                sma->sem_pending_last = &q->next;
}

static inline void remove_from_queue (struct semid_ds * sma,
                                      struct sem_queue * q)
{
        *(q->prev) = q->next;
        if (q->next)
                q->next->prev = q->prev;
        else /* sma->sem_pending_last == &q->next */
                sma->sem_pending_last = q->prev;
        q->prev = NULL; /* mark as removed */
}

/*
 * Determine whether a sequence of semaphore operations would succeed
 * all at once. Return 0 if yes, 1 if need to sleep, else return error code.
 */

static int try_atomic_semop (struct semid_ds * sma, struct sembuf * sops,
                             int nsops, struct sem_undo *un, int pid,
                             int do_undo)
{
        int result, sem_op;
        struct sembuf *sop;
        struct sem * curr;

        for (sop = sops; sop < sops + nsops; sop++) {
                curr = sma->sem_base + sop->sem_num;
                sem_op = sop->sem_op;

                if (!sem_op && curr->semval)
                        goto would_block;

                curr->sempid = (curr->sempid << 16) | pid;
                curr->semval += sem_op;
                if (sop->sem_flg & SEM_UNDO)
                        un->semadj[sop->sem_num] -= sem_op;

                if (curr->semval < 0)
                        goto would_block;
                if (curr->semval > SEMVMX)
                        goto out_of_range;
        }

        if (do_undo)
        {
                sop--;
                result = 0;
                goto undo;
        }

        sma->sem_otime = CURRENT_TIME;
        return 0;

out_of_range:
        result = -ERANGE;
        goto undo;

would_block:
        if (sop->sem_flg & IPC_NOWAIT)
                result = -EAGAIN;
        else
                result = 1;

undo:
        while (sop >= sops) {
                curr = sma->sem_base + sop->sem_num;
                curr->semval -= sop->sem_op;
                curr->sempid >>= 16;

                if (sop->sem_flg & SEM_UNDO)
                        un->semadj[sop->sem_num] += sop->sem_op;
                sop--;
        }

        return result;
}

/* Go through the pending queue for the indicated semaphore
 * looking for tasks that can be completed.
 */
static void update_queue (struct semid_ds * sma)
{
        int error;
        struct sem_queue * q;

        for (q = sma->sem_pending; q; q = q->next) {
                        
                if (q->status == 1)
                        continue; /* this one was woken up before */

                error = try_atomic_semop(sma, q->sops, q->nsops,
                                         q->undo, q->pid, q->alter);

                /* Does q->sleeper still need to sleep? */
                if (error <= 0) {
                                /* Found one, wake it up */
                        wake_up_interruptible(&q->sleeper);
                        if (error == 0 && q->alter) {
                                /* if q-> alter let it self try */
                                q->status = 1;
                                return;
                        }
                        q->status = error;
                        remove_from_queue(sma,q);
                }
        }
}

/* The following counts are associated to each semaphore:
 *   semncnt        number of tasks waiting on semval being nonzero
 *   semzcnt        number of tasks waiting on semval being zero
 * This model assumes that a task waits on exactly one semaphore.
 * Since semaphore operations are to be performed atomically, tasks actually
 * wait on a whole sequence of semaphores simultaneously.
 * The counts we return here are a rough approximation, but still
 * warrant that semncnt+semzcnt>0 if the task is on the pending queue.
 */
static int count_semncnt (struct semid_ds * sma, ushort semnum)
{
        int semncnt;
        struct sem_queue * q;

        semncnt = 0;
        for (q = sma->sem_pending; q; q = q->next) {
                struct sembuf * sops = q->sops;
                int nsops = q->nsops;
                int i;
                for (i = 0; i < nsops; i++)
                        if (sops[i].sem_num == semnum
                            && (sops[i].sem_op < 0)
                            && !(sops[i].sem_flg & IPC_NOWAIT))
                                semncnt++;
        }
        return semncnt;
}
static int count_semzcnt (struct semid_ds * sma, ushort semnum)
{
        int semzcnt;
        struct sem_queue * q;

        semzcnt = 0;
        for (q = sma->sem_pending; q; q = q->next) {
                struct sembuf * sops = q->sops;
                int nsops = q->nsops;
                int i;
                for (i = 0; i < nsops; i++)
                        if (sops[i].sem_num == semnum
                            && (sops[i].sem_op == 0)
                            && !(sops[i].sem_flg & IPC_NOWAIT))
                                semzcnt++;
        }
        return semzcnt;
}

/* Free a semaphore set. */
static void freeary (int id)
{
        struct semid_ds *sma = semary[id];
        struct sem_undo *un;
        struct sem_queue *q;

        /* Invalidate this semaphore set */
        sma->sem_perm.seq++;
        sem_seq = (sem_seq+1) % ((unsigned)(1<<31)/SEMMNI); /* increment, but avoid overflow */
        used_sems -= sma->sem_nsems;
        if (id == max_semid)
                while (max_semid && (semary[--max_semid] == IPC_UNUSED));
        semary[id] = (struct semid_ds *) IPC_UNUSED;
        used_semids--;

        /* Invalidate the existing undo structures for this semaphore set.
         * (They will be freed without any further action in sem_exit().)
         */
        for (un = sma->undo; un; un = un->id_next)
                un->semid = -1;

        /* Wake up all pending processes and let them fail with EIDRM. */
        for (q = sma->sem_pending; q; q = q->next) {
                q->status = -EIDRM;
                q->prev = NULL;
                wake_up_interruptible(&q->sleeper); /* doesn't sleep! */
        }

        kfree(sma);
}

asmlinkage int sys_semctl (int semid, int semnum, int cmd, union semun arg)
{
        struct semid_ds *buf = NULL;
        struct semid_ds tbuf;
        int i, id, val = 0;
        struct semid_ds *sma;
        struct ipc_perm *ipcp;
        struct sem *curr = NULL;
        struct sem_undo *un;
        unsigned int nsems;
        ushort *array = NULL;
        ushort sem_io[SEMMSL];
        int err = -EINVAL;

        lock_kernel();
        if (semid < 0 || semnum < 0 || cmd < 0)
                goto out;

        switch (cmd) {
        case IPC_INFO:
        case SEM_INFO:
        {
                struct seminfo seminfo, *tmp = arg.__buf;
                seminfo.semmni = SEMMNI;
                seminfo.semmns = SEMMNS;
                seminfo.semmsl = SEMMSL;
                seminfo.semopm = SEMOPM;
                seminfo.semvmx = SEMVMX;
                seminfo.semmnu = SEMMNU;
                seminfo.semmap = SEMMAP;
                seminfo.semume = SEMUME;
                seminfo.semusz = SEMUSZ;
                seminfo.semaem = SEMAEM;
                if (cmd == SEM_INFO) {
                        seminfo.semusz = used_semids;
                        seminfo.semaem = used_sems;
                }
                err = -EFAULT;
                if (copy_to_user (tmp, &seminfo, sizeof(struct seminfo))) 
                        goto out;
                err = max_semid;
                goto out;
        }

        case SEM_STAT:
                buf = arg.buf;
                err = -EINVAL;
                if (semid > max_semid)
                        goto out;
                sma = semary[semid];
                if (sma == IPC_UNUSED || sma == IPC_NOID)
                        goto out;
                err = -EACCES;
                if (ipcperms (&sma->sem_perm, S_IRUGO))
                        goto out;
                id = (unsigned int) sma->sem_perm.seq * SEMMNI + semid;
                tbuf.sem_perm   = sma->sem_perm;
                tbuf.sem_otime  = sma->sem_otime;
                tbuf.sem_ctime  = sma->sem_ctime;
                tbuf.sem_nsems  = sma->sem_nsems;
                err = -EFAULT;
                if (copy_to_user (buf, &tbuf, sizeof(*buf)) == 0)
                        err = id;
                goto out;
        }

        id = (unsigned int) semid % SEMMNI;
        sma = semary [id];
        err = -EINVAL;
        if (sma == IPC_UNUSED || sma == IPC_NOID)
                goto out;
        ipcp = &sma->sem_perm;
        nsems = sma->sem_nsems;
        err = -EIDRM;
        if (sma->sem_perm.seq != (unsigned int) semid / SEMMNI)
                goto out;

        switch (cmd) {
        case GETVAL:
        case GETPID:
        case GETNCNT:
        case GETZCNT:
        case SETVAL:
                err = -EINVAL;
                if (semnum >= nsems)
                        goto out;
                curr = &sma->sem_base[semnum];
                break;
        }

        switch (cmd) {
        case GETVAL:
        case GETPID:
        case GETNCNT:
        case GETZCNT:
        case GETALL:
                err = -EACCES;
                if (ipcperms (ipcp, S_IRUGO))
                        goto out;
                switch (cmd) {
                case GETVAL : err = curr->semval; goto out;
                case GETPID : err = curr->sempid & 0xffff; goto out;
                case GETNCNT: err = count_semncnt(sma,semnum); goto out;
                case GETZCNT: err = count_semzcnt(sma,semnum); goto out;
                case GETALL:
                        array = arg.array;
                        break;
                }
                break;
        case SETVAL:
                val = arg.val;
                err = -ERANGE;
                if (val > SEMVMX || val < 0)
                        goto out;
                break;
        case IPC_RMID:
                if (current->euid == ipcp->cuid || 
                    current->euid == ipcp->uid || capable(CAP_SYS_ADMIN)) {
                        freeary (id);
                        err = 0;
                        goto out;
                }
                err = -EPERM;
                goto out;
        case SETALL: /* arg is a pointer to an array of ushort */
                array = arg.array;
                err = -EFAULT;
                if (copy_from_user (sem_io, array, nsems*sizeof(ushort)))
                       goto out;
                err = 0;
                for (i = 0; i < nsems; i++)
                        if (sem_io[i] > SEMVMX) {
                                err = -ERANGE;
                                goto out;
                        }
                break;
        case IPC_STAT:
                buf = arg.buf;
                break;
        case IPC_SET:
                buf = arg.buf;
                err = -EFAULT;
                if(copy_from_user (&tbuf, buf, sizeof (*buf)))
                        goto out;
                break;
        }

        err = -EIDRM;
        if ((sma != semary[id]) || (sma->sem_perm.seq != (unsigned int) semid / SEMMNI))
                goto out;

        switch (cmd) {
        case GETALL:
                err = -EACCES;
                if (ipcperms (ipcp, S_IRUGO))
                        goto out;
                for (i = 0; i < sma->sem_nsems; i++)
                        sem_io[i] = sma->sem_base[i].semval;
                err = -EFAULT;
                if (copy_to_user (array, sem_io, nsems*sizeof(ushort)))
                        goto out;
                break;
        case SETVAL:
                err = -EACCES;
                if (ipcperms (ipcp, S_IWUGO))
                        goto out;
                for (un = sma->undo; un; un = un->id_next)
                        un->semadj[semnum] = 0;
                curr->semval = val;
                sma->sem_ctime = CURRENT_TIME;
                /* maybe some queued-up processes were waiting for this */
                update_queue(sma);
                break;
        case IPC_SET:
                if (current->euid == ipcp->cuid || 
                    current->euid == ipcp->uid || capable(CAP_SYS_ADMIN)) {
                        ipcp->uid = tbuf.sem_perm.uid;
                        ipcp->gid = tbuf.sem_perm.gid;
                        ipcp->mode = (ipcp->mode & ~S_IRWXUGO)
                                | (tbuf.sem_perm.mode & S_IRWXUGO);
                        sma->sem_ctime = CURRENT_TIME;
                        err = 0;
                        goto out;
                }
                err = -EPERM;
                goto out;
        case IPC_STAT:
                err = -EACCES;
                if (ipcperms (ipcp, S_IRUGO))
                        goto out;
                tbuf.sem_perm   = sma->sem_perm;
                tbuf.sem_otime  = sma->sem_otime;
                tbuf.sem_ctime  = sma->sem_ctime;
                tbuf.sem_nsems  = sma->sem_nsems;
                err = -EFAULT;
                if (copy_to_user (buf, &tbuf, sizeof(*buf)))
                        goto out;
                break;
        case SETALL:
                err = -EACCES;
                if (ipcperms (ipcp, S_IWUGO))
                        goto out;
                for (i = 0; i < nsems; i++)
                        sma->sem_base[i].semval = sem_io[i];
                for (un = sma->undo; un; un = un->id_next)
                        for (i = 0; i < nsems; i++)
                                un->semadj[i] = 0;
                sma->sem_ctime = CURRENT_TIME;
                /* maybe some queued-up processes were waiting for this */
                update_queue(sma);
                break;
        default:
                err = -EINVAL;
                goto out;
        }
        err = 0;
out:
        unlock_kernel();
        return err;
}


static struct sem_undo* freeundos(struct sem_undo* un)
{
        struct sem_undo* u;
        struct sem_undo** up;

        for (up = &current->semundo;(u=*up);up=&u->proc_next) {
                if(un==u) {
                        un=u->proc_next;
                        *up=un;
                        kfree(u);
                        return un;
                }
        }
        printk ("freeundos undo list error id=%d\n", un->semid);
        return un->proc_next;
}


asmlinkage int sys_semop (int semid, struct sembuf *tsops, unsigned nsops)
{
        int id, size, error = -EINVAL;
        struct semid_ds *sma;
        struct sembuf sops[SEMOPM], *sop;
        struct sem_undo *un;
        int undos = 0, decrease = 0, alter = 0;
        struct sem_queue queue;

        lock_kernel();
        if (nsops < 1 || semid < 0)
                goto out;
        error = -E2BIG;
        if (nsops > SEMOPM)
                goto out;
        error = -EFAULT;
        if (copy_from_user (sops, tsops, nsops * sizeof(*tsops)))
                goto out;
        id = (unsigned int) semid % SEMMNI;
        error = -EINVAL;
        if ((sma = semary[id]) == IPC_UNUSED || sma == IPC_NOID)
                goto out;
        error = -EIDRM;
        if (sma->sem_perm.seq != (unsigned int) semid / SEMMNI)
                goto out;

                error = -EFBIG;
        for (sop = sops; sop < sops + nsops; sop++) {
                if (sop->sem_num >= sma->sem_nsems)
                        goto out;
                if (sop->sem_flg & SEM_UNDO)
                        undos++;
                if (sop->sem_op < 0)
                        decrease = 1;
                if (sop->sem_op > 0)
                        alter = 1;
        }
        alter |= decrease;

        error = -EACCES;
        if (ipcperms(&sma->sem_perm, alter ? S_IWUGO : S_IRUGO))
                goto out;
        if (undos) {
                /* Make sure we have an undo structure
                 * for this process and this semaphore set.
                 */
                un = current->semundo;
                while(un != NULL) {
                        if(un->semid==semid)
                                break;
                        if(un->semid==-1)
                                un=freeundos(un);
                        else
                        un=un->proc_next;
                }
                if (!un) {
                        size = sizeof(struct sem_undo) + sizeof(short)*sma->sem_nsems;
                        un = (struct sem_undo *) kmalloc(size, GFP_ATOMIC);
                        if (!un) {
                                error = -ENOMEM;
                                goto out;
                        }
                        memset(un, 0, size);
                        un->semadj = (short *) &un[1];
                        un->semid = semid;
                        un->proc_next = current->semundo;
                        current->semundo = un;
                        un->id_next = sma->undo;
                        sma->undo = un;
                }
        } else
                un = NULL;

        error = try_atomic_semop (sma, sops, nsops, un, current->pid, 0);
        if (error <= 0)
                goto update;

        /* We need to sleep on this operation, so we put the current
         * task into the pending queue and go to sleep.
         */
                
        queue.sma = sma;
        queue.sops = sops;
        queue.nsops = nsops;
        queue.undo = un;
        queue.pid = current->pid;
        queue.alter = decrease;
        current->semsleeping = &queue;
        if (alter)
                append_to_queue(sma ,&queue);
        else
                prepend_to_queue(sma ,&queue);

        for (;;) {
                queue.status = -EINTR;
                queue.sleeper = NULL;
                interruptible_sleep_on(&queue.sleeper);

                /*
                 * If queue.status == 1 we were woken up and
                 * have to retry else we simply return.
                 * If an interrupt occurred we have to clean up the
                 * queue
                 *
                 */
                if (queue.status == 1)
                {
                        error = try_atomic_semop (sma, sops, nsops, un,
                                                  current->pid,0);
                        if (error <= 0) 
                                break;
                } else {
                        error = queue.status;;
                        if (queue.prev) /* got Interrupt */
                                break;
                        /* Everything done by update_queue */
                        current->semsleeping = NULL;
                        goto out;
                }
        }
        current->semsleeping = NULL;
        remove_from_queue(sma,&queue);
update:
        if (alter)
                update_queue (sma);
out:
        unlock_kernel();
        return error;
}

/*
 * add semadj values to semaphores, free undo structures.
 * undo structures are not freed when semaphore arrays are destroyed
 * so some of them may be out of date.
 * IMPLEMENTATION NOTE: There is some confusion over whether the
 * set of adjustments that needs to be done should be done in an atomic
 * manner or not. That is, if we are attempting to decrement the semval
 * should we queue up and wait until we can do so legally?
 * The original implementation attempted to do this (queue and wait).
 * The current implementation does not do so. The POSIX standard
 * and SVID should be consulted to determine what behavior is mandated.
 */
void sem_exit (void)
{
        struct sem_queue *q;
        struct sem_undo *u, *un = NULL, **up, **unp;
        struct semid_ds *sma;
        int nsems, i;

        /* If the current process was sleeping for a semaphore,
         * remove it from the queue.
         */
        if ((q = current->semsleeping)) {
                if (q->prev)
                        remove_from_queue(q->sma,q);
                current->semsleeping = NULL;
        }

        for (up = &current->semundo; (u = *up); *up = u->proc_next, kfree(u)) {
                if (u->semid == -1)
                        continue;
                sma = semary[(unsigned int) u->semid % SEMMNI];
                if (sma == IPC_UNUSED || sma == IPC_NOID)
                        continue;
                if (sma->sem_perm.seq != (unsigned int) u->semid / SEMMNI)
                        continue;
                /* remove u from the sma->undo list */
                for (unp = &sma->undo; (un = *unp); unp = &un->id_next) {
                        if (u == un)
                                goto found;
                }
                printk ("sem_exit undo list error id=%d\n", u->semid);
                break;
found:
                *unp = un->id_next;
                /* perform adjustments registered in u */
                nsems = sma->sem_nsems;
                for (i = 0; i < nsems; i++) {
                        struct sem * sem = &sma->sem_base[i];
                        sem->semval += u->semadj[i];
                        if (sem->semval < 0)
                                sem->semval = 0; /* shouldn't happen */
                        sem->sempid = current->pid;
                }
                sma->sem_otime = CURRENT_TIME;
                /* maybe some queued-up processes were waiting for this */
                update_queue(sma);
        }
        current->semundo = NULL;
}