Merge with Linux 2.5.48.

[linux-2.6/linux-mips.git] / ipc / sem.c

/*
 * 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 reacquire 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.
 *
 * /proc/sysvipc/sem support (c) 1999 Dragos Acostachioaie <dragos@iname.com>
 *
 * SMP-threaded, sysctl's added
 * (c) 1999 Manfred Spraul <manfreds@colorfullife.com>
 * Enforced range limit on SEM_UNDO
 * (c) 2001 Red Hat Inc <alan@redhat.com>
 */

#include <linux/config.h>
#include <linux/slab.h>
#include <linux/spinlock.h>
#include <linux/init.h>
#include <linux/proc_fs.h>
#include <linux/smp_lock.h>
#include <linux/security.h>
#include <asm/uaccess.h>
#include "util.h"


#define sem_lock(id)    ((struct sem_array*)ipc_lock(&sem_ids,id))
#define sem_unlock(sma) ipc_unlock(&(sma)->sem_perm)
#define sem_rmid(id)    ((struct sem_array*)ipc_rmid(&sem_ids,id))
#define sem_checkid(sma, semid) \
        ipc_checkid(&sem_ids,&sma->sem_perm,semid)
#define sem_buildid(id, seq) \
        ipc_buildid(&sem_ids, id, seq)
static struct ipc_ids sem_ids;

static int newary (key_t, int, int);
static void freeary (int id);
#ifdef CONFIG_PROC_FS
static int sysvipc_sem_read_proc(char *buffer, char **start, off_t offset, int length, int *eof, void *data);
#endif

#define SEMMSL_FAST     256 /* 512 bytes on stack */
#define SEMOPM_FAST     64  /* ~ 372 bytes on stack */

/*
 * linked list protection:
 *      sem_undo.id_next,
 *      sem_array.sem_pending{,last},
 *      sem_array.sem_undo: sem_lock() for read/write
 *      sem_undo.proc_next: only "current" is allowed to read/write that field.
 *      
 */

int sem_ctls[4] = {SEMMSL, SEMMNS, SEMOPM, SEMMNI};
#define sc_semmsl       (sem_ctls[0])
#define sc_semmns       (sem_ctls[1])
#define sc_semopm       (sem_ctls[2])
#define sc_semmni       (sem_ctls[3])

static int used_sems;

void __init sem_init (void)
{
        used_sems = 0;
        ipc_init_ids(&sem_ids,sc_semmni);

#ifdef CONFIG_PROC_FS
        create_proc_read_entry("sysvipc/sem", 0, 0, sysvipc_sem_read_proc, NULL);
#endif
}

static int newary (key_t key, int nsems, int semflg)
{
        int id;
        int retval;
        struct sem_array *sma;
        int size;

        if (!nsems)
                return -EINVAL;
        if (used_sems + nsems > sc_semmns)
                return -ENOSPC;

        size = sizeof (*sma) + nsems * sizeof (struct sem);
        sma = ipc_rcu_alloc(size);
        if (!sma) {
                return -ENOMEM;
        }
        memset (sma, 0, size);

        sma->sem_perm.mode = (semflg & S_IRWXUGO);
        sma->sem_perm.key = key;

        sma->sem_perm.security = NULL;
        retval = security_ops->sem_alloc_security(sma);
        if (retval) {
                ipc_rcu_free(sma, size);
                return retval;
        }

        id = ipc_addid(&sem_ids, &sma->sem_perm, sc_semmni);
        if(id == -1) {
                security_ops->sem_free_security(sma);
                ipc_rcu_free(sma, size);
                return -ENOSPC;
        }
        used_sems += nsems;

        sma->sem_base = (struct sem *) &sma[1];
        /* sma->sem_pending = NULL; */
        sma->sem_pending_last = &sma->sem_pending;
        /* sma->undo = NULL; */
        sma->sem_nsems = nsems;
        sma->sem_ctime = get_seconds();
        sem_unlock(sma);

        return sem_buildid(id, sma->sem_perm.seq);
}

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

        if (nsems < 0 || nsems > sc_semmsl)
                return -EINVAL;
        down(&sem_ids.sem);
        
        if (key == IPC_PRIVATE) {
                err = newary(key, nsems, semflg);
        } else if ((id = ipc_findkey(&sem_ids, 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 = sem_lock(id);
                if(sma==NULL)
                        BUG();
                if (nsems > sma->sem_nsems)
                        err = -EINVAL;
                else if (ipcperms(&sma->sem_perm, semflg))
                        err = -EACCES;
                else
                        err = sem_buildid(id, sma->sem_perm.seq);
                sem_unlock(sma);
        }

        up(&sem_ids.sem);
        return err;
}

/* doesn't acquire the sem_lock on error! */
static int sem_revalidate(int semid, struct sem_array* sma, int nsems, short flg)
{
        struct sem_array* smanew;

        smanew = sem_lock(semid);
        if(smanew==NULL)
                return -EIDRM;
        if(smanew != sma || sem_checkid(sma,semid) || sma->sem_nsems != nsems) {
                sem_unlock(smanew);
                return -EIDRM;
        }

        if (ipcperms(&sma->sem_perm, flg)) {
                sem_unlock(smanew);
                return -EACCES;
        }
        return 0;
}
/* 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 sem_array * 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 sem_array * 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 sem_array * 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 sem_array * 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)
                {
                        int undo = un->semadj[sop->sem_num] - sem_op;
                        /*
                         *      Exceeding the undo range is an error.
                         */
                        if (undo < (-SEMAEM - 1) || undo > SEMAEM)
                        {
                                /* Don't undo the undo */
                                sop->sem_flg &= ~SEM_UNDO;
                                goto out_of_range;
                        }
                        un->semadj[sop->sem_num] = undo;
                }
                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 = get_seconds();
        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 sem_array * 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_process(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 sem_array * 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 sem_array * 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 sem_array *sma;
        struct sem_undo *un;
        struct sem_queue *q;
        int size;

        sma = sem_rmid(id);

        /* Invalidate the existing undo structures for this semaphore set.
         * (They will be freed without any further action in sem_exit()
         * or during the next semop.)
         */
        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_process(q->sleeper); /* doesn't sleep */
        }
        sem_unlock(sma);

        used_sems -= sma->sem_nsems;
        size = sizeof (*sma) + sma->sem_nsems * sizeof (struct sem);
        security_ops->sem_free_security(sma);
        ipc_rcu_free(sma, size);
}

static unsigned long copy_semid_to_user(void *buf, struct semid64_ds *in, int version)
{
        switch(version) {
        case IPC_64:
                return copy_to_user(buf, in, sizeof(*in));
        case IPC_OLD:
            {
                struct semid_ds out;

                ipc64_perm_to_ipc_perm(&in->sem_perm, &out.sem_perm);

                out.sem_otime   = in->sem_otime;
                out.sem_ctime   = in->sem_ctime;
                out.sem_nsems   = in->sem_nsems;

                return copy_to_user(buf, &out, sizeof(out));
            }
        default:
                return -EINVAL;
        }
}

static int semctl_nolock(int semid, int semnum, int cmd, int version, union semun arg)
{
        int err = -EINVAL;
        struct sem_array *sma;

        switch(cmd) {
        case IPC_INFO:
        case SEM_INFO:
        {
                struct seminfo seminfo;
                int max_id;

                memset(&seminfo,0,sizeof(seminfo));
                seminfo.semmni = sc_semmni;
                seminfo.semmns = sc_semmns;
                seminfo.semmsl = sc_semmsl;
                seminfo.semopm = sc_semopm;
                seminfo.semvmx = SEMVMX;
                seminfo.semmnu = SEMMNU;
                seminfo.semmap = SEMMAP;
                seminfo.semume = SEMUME;
                down(&sem_ids.sem);
                if (cmd == SEM_INFO) {
                        seminfo.semusz = sem_ids.in_use;
                        seminfo.semaem = used_sems;
                } else {
                        seminfo.semusz = SEMUSZ;
                        seminfo.semaem = SEMAEM;
                }
                max_id = sem_ids.max_id;
                up(&sem_ids.sem);
                if (copy_to_user (arg.__buf, &seminfo, sizeof(struct seminfo))) 
                        return -EFAULT;
                return (max_id < 0) ? 0: max_id;
        }
        case SEM_STAT:
        {
                struct semid64_ds tbuf;
                int id;

                if(semid >= sem_ids.size)
                        return -EINVAL;

                memset(&tbuf,0,sizeof(tbuf));

                sma = sem_lock(semid);
                if(sma == NULL)
                        return -EINVAL;

                err = -EACCES;
                if (ipcperms (&sma->sem_perm, S_IRUGO))
                        goto out_unlock;
                id = sem_buildid(semid, sma->sem_perm.seq);

                kernel_to_ipc64_perm(&sma->sem_perm, &tbuf.sem_perm);
                tbuf.sem_otime  = sma->sem_otime;
                tbuf.sem_ctime  = sma->sem_ctime;
                tbuf.sem_nsems  = sma->sem_nsems;
                sem_unlock(sma);
                if (copy_semid_to_user (arg.buf, &tbuf, version))
                        return -EFAULT;
                return id;
        }
        default:
                return -EINVAL;
        }
        return err;
out_unlock:
        sem_unlock(sma);
        return err;
}

static int semctl_main(int semid, int semnum, int cmd, int version, union semun arg)
{
        struct sem_array *sma;
        struct sem* curr;
        int err;
        ushort fast_sem_io[SEMMSL_FAST];
        ushort* sem_io = fast_sem_io;
        int nsems;

        sma = sem_lock(semid);
        if(sma==NULL)
                return -EINVAL;

        nsems = sma->sem_nsems;

        err=-EIDRM;
        if (sem_checkid(sma,semid))
                goto out_unlock;

        err = -EACCES;
        if (ipcperms (&sma->sem_perm, (cmd==SETVAL||cmd==SETALL)?S_IWUGO:S_IRUGO))
                goto out_unlock;

        switch (cmd) {
        case GETALL:
        {
                ushort *array = arg.array;
                int i;

                if(nsems > SEMMSL_FAST) {
                        sem_unlock(sma);                        
                        sem_io = ipc_alloc(sizeof(ushort)*nsems);
                        if(sem_io == NULL)
                                return -ENOMEM;
                        err = sem_revalidate(semid, sma, nsems, S_IRUGO);
                        if(err)
                                goto out_free;
                }

                for (i = 0; i < sma->sem_nsems; i++)
                        sem_io[i] = sma->sem_base[i].semval;
                sem_unlock(sma);
                err = 0;
                if(copy_to_user(array, sem_io, nsems*sizeof(ushort)))
                        err = -EFAULT;
                goto out_free;
        }
        case SETALL:
        {
                int i;
                struct sem_undo *un;

                sem_unlock(sma);

                if(nsems > SEMMSL_FAST) {
                        sem_io = ipc_alloc(sizeof(ushort)*nsems);
                        if(sem_io == NULL)
                                return -ENOMEM;
                }

                if (copy_from_user (sem_io, arg.array, nsems*sizeof(ushort))) {
                        err = -EFAULT;
                        goto out_free;
                }

                for (i = 0; i < nsems; i++) {
                        if (sem_io[i] > SEMVMX) {
                                err = -ERANGE;
                                goto out_free;
                        }
                }
                err = sem_revalidate(semid, sma, nsems, S_IWUGO);
                if(err)
                        goto out_free;

                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 = get_seconds();
                /* maybe some queued-up processes were waiting for this */
                update_queue(sma);
                err = 0;
                goto out_unlock;
        }
        case IPC_STAT:
        {
                struct semid64_ds tbuf;
                memset(&tbuf,0,sizeof(tbuf));
                kernel_to_ipc64_perm(&sma->sem_perm, &tbuf.sem_perm);
                tbuf.sem_otime  = sma->sem_otime;
                tbuf.sem_ctime  = sma->sem_ctime;
                tbuf.sem_nsems  = sma->sem_nsems;
                sem_unlock(sma);
                if (copy_semid_to_user (arg.buf, &tbuf, version))
                        return -EFAULT;
                return 0;
        }
        /* GETVAL, GETPID, GETNCTN, GETZCNT, SETVAL: fall-through */
        }
        err = -EINVAL;
        if(semnum < 0 || semnum >= nsems)
                goto out_unlock;

        curr = &sma->sem_base[semnum];

        switch (cmd) {
        case GETVAL:
                err = curr->semval;
                goto out_unlock;
        case GETPID:
                err = curr->sempid & 0xffff;
                goto out_unlock;
        case GETNCNT:
                err = count_semncnt(sma,semnum);
                goto out_unlock;
        case GETZCNT:
                err = count_semzcnt(sma,semnum);
                goto out_unlock;
        case SETVAL:
        {
                int val = arg.val;
                struct sem_undo *un;
                err = -ERANGE;
                if (val > SEMVMX || val < 0)
                        goto out_unlock;

                for (un = sma->undo; un; un = un->id_next)
                        un->semadj[semnum] = 0;
                curr->semval = val;
                curr->sempid = current->pid;
                sma->sem_ctime = get_seconds();
                /* maybe some queued-up processes were waiting for this */
                update_queue(sma);
                err = 0;
                goto out_unlock;
        }
        }
out_unlock:
        sem_unlock(sma);
out_free:
        if(sem_io != fast_sem_io)
                ipc_free(sem_io, sizeof(ushort)*nsems);
        return err;
}

struct sem_setbuf {
        uid_t   uid;
        gid_t   gid;
        mode_t  mode;
};

static inline unsigned long copy_semid_from_user(struct sem_setbuf *out, void *buf, int version)
{
        switch(version) {
        case IPC_64:
            {
                struct semid64_ds tbuf;

                if(copy_from_user(&tbuf, buf, sizeof(tbuf)))
                        return -EFAULT;

                out->uid        = tbuf.sem_perm.uid;
                out->gid        = tbuf.sem_perm.gid;
                out->mode       = tbuf.sem_perm.mode;

                return 0;
            }
        case IPC_OLD:
            {
                struct semid_ds tbuf_old;

                if(copy_from_user(&tbuf_old, buf, sizeof(tbuf_old)))
                        return -EFAULT;

                out->uid        = tbuf_old.sem_perm.uid;
                out->gid        = tbuf_old.sem_perm.gid;
                out->mode       = tbuf_old.sem_perm.mode;

                return 0;
            }
        default:
                return -EINVAL;
        }
}

static int semctl_down(int semid, int semnum, int cmd, int version, union semun arg)
{
        struct sem_array *sma;
        int err;
        struct sem_setbuf setbuf;
        struct kern_ipc_perm *ipcp;

        if(cmd == IPC_SET) {
                if(copy_semid_from_user (&setbuf, arg.buf, version))
                        return -EFAULT;
        }
        sma = sem_lock(semid);
        if(sma==NULL)
                return -EINVAL;

        if (sem_checkid(sma,semid)) {
                err=-EIDRM;
                goto out_unlock;
        }       
        ipcp = &sma->sem_perm;
        
        if (current->euid != ipcp->cuid && 
            current->euid != ipcp->uid && !capable(CAP_SYS_ADMIN)) {
                err=-EPERM;
                goto out_unlock;
        }

        switch(cmd){
        case IPC_RMID:
                freeary(semid);
                err = 0;
                break;
        case IPC_SET:
                ipcp->uid = setbuf.uid;
                ipcp->gid = setbuf.gid;
                ipcp->mode = (ipcp->mode & ~S_IRWXUGO)
                                | (setbuf.mode & S_IRWXUGO);
                sma->sem_ctime = get_seconds();
                sem_unlock(sma);
                err = 0;
                break;
        default:
                sem_unlock(sma);
                err = -EINVAL;
                break;
        }
        return err;

out_unlock:
        sem_unlock(sma);
        return err;
}

asmlinkage long sys_semctl (int semid, int semnum, int cmd, union semun arg)
{
        int err = -EINVAL;
        int version;

        if (semid < 0)
                return -EINVAL;

        version = ipc_parse_version(&cmd);

        switch(cmd) {
        case IPC_INFO:
        case SEM_INFO:
        case SEM_STAT:
                err = semctl_nolock(semid,semnum,cmd,version,arg);
                return err;
        case GETALL:
        case GETVAL:
        case GETPID:
        case GETNCNT:
        case GETZCNT:
        case IPC_STAT:
        case SETVAL:
        case SETALL:
                err = semctl_main(semid,semnum,cmd,version,arg);
                return err;
        case IPC_RMID:
        case IPC_SET:
                down(&sem_ids.sem);
                err = semctl_down(semid,semnum,cmd,version,arg);
                up(&sem_ids.sem);
                return err;
        default:
                return -EINVAL;
        }
}

static inline void lock_semundo(void)
{
        struct sem_undo_list *undo_list;

        undo_list = current->sysvsem.undo_list;
        if ((undo_list != NULL) && (atomic_read(&undo_list->refcnt) != 1))
                spin_lock(&undo_list->lock);
}

/* This code has an interaction with copy_semundo().
 * Consider; two tasks are sharing the undo_list. task1
 * acquires the undo_list lock in lock_semundo().  If task2 now
 * exits before task1 releases the lock (by calling
 * unlock_semundo()), then task1 will never call spin_unlock().
 * This leave the sem_undo_list in a locked state.  If task1 now creats task3
 * and once again shares the sem_undo_list, the sem_undo_list will still be
 * locked, and future SEM_UNDO operations will deadlock.  This case is
 * dealt with in copy_semundo() by having it reinitialize the spin lock when 
 * the refcnt goes from 1 to 2.
 */
static inline void unlock_semundo(void)
{
        struct sem_undo_list *undo_list;

        undo_list = current->sysvsem.undo_list;
        if ((undo_list != NULL) && (atomic_read(&undo_list->refcnt) != 1))
                spin_unlock(&undo_list->lock);
}


/* If the task doesn't already have a undo_list, then allocate one
 * here.  We guarantee there is only one thread using this undo list,
 * and current is THE ONE
 *
 * If this allocation and assignment succeeds, but later
 * portions of this code fail, there is no need to free the sem_undo_list.
 * Just let it stay associated with the task, and it'll be freed later
 * at exit time.
 *
 * This can block, so callers must hold no locks.
 */
static inline int get_undo_list(struct sem_undo_list **undo_listp)
{
        struct sem_undo_list *undo_list;
        int size;

        undo_list = current->sysvsem.undo_list;
        if (!undo_list) {
                size = sizeof(struct sem_undo_list);
                undo_list = (struct sem_undo_list *) kmalloc(size, GFP_KERNEL);
                if (undo_list == NULL)
                        return -ENOMEM;
                memset(undo_list, 0, size);
                /* don't initialize unodhd->lock here.  It's done
                 * in copy_semundo() instead.
                 */
                atomic_set(&undo_list->refcnt, 1);
                current->sysvsem.undo_list = undo_list;
        }
        *undo_listp = undo_list;
        return 0;
}

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

        for(up = &current->sysvsem.undo_list->proc_list;(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;
}

static inline struct sem_undo *find_undo(int semid)
{
        struct sem_undo *un;

        un = NULL;
        if (current->sysvsem.undo_list != NULL) {
                un = current->sysvsem.undo_list->proc_list;
        }
        while(un != NULL) {
                if(un->semid==semid)
                        break;
                if(un->semid==-1)
                        un=freeundos(un);
                 else
                        un=un->proc_next;
        }
        return un;
}

/* returns without sem_lock and semundo list locks on error! */
static int alloc_undo(struct sem_array *sma, struct sem_undo** unp, int semid, int alter)
{
        int size, nsems, error;
        struct sem_undo *un, *new_un;
        struct sem_undo_list *undo_list;
        unsigned long   saved_add_count;


        nsems = sma->sem_nsems;
        saved_add_count = 0;
        if (current->sysvsem.undo_list != NULL)
                saved_add_count = current->sysvsem.undo_list->add_count;
        sem_unlock(sma);
        unlock_semundo();

        error = get_undo_list(&undo_list);
        if (error)
                return error;

        size = sizeof(struct sem_undo) + sizeof(short)*nsems;
        un = (struct sem_undo *) kmalloc(size, GFP_KERNEL);
        if (!un)
                return -ENOMEM;

        memset(un, 0, size);
        lock_semundo();
        error = sem_revalidate(semid, sma, nsems, alter ? S_IWUGO : S_IRUGO);
        if(error) {
                unlock_semundo();
                kfree(un);
                return error;
        }


        /* alloc_undo has just
         * released all locks and reacquired them. 
         * But, another thread may have
         * added the semundo we were looking for
         * during that time.
         * So, we check for it again.
         * only initialize and add the new one
         * if we don't discover one.
         */
        new_un = NULL;
        if (current->sysvsem.undo_list->add_count != saved_add_count)
                new_un = find_undo(semid);

        if (new_un != NULL) {
                if (sma->undo != new_un)
                        BUG();
                kfree(un);
                un = new_un;
        } else {
                current->sysvsem.undo_list->add_count++;
                un->semadj = (short *) &un[1];
                un->semid = semid;
                un->proc_next = undo_list->proc_list;
                undo_list->proc_list = un;
                un->id_next = sma->undo;
                sma->undo = un;
        }
        *unp = un;
        return 0;
}

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


        if (nsops < 1 || semid < 0)
                return -EINVAL;
        if (nsops > sc_semopm)
                return -E2BIG;
        if(nsops > SEMOPM_FAST) {
                sops = kmalloc(sizeof(*sops)*nsops,GFP_KERNEL);
                if(sops==NULL)
                        return -ENOMEM;
        }
        if (copy_from_user (sops, tsops, nsops * sizeof(*tsops))) {
                error=-EFAULT;
                goto out_free;
        }
        lock_semundo();
        sma = sem_lock(semid);
        error=-EINVAL;
        if(sma==NULL)
                goto out_semundo_free;
        error = -EIDRM;
        if (sem_checkid(sma,semid))
                goto out_unlock_semundo_free;
        error = -EFBIG;
        for (sop = sops; sop < sops + nsops; sop++) {
                if (sop->sem_num >= sma->sem_nsems)
                        goto out_unlock_semundo_free;
                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_unlock_semundo_free;
        if (undos) {
                /* Make sure we have an undo structure
                 * for this process and this semaphore set.
                 */
                
                un = find_undo(semid);
                if (!un) {
                        error = alloc_undo(sma,&un,semid,alter);
                        if (error)
                                goto out_free;

                }
        } 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;
        queue.id = semid;
        if (alter)
                append_to_queue(sma ,&queue);
        else
                prepend_to_queue(sma ,&queue);
        current->sysvsem.sleep_list = &queue;

        for (;;) {
                queue.status = -EINTR;
                queue.sleeper = current;
                current->state = TASK_INTERRUPTIBLE;
                sem_unlock(sma);
                unlock_semundo();

                schedule();

                lock_semundo();
                sma = sem_lock(semid);
                if(sma==NULL) {
                        if(queue.prev != NULL)
                                BUG();
                        current->sysvsem.sleep_list = NULL;
                        error = -EIDRM;
                        goto out_semundo_free;
                }
                /*
                 * If queue.status == 1 we where 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->sysvsem.sleep_list = NULL;
                        goto out_unlock_semundo_free;
                }
        }
        current->sysvsem.sleep_list = NULL;
        remove_from_queue(sma,&queue);
update:
        if (alter)
                update_queue (sma);
out_unlock_semundo_free:
        sem_unlock(sma);
out_semundo_free:
        unlock_semundo();
out_free:
        if(sops != fast_sops)
                kfree(sops);
        return error;
}

/* If CLONE_SYSVSEM is set, establish sharing of SEM_UNDO state between
 * parent and child tasks.
 *
 * See the notes above unlock_semundo() regarding the spin_lock_init()
 * in this code.  Initialize the undo_list->lock here instead of get_undo_list()
 * because of the reasoning in the comment above unlock_semundo.
 */

int copy_semundo(unsigned long clone_flags, struct task_struct *tsk)
{
        struct sem_undo_list *undo_list;
        int error;

        if (clone_flags & CLONE_SYSVSEM) {
                error = get_undo_list(&undo_list);
                if (error)
                        return error;
                if (atomic_read(&undo_list->refcnt) == 1)
                        spin_lock_init(&undo_list->lock);
                atomic_inc(&undo_list->refcnt);
                tsk->sysvsem.undo_list = undo_list;
        } else 
                tsk->sysvsem.undo_list = NULL;

        return 0;
}

static inline void __exit_semundo(struct task_struct *tsk)
{
        struct sem_undo_list *undo_list;

        undo_list = tsk->sysvsem.undo_list;
        if (!atomic_dec_and_test(&undo_list->refcnt))
                kfree(undo_list);
}

void exit_semundo(struct task_struct *tsk)
{
        if (tsk->sysvsem.undo_list != NULL)
                __exit_semundo(tsk);
}

/*
 * 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 sem_array *sma;
        struct sem_undo_list *undo_list;
        int nsems, i;

        lock_kernel();

        /* If the current process was sleeping for a semaphore,
         * remove it from the queue.
         */
        if ((q = current->sysvsem.sleep_list)) {
                int semid = q->id;
                sma = sem_lock(semid);
                current->sysvsem.sleep_list = NULL;

                if (q->prev) {
                        if(sma==NULL)
                                BUG();
                        remove_from_queue(q->sma,q);
                }
                if(sma!=NULL)
                        sem_unlock(sma);
        }

        undo_list = current->sysvsem.undo_list;
        if ((undo_list == NULL) || (atomic_read(&undo_list->refcnt) != 1)) {
                unlock_kernel();
                return;
        }

        /* There's no need to hold the semundo list lock, as current
         * is the last task exiting for this undo list.
         */
        for (up = &undo_list->proc_list; (u = *up); *up = u->proc_next, kfree(u)) {
                int semid = u->semid;
                if(semid == -1)
                        continue;
                sma = sem_lock(semid);
                if (sma == NULL)
                        continue;

                if (u->semid == -1)
                        goto next_entry;

                if (sem_checkid(sma,u->semid))
                        goto next_entry;

                /* 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);
                goto next_entry;
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 = get_seconds();
                /* maybe some queued-up processes were waiting for this */
                update_queue(sma);
next_entry:
                sem_unlock(sma);
        }
        __exit_semundo(current);

        unlock_kernel();
}

#ifdef CONFIG_PROC_FS
static int sysvipc_sem_read_proc(char *buffer, char **start, off_t offset, int length, int *eof, void *data)
{
        off_t pos = 0;
        off_t begin = 0;
        int i, len = 0;

        len += sprintf(buffer, "       key      semid perms      nsems   uid   gid  cuid  cgid      otime      ctime\n");
        down(&sem_ids.sem);

        for(i = 0; i <= sem_ids.max_id; i++) {
                struct sem_array *sma;
                sma = sem_lock(i);
                if(sma) {
                        len += sprintf(buffer + len, "%10d %10d  %4o %10lu %5u %5u %5u %5u %10lu %10lu\n",
                                sma->sem_perm.key,
                                sem_buildid(i,sma->sem_perm.seq),
                                sma->sem_perm.mode,
                                sma->sem_nsems,
                                sma->sem_perm.uid,
                                sma->sem_perm.gid,
                                sma->sem_perm.cuid,
                                sma->sem_perm.cgid,
                                sma->sem_otime,
                                sma->sem_ctime);
                        sem_unlock(sma);

                        pos += len;
                        if(pos < offset) {
                                len = 0;
                                begin = pos;
                        }
                        if(pos > offset + length)
                                goto done;
                }
        }
        *eof = 1;
done:
        up(&sem_ids.sem);
        *start = buffer + (offset - begin);
        len -= (offset - begin);
        if(len > length)
                len = length;
        if(len < 0)
                len = 0;
        return len;
}
#endif