lib/libnisdb/nisdb_rw.c

	nisdb_rw.c revision a87701e9837f8a9ee9e4c4d3186295c0e29f743f
/*
 * CDDL HEADER START
 *
 * The contents of this file are subject to the terms of the
 * Common Development and Distribution License, Version 1.0 only
 * (the "License").  You may not use this file except in compliance
 * with the License.
 *
 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
 * or http://www.opensolaris.org/os/licensing.
 * See the License for the specific language governing permissions
 * and limitations under the License.
 *
 * When distributing Covered Code, include this CDDL HEADER in each
 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
 * If applicable, add the following below this CDDL HEADER, with the
 * fields enclosed by brackets "[]" replaced with your own identifying
 * information: Portions Copyright [yyyy] [name of copyright owner]
 *
 * CDDL HEADER END
 */
/*
 * Copyright 2015 Gary Mills
 * Copyright (c) 2001 by Sun Microsystems, Inc.
 * All rights reserved.
 */

#include <stdio.h>
#include <rpc/types.h>
#include <rpc/xdr.h>
#include "db_dictionary_c.h"
#include "nisdb_rw.h"
#include "nisdb_ldap.h"

/*
 * Nesting-safe RW locking functions. Return 0 when successful, an
 * error number from the E-series when not.
 */

int
__nisdb_rwinit(__nisdb_rwlock_t *rw) {

    int ret;

    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if ((ret = mutex_init(&rw->mutex, USYNC_THREAD, 0)) != 0)
        return (ret);
    if ((ret = cond_init(&rw->cv, USYNC_THREAD, 0)) != 0)
        return (ret);
    rw->destroyed = 0;

    /*
     * If we allow read-to-write lock migration, there's a potential
     * race condition if two or more threads want to upgrade at the
     * same time. The simple and safe (but crude and possibly costly)
     * method to fix this is to always use exclusive locks, and so
     * that has to be the default.
     *
     * There are two conditions under which it is safe to set
     * 'force_write' to zero for a certain lock structure:
     *
     * (1)  The lock will never be subject to migration, or
     *
     * (2)  It's OK if the data protected by the lock has changed
     *  (a) Every time the lock (read or write) has been
     *      acquired (even if the lock already was held by
     *      the thread), and
     *  (b) After every call to a function that might have
     *      acquired the lock.
     */
    rw->force_write = NISDB_FORCE_WRITE;

    rw->writer_count = rw->reader_count = rw->reader_blocked = 0;
    rw->writer.id = rw->reader.id = INV_PTHREAD_ID;
    rw->writer.count = rw->reader.count = 0;
    rw->writer.next = rw->reader.next = 0;

    return (0);
}


static __nisdb_rl_t *
find_reader(pthread_t id, __nisdb_rwlock_t *rw) {

    __nisdb_rl_t    *rr;

    for (rr = &rw->reader; rr != 0; rr = rr->next) {
        if (rr->id == INV_PTHREAD_ID) {
            rr = 0;
            break;
        }
        if (rr->id == id)
            break;
    }

    return (rr);
}


int
__nisdb_rw_readlock_ok(__nisdb_rwlock_t *rw) {
    int     ret;

    if (rw == 0)
        return (EFAULT);

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    /*
     * Only allow changing 'force_write' when it's really safe; i.e.,
     * the lock hasn't been destroyed, and there are no readers.
     */
    if (rw->destroyed == 0 && rw->reader_count == 0) {
        rw->force_write = 0;
        ret = 0;
    } else {
        ret = EBUSY;
    }

    (void) mutex_unlock(&rw->mutex);

    return (ret);
}


int
__nisdb_rw_force_writelock(__nisdb_rwlock_t *rw) {
    int     ret;

    if (rw == 0 || rw->destroyed != 0)
        return (ESHUTDOWN);

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    /*
     * Only allow changing 'force_write' when it's really safe; i.e.,
     * the lock hasn't been destroyed, and there are no readers.
     */
    if (rw->destroyed == 0 && rw->reader_count == 0) {
        rw->force_write = 1;
        ret = 0;
    } else {
        ret = EBUSY;
    }

    (void) mutex_unlock(&rw->mutex);

    return (ret);
}


int
__nisdb_wlock_trylock(__nisdb_rwlock_t *rw, int trylock) {

    int     ret;
    pthread_t   myself = pthread_self();
    int     all_readers_blocked = 0;
    __nisdb_rl_t    *rr = 0;

    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        /* This shouldn't happen */
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    if (rw->destroyed != 0) {
        (void) mutex_unlock(&rw->mutex);
        return (ESHUTDOWN);
    }

    /* Simplest (and probably most common) case: no readers or writers */
    if (rw->reader_count == 0 && rw->writer_count == 0) {
        rw->writer_count = 1;
        rw->writer.id = myself;
        rw->writer.count = 1;
        return (mutex_unlock(&rw->mutex));
    }

    /*
     * Need to know if we're holding a read lock already, and if
     * all other readers are blocked waiting for the mutex.
     */
    if (rw->reader_count > 0) {
        if ((rr = find_reader(myself, rw)) != 0) {
            if (rr->count) {
                /*
                 * We're already holding a read lock, so
                 * if the number of readers equals the number
                 * of blocked readers plus one, all other
                 * readers are blocked.
                 */
                if (rw->reader_count ==
                        (rw->reader_blocked + 1))
                    all_readers_blocked = 1;
            } else {
                /*
                 * We're not holding a read lock, so the
                 * number of readers should equal the number
                 * of blocked readers if all readers are
                 * blocked.
                 */
                if (rw->reader_count == rw->reader_blocked)
                    all_readers_blocked = 1;
            }
        }
    }

    /* Wait for reader(s) or writer to finish */
    while (1) {
        /*
         * We can stop looping if one of the following holds:
         *  - No readers, no writers
         *  - No writers (or writer is myself), and one of:
         *      - No readers
         *      - One reader, and it's us
         *      - N readers, but all blocked on the mutex
         */
        if (
            (rw->writer_count == 0 && rw->reader_count == 0) ||
            (((rw->writer_count == 0 || rw->writer.id == myself) &&
            (rw->reader_count == 0)) ||
            (rw->reader_count == 1 &&
            rw->reader.id == myself))) {
            break;
        }
        /*
         * Provided that all readers are blocked on the mutex
         * we break a potential dead-lock by acquiring the
         * write lock.
         */
        if (all_readers_blocked) {
            if (rw->writer_count == 0 || rw->writer.id == myself) {
                break;
            }
        }

        /*
         * If 'trylock' is set, tell the caller that we'd have to
         * block to obtain the lock.
         */
        if (trylock) {
            (void) mutex_unlock(&rw->mutex);
            return (EBUSY);
        }

        /* If we're also a reader, indicate that we're blocking */
        if (rr != 0) {
            rr->wait = 1;
            rw->reader_blocked++;
        }
        if ((ret = cond_wait(&rw->cv, &rw->mutex)) != 0) {
            if (rr != 0) {
                rr->wait = 0;
                if (rw->reader_blocked > 0)
                    rw->reader_blocked--;
#ifdef  NISDB_MT_DEBUG
                else
                    abort();
#endif  /* NISDB_MT_DEBUG */
            }
            (void) mutex_unlock(&rw->mutex);
            return (ret);
        }
        if (rr != 0) {
            rr->wait = 0;
            if (rw->reader_blocked > 0)
                rw->reader_blocked--;
#ifdef  NISDB_MT_DEBUG
            else
                abort();
#endif  /* NISDB_MT_DEBUG */
        }
    }

    /* OK to grab the write lock */
    rw->writer.id = myself;
    /* Increment lock depth */
    rw->writer.count++;
    /* Set number of writers (doesn't increase with lock depth) */
    if (rw->writer_count == 0)
        rw->writer_count = 1;

    return (mutex_unlock(&rw->mutex));
}

int
__nisdb_wlock(__nisdb_rwlock_t *rw) {
    return (__nisdb_wlock_trylock(rw, 0));
}


static __nisdb_rl_t *
increment_reader(pthread_t id, __nisdb_rwlock_t *rw) {

    __nisdb_rl_t    *rr;

    for (rr = &rw->reader; rr != 0; rr = rr->next) {
        if (rr->id == id || rr->id == INV_PTHREAD_ID)
            break;
    }
    if (rw->reader_count == 0 && rr == &rw->reader) {
        /* No previous reader */
        rr->id = id;
        rw->reader_count = 1;
    } else if (rr == 0) {
        if ((rr = malloc(sizeof (__nisdb_rl_t))) == 0)
            return (0);
        rr->id = id;
        rr->count = 0;
        /*
         * For insertion simplicity, make it the second item
         * on the list.
         */
        rr->next = rw->reader.next;
        rw->reader.next = rr;
        rw->reader_count++;
    }
    rr->count++;

    return (rr);
}


int
__nisdb_rlock(__nisdb_rwlock_t *rw) {

    int     ret;
    pthread_t   myself = pthread_self();
    __nisdb_rl_t    *rr;

    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        /* This shouldn't happen */
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if (rw->force_write)
        return (__nisdb_wlock(rw));

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    if (rw->destroyed != 0) {
        (void) mutex_unlock(&rw->mutex);
        return (ESHUTDOWN);
    }

    rr = find_reader(myself, rw);

    /* Wait for writer to complete; writer == myself also OK */
    while (rw->writer_count > 0 && rw->writer.id != myself) {
        if (rr != 0) {
            rr->wait = 1;
            rw->reader_blocked++;
        }
        if ((ret = cond_wait(&rw->cv, &rw->mutex)) != 0) {
            if (rr != 0) {
                rr->wait = 0;
                if (rw->reader_blocked > 0)
                    rw->reader_blocked--;
#ifdef  NISDB_MT_DEBUG
                else
                    abort();
#endif  /* NISDB_MT_DEBUG */
            }
            (void) mutex_unlock(&rw->mutex);
            return (ret);
        }
        if (rr != 0) {
            rr->wait = 0;
            if (rw->reader_blocked > 0)
                rw->reader_blocked--;
#ifdef  NISDB_MT_DEBUG
            else
                abort();
#endif  /* NISDB_MT_DEBUG */
        }
    }

    rr = increment_reader(myself, rw);
    ret = mutex_unlock(&rw->mutex);
    return ((rr == 0) ? ENOMEM : ret);
}


int
__nisdb_wulock(__nisdb_rwlock_t *rw) {

    int     ret;
    pthread_t   myself = pthread_self();

    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        /* This shouldn't happen */
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    if (rw->destroyed != 0) {
        (void) mutex_unlock(&rw->mutex);
        return (ESHUTDOWN);
    }

    /* Sanity check */
    if (rw->writer_count == 0 ||
        rw->writer.id != myself || rw->writer.count == 0) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        (void) mutex_unlock(&rw->mutex);
        return (ENOLCK);
    }

    rw->writer.count--;
    if (rw->writer.count == 0) {
        rw->writer.id = INV_PTHREAD_ID;
        rw->writer_count = 0;
        if ((ret = cond_broadcast(&rw->cv)) != 0) {
            (void) mutex_unlock(&rw->mutex);
            return (ret);
        }
    }

    return (mutex_unlock(&rw->mutex));
}


int
__nisdb_rulock(__nisdb_rwlock_t *rw) {

    int     ret;
    pthread_t   myself = pthread_self();
    __nisdb_rl_t    *rr, *prev;

    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if (rw->force_write)
        return (__nisdb_wulock(rw));

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    if (rw->destroyed != 0) {
        (void) mutex_unlock(&rw->mutex);
        return (ESHUTDOWN);
    }

    /* Sanity check */
    if (rw->reader_count == 0 ||
        (rw->writer_count > 0 && rw->writer.id != myself)) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        (void) mutex_unlock(&rw->mutex);
        return (ENOLCK);
    }

    /* Find the reader record */
    for (rr = &rw->reader, prev = 0; rr != 0; prev = rr, rr = rr->next) {
        if (rr->id == myself)
            break;
    }

    if (rr == 0 || rr->count == 0) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        (void) mutex_unlock(&rw->mutex);
        return (ENOLCK);
    }

    rr->count--;
    if (rr->count == 0) {
        if (rr != &rw->reader) {
            /* Remove item from list and free it */
            prev->next = rr->next;
            free(rr);
        } else {
            /*
             * First record: copy second to first, and free second
             * record.
             */
            if (rr->next != 0) {
                rr = rr->next;
                rw->reader.id = rr->id;
                rw->reader.count = rr->count;
                rw->reader.next = rr->next;
                free(rr);
            } else {
                /* Decomission the first record */
                rr->id = INV_PTHREAD_ID;
            }
        }
        rw->reader_count--;
    }

    /* If there are no readers, wake up any waiting writer */
    if (rw->reader_count == 0) {
        if ((ret = cond_broadcast(&rw->cv)) != 0) {
            (void) mutex_unlock(&rw->mutex);
            return (ret);
        }
    }

    return (mutex_unlock(&rw->mutex));
}


/* Return zero if write lock held by this thread, non-zero otherwise */
int
__nisdb_assert_wheld(__nisdb_rwlock_t *rw) {

    int ret;


    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    if (rw->destroyed != 0) {
        (void) mutex_unlock(&rw->mutex);
        return (ESHUTDOWN);
    }

    if (rw->writer_count == 0 || rw->writer.id != pthread_self()) {
        ret = mutex_unlock(&rw->mutex);
        return ((ret == 0) ? -1 : ret);
    }

    /*
     * We're holding the lock, so we should return zero. Since
     * that's what mutex_unlock() does if it succeeds, we just
     * return the value of mutex_unlock().
     */
    return (mutex_unlock(&rw->mutex));
}


/* Return zero if read lock held by this thread, non-zero otherwise */
int
__nisdb_assert_rheld(__nisdb_rwlock_t *rw) {

    int     ret;
    pthread_t   myself = pthread_self();
    __nisdb_rl_t    *rr;


    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if (rw->force_write)
        return (__nisdb_assert_wheld(rw));

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    if (rw->destroyed != 0) {
        (void) mutex_unlock(&rw->mutex);
        return (ESHUTDOWN);
    }

    /* Write lock also OK */
    if (rw->writer_count > 0 && rw->writer.id == myself) {
        (void) mutex_unlock(&rw->mutex);
        return (0);
    }

    if (rw->reader_count == 0) {
        (void) mutex_unlock(&rw->mutex);
        return (EBUSY);
    }

    rr = &rw->reader;
    do {
        if (rr->id == myself) {
            (void) mutex_unlock(&rw->mutex);
            return (0);
        }
        rr = rr->next;
    } while (rr != 0);

    ret = mutex_unlock(&rw->mutex);
    return ((ret == 0) ? EBUSY : ret);
}


int
__nisdb_destroy_lock(__nisdb_rwlock_t *rw) {

    int     ret;
    pthread_t   myself = pthread_self();


    if (rw == 0) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        return (EFAULT);
    }

    if (rw->destroyed != 0)
        return (ESHUTDOWN);

    if ((ret = mutex_lock(&rw->mutex)) != 0)
        return (ret);

    if (rw->destroyed != 0) {
        (void) mutex_unlock(&rw->mutex);
        return (ESHUTDOWN);
    }

    /*
     * Only proceed if if there are neither readers nor writers
     * other than this thread. Also, no nested locks may be in
     * effect.
     */
    if (((rw->writer_count > 0 &&
        (rw->writer.id != myself || rw->writer.count != 1)) ||
        (rw->reader_count > 0 &&
        !(rw->reader_count == 1 && rw->reader.id == myself &&
        rw->reader.count == 1))) ||
        (rw->writer_count > 0 && rw->reader_count > 0)) {
#ifdef  NISDB_MT_DEBUG
        abort();
#endif  /* NISDB_MT_DEBUG */
        (void) mutex_unlock(&rw->mutex);
        return (ENOLCK);
    }

    /*
     * Mark lock destroyed, so that any thread waiting on the mutex
     * will know what's what. Of course, this is a bit iffy, since
     * we're probably being called from a destructor, and the structure
     * where we live will soon cease to exist (i.e., be freed and
     * perhaps re-used). Still, we can only do our best, and give
     * those other threads the best chance possible.
     */
    rw->destroyed++;

    return (mutex_unlock(&rw->mutex));
}

void
__nisdb_lock_report(__nisdb_rwlock_t *rw) {
    char        *myself = "__nisdb_lock_report";

    if (rw == 0) {
        printf("%s: NULL argument\n", myself);
        return;
    }

    if (rw->destroyed)
        printf("0x%x: DESTROYED\n", rw);

    printf("0x%x: Read locking %s\n",
        rw, rw->force_write ? "disallowed" : "allowed");

    if (rw->writer_count == 0)
        printf("0x%x: No writer\n", rw);
    else if (rw->writer_count == 1) {
        printf("0x%x: Write locked by %d, depth = %d\n",
            rw, rw->writer.id, rw->writer.count);
        if (rw->writer.wait)
            printf("0x%x:\tWriter blocked\n", rw);
    } else
        printf("0x%x: Invalid writer count = %d\n",
            rw, rw->writer_count);

    if (rw->reader_count == 0)
        printf("0x%x: No readers\n", rw);
    else {
        __nisdb_rl_t    *r;

        printf("0x%x: %d readers, %d blocked\n",
            rw, rw->reader_count, rw->reader_blocked);
        for (r = &rw->reader; r != 0; r = r->next) {
            printf("0x%x:\tthread %d, depth = %d%s\n",
                rw, r->id, r->count,
                (r->wait ? " (blocked)" : ""));
        }
    }
}