InfoMagic Source Code 1993 July/THE_SOURCE_CODE_CD_ROM.iso/bsd_srcs/sys/vm/kern_lock.c

C/C++ Source or Header | 1991-04-21 | 11.7 KB | 534 lines

/*
 * Copyright (c) 1991 Regents of the University of California.
 * All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * The Mach Operating System project at Carnegie-Mellon University.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *    This product includes software developed by the University of
 *    California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *    @(#)kern_lock.c    7.4 (Berkeley) 4/21/91
 *
 *
 * Copyright (c) 1987, 1990 Carnegie-Mellon University.
 * All rights reserved.
 *
 * Authors: Avadis Tevanian, Jr., Michael Wayne Young
 *
 * Permission to use, copy, modify and distribute this software and
 * its documentation is hereby granted, provided that both the copyright
 * notice and this permission notice appear in all copies of the
 * software, derivative works or modified versions, and any portions
 * thereof, and that both notices appear in supporting documentation.
 *
 * CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
 * CONDITION.  CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND
 * FOR ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
 *
 * Carnegie Mellon requests users of this software to return to
 *
 *  Software Distribution Coordinator  or  [e-mail]
 *  School of Computer Science
 *  Carnegie Mellon University
 *  Pittsburgh PA 15213-3890
 *
 * any improvements or extensions that they make and grant Carnegie the
 * rights to redistribute these changes.
 */

/*
 *    Locking primitives implementation
 */

#include "param.h"
#include "vm_param.h"
#include "lock.h"

/* XXX */
#include "proc.h"
typedef    int *thread_t;
#define    current_thread()    ((thread_t)&curproc->p_thread)
/* XXX */

#if    NCPUS > 1

/*
 *    Module:        lock
 *    Function:
 *        Provide reader/writer sychronization.
 *    Implementation:
 *        Simple interlock on a bit.  Readers first interlock
 *        increment the reader count, then let go.  Writers hold
 *        the interlock (thus preventing further readers), and
 *        wait for already-accepted readers to go away.
 */

/*
 *    The simple-lock routines are the primitives out of which
 *    the lock package is built.  The implementation is left
 *    to the machine-dependent code.
 */

#ifdef    notdef
/*
 *    A sample implementation of simple locks.
 *    assumes:
 *        boolean_t test_and_set(boolean_t *)
 *            indivisibly sets the boolean to TRUE
 *            and returns its old value
 *        and that setting a boolean to FALSE is indivisible.
 */
/*
 *    simple_lock_init initializes a simple lock.  A simple lock
 *    may only be used for exclusive locks.
 */

void simple_lock_init(l)
    simple_lock_t    l;
{
    *(boolean_t *)l = FALSE;
}

void simple_lock(l)
    simple_lock_t    l;
{
    while (test_and_set((boolean_t *)l))
        continue;
}

void simple_unlock(l)
    simple_lock_t    l;
{
    *(boolean_t *)l = FALSE;
}

boolean_t simple_lock_try(l)
    simple_lock_t    l;
{
        return (!test_and_set((boolean_t *)l));
}
#endif    notdef
#endif    NCPUS > 1

#if    NCPUS > 1
int lock_wait_time = 100;
#else    NCPUS > 1

    /*
     *     It is silly to spin on a uni-processor as if we
     *    thought something magical would happen to the
     *    want_write bit while we are executing.
     */
int lock_wait_time = 0;
#endif    NCPUS > 1


/*
 *    Routine:    lock_init
 *    Function:
 *        Initialize a lock; required before use.
 *        Note that clients declare the "struct lock"
 *        variables and then initialize them, rather
 *        than getting a new one from this module.
 */
void lock_init(l, can_sleep)
    lock_t        l;
    boolean_t    can_sleep;
{
    bzero(l, sizeof(lock_data_t));
    simple_lock_init(&l->interlock);
    l->want_write = FALSE;
    l->want_upgrade = FALSE;
    l->read_count = 0;
    l->can_sleep = can_sleep;
    l->thread = (char *)-1;        /* XXX */
    l->recursion_depth = 0;
}

void lock_sleepable(l, can_sleep)
    lock_t        l;
    boolean_t    can_sleep;
{
    simple_lock(&l->interlock);
    l->can_sleep = can_sleep;
    simple_unlock(&l->interlock);
}


/*
 *    Sleep locks.  These use the same data structure and algorithm
 *    as the spin locks, but the process sleeps while it is waiting
 *    for the lock.  These work on uniprocessor systems.
 */

void lock_write(l)
    register lock_t    l;
{
    register int    i;

    simple_lock(&l->interlock);

    if (((thread_t)l->thread) == current_thread()) {
        /*
         *    Recursive lock.
         */
        l->recursion_depth++;
        simple_unlock(&l->interlock);
        return;
    }

    /*
     *    Try to acquire the want_write bit.
     */
    while (l->want_write) {
        if ((i = lock_wait_time) > 0) {
            simple_unlock(&l->interlock);
            while (--i > 0 && l->want_write)
                continue;
            simple_lock(&l->interlock);
        }

        if (l->can_sleep && l->want_write) {
            l->waiting = TRUE;
            thread_sleep((int) l, &l->interlock, FALSE);
            simple_lock(&l->interlock);
        }
    }
    l->want_write = TRUE;

    /* Wait for readers (and upgrades) to finish */

    while ((l->read_count != 0) || l->want_upgrade) {
        if ((i = lock_wait_time) > 0) {
            simple_unlock(&l->interlock);
            while (--i > 0 && (l->read_count != 0 ||
                    l->want_upgrade))
                continue;
            simple_lock(&l->interlock);
        }

        if (l->can_sleep && (l->read_count != 0 || l->want_upgrade)) {
            l->waiting = TRUE;
            thread_sleep((int) l, &l->interlock, FALSE);
            simple_lock(&l->interlock);
        }
    }
    simple_unlock(&l->interlock);
}

void lock_done(l)
    register lock_t    l;
{
    simple_lock(&l->interlock);

    if (l->read_count != 0)
        l->read_count--;
    else
    if (l->recursion_depth != 0)
        l->recursion_depth--;
    else
    if (l->want_upgrade)
         l->want_upgrade = FALSE;
    else
         l->want_write = FALSE;

    if (l->waiting) {
        l->waiting = FALSE;
        thread_wakeup((int) l);
    }
    simple_unlock(&l->interlock);
}

void lock_read(l)
    register lock_t    l;
{
    register int    i;

    simple_lock(&l->interlock);

    if (((thread_t)l->thread) == current_thread()) {
        /*
         *    Recursive lock.
         */
        l->read_count++;
        simple_unlock(&l->interlock);
        return;
    }

    while (l->want_write || l->want_upgrade) {
        if ((i = lock_wait_time) > 0) {
            simple_unlock(&l->interlock);
            while (--i > 0 && (l->want_write || l->want_upgrade))
                continue;
            simple_lock(&l->interlock);
        }

        if (l->can_sleep && (l->want_write || l->want_upgrade)) {
            l->waiting = TRUE;
            thread_sleep((int) l, &l->interlock, FALSE);
            simple_lock(&l->interlock);
        }
    }

    l->read_count++;
    simple_unlock(&l->interlock);
}

/*
 *    Routine:    lock_read_to_write
 *    Function:
 *        Improves a read-only lock to one with
 *        write permission.  If another reader has
 *        already requested an upgrade to a write lock,
 *        no lock is held upon return.
 *
 *        Returns TRUE if the upgrade *failed*.
 */
boolean_t lock_read_to_write(l)
    register lock_t    l;
{
    register int    i;

    simple_lock(&l->interlock);

    l->read_count--;

    if (((thread_t)l->thread) == current_thread()) {
        /*
         *    Recursive lock.
         */
        l->recursion_depth++;
        simple_unlock(&l->interlock);
        return(FALSE);
    }

    if (l->want_upgrade) {
        /*
         *    Someone else has requested upgrade.
         *    Since we've released a read lock, wake
         *    him up.
         */
        if (l->waiting) {
            l->waiting = FALSE;
            thread_wakeup((int) l);
        }

        simple_unlock(&l->interlock);
        return (TRUE);
    }

    l->want_upgrade = TRUE;

    while (l->read_count != 0) {
        if ((i = lock_wait_time) > 0) {
            simple_unlock(&l->interlock);
            while (--i > 0 && l->read_count != 0)
                continue;
            simple_lock(&l->interlock);
        }

        if (l->can_sleep && l->read_count != 0) {
            l->waiting = TRUE;
            thread_sleep((int) l, &l->interlock, FALSE);
            simple_lock(&l->interlock);
        }
    }

    simple_unlock(&l->interlock);
    return (FALSE);
}

void lock_write_to_read(l)
    register lock_t    l;
{
    simple_lock(&l->interlock);

    l->read_count++;
    if (l->recursion_depth != 0)
        l->recursion_depth--;
    else
    if (l->want_upgrade)
        l->want_upgrade = FALSE;
    else
         l->want_write = FALSE;

    if (l->waiting) {
        l->waiting = FALSE;
        thread_wakeup((int) l);
    }

    simple_unlock(&l->interlock);
}


/*
 *    Routine:    lock_try_write
 *    Function:
 *        Tries to get a write lock.
 *
 *        Returns FALSE if the lock is not held on return.
 */

boolean_t lock_try_write(l)
    register lock_t    l;
{

    simple_lock(&l->interlock);

    if (((thread_t)l->thread) == current_thread()) {
        /*
         *    Recursive lock
         */
        l->recursion_depth++;
        simple_unlock(&l->interlock);
        return(TRUE);
    }

    if (l->want_write || l->want_upgrade || l->read_count) {
        /*
         *    Can't get lock.
         */
        simple_unlock(&l->interlock);
        return(FALSE);
    }

    /*
     *    Have lock.
     */

    l->want_write = TRUE;
    simple_unlock(&l->interlock);
    return(TRUE);
}

/*
 *    Routine:    lock_try_read
 *    Function:
 *        Tries to get a read lock.
 *
 *        Returns FALSE if the lock is not held on return.
 */

boolean_t lock_try_read(l)
    register lock_t    l;
{
    simple_lock(&l->interlock);

    if (((thread_t)l->thread) == current_thread()) {
        /*
         *    Recursive lock
         */
        l->read_count++;
        simple_unlock(&l->interlock);
        return(TRUE);
    }

    if (l->want_write || l->want_upgrade) {
        simple_unlock(&l->interlock);
        return(FALSE);
    }

    l->read_count++;
    simple_unlock(&l->interlock);
    return(TRUE);
}

/*
 *    Routine:    lock_try_read_to_write
 *    Function:
 *        Improves a read-only lock to one with
 *        write permission.  If another reader has
 *        already requested an upgrade to a write lock,
 *        the read lock is still held upon return.
 *
 *        Returns FALSE if the upgrade *failed*.
 */
boolean_t lock_try_read_to_write(l)
    register lock_t    l;
{

    simple_lock(&l->interlock);

    if (((thread_t)l->thread) == current_thread()) {
        /*
         *    Recursive lock
         */
        l->read_count--;
        l->recursion_depth++;
        simple_unlock(&l->interlock);
        return(TRUE);
    }

    if (l->want_upgrade) {
        simple_unlock(&l->interlock);
        return(FALSE);
    }
    l->want_upgrade = TRUE;
    l->read_count--;

    while (l->read_count != 0) {
        l->waiting = TRUE;
        thread_sleep((int) l, &l->interlock, FALSE);
        simple_lock(&l->interlock);
    }

    simple_unlock(&l->interlock);
    return(TRUE);
}

/*
 *    Allow a process that has a lock for write to acquire it
 *    recursively (for read, write, or update).
 */
void lock_set_recursive(l)
    lock_t        l;
{
    simple_lock(&l->interlock);
    if (!l->want_write) {
        panic("lock_set_recursive: don't have write lock");
    }
    l->thread = (char *) current_thread();
    simple_unlock(&l->interlock);
}

/*
 *    Prevent a lock from being re-acquired.
 */
void lock_clear_recursive(l)
    lock_t        l;
{
    simple_lock(&l->interlock);
    if (((thread_t) l->thread) != current_thread()) {
        panic("lock_clear_recursive: wrong thread");
    }
    if (l->recursion_depth == 0)
        l->thread = (char *)-1;        /* XXX */
    simple_unlock(&l->interlock);
}