lib/isc/rwlock.c

	rwlock.c revision 5fa46bc91672ef5737aee6f99763161511566c24
/*
 * Copyright (C) 2004, 2005, 2007, 2009, 2011, 2012  Internet Systems Consortium, Inc. ("ISC")
 * Copyright (C) 1998-2001, 2003  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 * PERFORMANCE OF THIS SOFTWARE.
 */

/* $Id$ */

/*! \file */

#include <config.h>

#include <stddef.h>

#include <isc/atomic.h>
#include <isc/magic.h>
#include <isc/msgs.h>
#include <isc/platform.h>
#include <isc/rwlock.h>
#include <isc/util.h>

#define RWLOCK_MAGIC        ISC_MAGIC('R', 'W', 'L', 'k')
#define VALID_RWLOCK(rwl)   ISC_MAGIC_VALID(rwl, RWLOCK_MAGIC)

#ifdef ISC_PLATFORM_USETHREADS

#ifndef RWLOCK_DEFAULT_READ_QUOTA
#define RWLOCK_DEFAULT_READ_QUOTA 4
#endif

#ifndef RWLOCK_DEFAULT_WRITE_QUOTA
#define RWLOCK_DEFAULT_WRITE_QUOTA 4
#endif

#ifdef ISC_RWLOCK_TRACE
#include <stdio.h>      /* Required for fprintf/stderr. */
#include <isc/thread.h>     /* Required for isc_thread_self(). */

static void
print_lock(const char *operation, isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    fprintf(stderr,
        isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                   ISC_MSG_PRINTLOCK,
                   "rwlock %p thread %lu %s(%s): %s, %u active, "
                   "%u granted, %u rwaiting, %u wwaiting\n"),
        rwl, isc_thread_self(), operation,
        (type == isc_rwlocktype_read ?
         isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                ISC_MSG_READ, "read") :
         isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                ISC_MSG_WRITE, "write")),
        (rwl->type == isc_rwlocktype_read ?
         isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                ISC_MSG_READING, "reading") :
         isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                ISC_MSG_WRITING, "writing")),
        rwl->active, rwl->granted, rwl->readers_waiting,
        rwl->writers_waiting);
}
#endif

isc_result_t
isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
        unsigned int write_quota)
{
    isc_result_t result;

    REQUIRE(rwl != NULL);

    /*
     * In case there's trouble initializing, we zero magic now.  If all
     * goes well, we'll set it to RWLOCK_MAGIC.
     */
    rwl->magic = 0;

#if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
    rwl->write_requests = 0;
    rwl->write_completions = 0;
    rwl->cnt_and_flag = 0;
    rwl->readers_waiting = 0;
    rwl->write_granted = 0;
    if (read_quota != 0) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "read quota is not supported");
    }
    if (write_quota == 0)
        write_quota = RWLOCK_DEFAULT_WRITE_QUOTA;
    rwl->write_quota = write_quota;
#else
    rwl->type = isc_rwlocktype_read;
    rwl->original = isc_rwlocktype_none;
    rwl->active = 0;
    rwl->granted = 0;
    rwl->readers_waiting = 0;
    rwl->writers_waiting = 0;
    if (read_quota == 0)
        read_quota = RWLOCK_DEFAULT_READ_QUOTA;
    rwl->read_quota = read_quota;
    if (write_quota == 0)
        write_quota = RWLOCK_DEFAULT_WRITE_QUOTA;
    rwl->write_quota = write_quota;
#endif

    result = isc_mutex_init(&rwl->lock);
    if (result != ISC_R_SUCCESS)
        return (result);

    result = isc_condition_init(&rwl->readable);
    if (result != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_condition_init(readable) %s: %s",
                 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                        ISC_MSG_FAILED, "failed"),
                 isc_result_totext(result));
        result = ISC_R_UNEXPECTED;
        goto destroy_lock;
    }
    result = isc_condition_init(&rwl->writeable);
    if (result != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_condition_init(writeable) %s: %s",
                 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                        ISC_MSG_FAILED, "failed"),
                 isc_result_totext(result));
        result = ISC_R_UNEXPECTED;
        goto destroy_rcond;
    }

    rwl->magic = RWLOCK_MAGIC;

    return (ISC_R_SUCCESS);

  destroy_rcond:
    (void)isc_condition_destroy(&rwl->readable);
  destroy_lock:
    DESTROYLOCK(&rwl->lock);

    return (result);
}

void
isc_rwlock_destroy(isc_rwlock_t *rwl) {
    REQUIRE(VALID_RWLOCK(rwl));

#if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
    REQUIRE(rwl->write_requests == rwl->write_completions &&
        rwl->cnt_and_flag == 0 && rwl->readers_waiting == 0);
#else
    LOCK(&rwl->lock);
    REQUIRE(rwl->active == 0 &&
        rwl->readers_waiting == 0 &&
        rwl->writers_waiting == 0);
    UNLOCK(&rwl->lock);
#endif

    rwl->magic = 0;
    (void)isc_condition_destroy(&rwl->readable);
    (void)isc_condition_destroy(&rwl->writeable);
    DESTROYLOCK(&rwl->lock);
}

#if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)

/*
 * When some architecture-dependent atomic operations are available,
 * rwlock can be more efficient than the generic algorithm defined below.
 * The basic algorithm is described in the following URL:
 *   http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html
 *
 * The key is to use the following integer variables modified atomically:
 *   write_requests, write_completions, and cnt_and_flag.
 *
 * write_requests and write_completions act as a waiting queue for writers
 * in order to ensure the FIFO order.  Both variables begin with the initial
 * value of 0.  When a new writer tries to get a write lock, it increments
 * write_requests and gets the previous value of the variable as a "ticket".
 * When write_completions reaches the ticket number, the new writer can start
 * writing.  When the writer completes its work, it increments
 * write_completions so that another new writer can start working.  If the
 * write_requests is not equal to write_completions, it means a writer is now
 * working or waiting.  In this case, a new readers cannot start reading, or
 * in other words, this algorithm basically prefers writers.
 *
 * cnt_and_flag is a "lock" shared by all readers and writers.  This integer
 * variable is a kind of structure with two members: writer_flag (1 bit) and
 * reader_count (31 bits).  The writer_flag shows whether a writer is working,
 * and the reader_count shows the number of readers currently working or almost
 * ready for working.  A writer who has the current "ticket" tries to get the
 * lock by exclusively setting the writer_flag to 1, provided that the whole
 * 32-bit is 0 (meaning no readers or writers working).  On the other hand,
 * a new reader tries to increment the "reader_count" field provided that
 * the writer_flag is 0 (meaning there is no writer working).
 *
 * If some of the above operations fail, the reader or the writer sleeps
 * until the related condition changes.  When a working reader or writer
 * completes its work, some readers or writers are sleeping, and the condition
 * that suspended the reader or writer has changed, it wakes up the sleeping
 * readers or writers.
 *
 * As already noted, this algorithm basically prefers writers.  In order to
 * prevent readers from starving, however, the algorithm also introduces the
 * "writer quota" (Q).  When Q consecutive writers have completed their work,
 * suspending readers, the last writer will wake up the readers, even if a new
 * writer is waiting.
 *
 * Implementation specific note: due to the combination of atomic operations
 * and a mutex lock, ordering between the atomic operation and locks can be
 * very sensitive in some cases.  In particular, it is generally very important
 * to check the atomic variable that requires a reader or writer to sleep after
 * locking the mutex and before actually sleeping; otherwise, it could be very
 * likely to cause a deadlock.  For example, assume "var" is a variable
 * atomically modified, then the corresponding code would be:
 *  if (var == need_sleep) {
 *      LOCK(lock);
 *      if (var == need_sleep)
 *          WAIT(cond, lock);
 *      UNLOCK(lock);
 *  }
 * The second check is important, since "var" is protected by the atomic
 * operation, not by the mutex, and can be changed just before sleeping.
 * (The first "if" could be omitted, but this is also important in order to
 * make the code efficient by avoiding the use of the mutex unless it is
 * really necessary.)
 */

#define WRITER_ACTIVE   0x1
#define READER_INCR 0x2

isc_result_t
isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    isc_int32_t cntflag;

    REQUIRE(VALID_RWLOCK(rwl));

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_PRELOCK, "prelock"), rwl, type);
#endif

    if (type == isc_rwlocktype_read) {
        if (rwl->write_requests != rwl->write_completions) {
            /* there is a waiting or active writer */
            LOCK(&rwl->lock);
            if (rwl->write_requests != rwl->write_completions) {
                rwl->readers_waiting++;
                WAIT(&rwl->readable, &rwl->lock);
                rwl->readers_waiting--;
            }
            UNLOCK(&rwl->lock);
        }

        cntflag = isc_atomic_xadd(&rwl->cnt_and_flag, READER_INCR);
        POST(cntflag);
        while (1) {
            if ((rwl->cnt_and_flag & WRITER_ACTIVE) == 0)
                break;

            /* A writer is still working */
            LOCK(&rwl->lock);
            rwl->readers_waiting++;
            if ((rwl->cnt_and_flag & WRITER_ACTIVE) != 0)
                WAIT(&rwl->readable, &rwl->lock);
            rwl->readers_waiting--;
            UNLOCK(&rwl->lock);

            /*
             * Typically, the reader should be able to get a lock
             * at this stage:
             *   (1) there should have been no pending writer when
             *       the reader was trying to increment the
             *       counter; otherwise, the writer should be in
             *       the waiting queue, preventing the reader from
             *       proceeding to this point.
             *   (2) once the reader increments the counter, no
             *       more writer can get a lock.
             * Still, it is possible another writer can work at
             * this point, e.g. in the following scenario:
             *   A previous writer unlocks the writer lock.
             *   This reader proceeds to point (1).
             *   A new writer appears, and gets a new lock before
             *   the reader increments the counter.
             *   The reader then increments the counter.
             *   The previous writer notices there is a waiting
             *   reader who is almost ready, and wakes it up.
             * So, the reader needs to confirm whether it can now
             * read explicitly (thus we loop).  Note that this is
             * not an infinite process, since the reader has
             * incremented the counter at this point.
             */
        }

        /*
         * If we are temporarily preferred to writers due to the writer
         * quota, reset the condition (race among readers doesn't
         * matter).
         */
        rwl->write_granted = 0;
    } else {
        isc_int32_t prev_writer;

        /* enter the waiting queue, and wait for our turn */
        prev_writer = isc_atomic_xadd(&rwl->write_requests, 1);
        while (rwl->write_completions != prev_writer) {
            LOCK(&rwl->lock);
            if (rwl->write_completions != prev_writer) {
                WAIT(&rwl->writeable, &rwl->lock);
                UNLOCK(&rwl->lock);
                continue;
            }
            UNLOCK(&rwl->lock);
            break;
        }

        while (1) {
            cntflag = isc_atomic_cmpxchg(&rwl->cnt_and_flag, 0,
                             WRITER_ACTIVE);
            if (cntflag == 0)
                break;

            /* Another active reader or writer is working. */
            LOCK(&rwl->lock);
            if (rwl->cnt_and_flag != 0)
                WAIT(&rwl->writeable, &rwl->lock);
            UNLOCK(&rwl->lock);
        }

        INSIST((rwl->cnt_and_flag & WRITER_ACTIVE) != 0);
        rwl->write_granted++;
    }

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_POSTLOCK, "postlock"), rwl, type);
#endif

    return (ISC_R_SUCCESS);
}

isc_result_t
isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    isc_int32_t cntflag;

    REQUIRE(VALID_RWLOCK(rwl));

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_PRELOCK, "prelock"), rwl, type);
#endif

    if (type == isc_rwlocktype_read) {
        /* If a writer is waiting or working, we fail. */
        if (rwl->write_requests != rwl->write_completions)
            return (ISC_R_LOCKBUSY);

        /* Otherwise, be ready for reading. */
        cntflag = isc_atomic_xadd(&rwl->cnt_and_flag, READER_INCR);
        if ((cntflag & WRITER_ACTIVE) != 0) {
            /*
             * A writer is working.  We lose, and cancel the read
             * request.
             */
            cntflag = isc_atomic_xadd(&rwl->cnt_and_flag,
                          -READER_INCR);
            /*
             * If no other readers are waiting and we've suspended
             * new writers in this short period, wake them up.
             */
            if (cntflag == READER_INCR &&
                rwl->write_completions != rwl->write_requests) {
                LOCK(&rwl->lock);
                BROADCAST(&rwl->writeable);
                UNLOCK(&rwl->lock);
            }

            return (ISC_R_LOCKBUSY);
        }
    } else {
        /* Try locking without entering the waiting queue. */
        cntflag = isc_atomic_cmpxchg(&rwl->cnt_and_flag, 0,
                         WRITER_ACTIVE);
        if (cntflag != 0)
            return (ISC_R_LOCKBUSY);

        /*
         * XXXJT: jump into the queue, possibly breaking the writer
         * order.
         */
        (void)isc_atomic_xadd(&rwl->write_completions, -1);

        rwl->write_granted++;
    }

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_POSTLOCK, "postlock"), rwl, type);
#endif

    return (ISC_R_SUCCESS);
}

isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
    isc_int32_t prevcnt;

    REQUIRE(VALID_RWLOCK(rwl));

    /* Try to acquire write access. */
    prevcnt = isc_atomic_cmpxchg(&rwl->cnt_and_flag,
                     READER_INCR, WRITER_ACTIVE);
    /*
     * There must have been no writer, and there must have been at least
     * one reader.
     */
    INSIST((prevcnt & WRITER_ACTIVE) == 0 &&
           (prevcnt & ~WRITER_ACTIVE) != 0);

    if (prevcnt == READER_INCR) {
        /*
         * We are the only reader and have been upgraded.
         * Now jump into the head of the writer waiting queue.
         */
        (void)isc_atomic_xadd(&rwl->write_completions, -1);
    } else
        return (ISC_R_LOCKBUSY);

    return (ISC_R_SUCCESS);

}

void
isc_rwlock_downgrade(isc_rwlock_t *rwl) {
    isc_int32_t prev_readers;

    REQUIRE(VALID_RWLOCK(rwl));

    /* Become an active reader. */
    prev_readers = isc_atomic_xadd(&rwl->cnt_and_flag, READER_INCR);
    /* We must have been a writer. */
    INSIST((prev_readers & WRITER_ACTIVE) != 0);

    /* Complete write */
    (void)isc_atomic_xadd(&rwl->cnt_and_flag, -WRITER_ACTIVE);
    (void)isc_atomic_xadd(&rwl->write_completions, 1);

    /* Resume other readers */
    LOCK(&rwl->lock);
    if (rwl->readers_waiting > 0)
        BROADCAST(&rwl->readable);
    UNLOCK(&rwl->lock);
}

isc_result_t
isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    isc_int32_t prev_cnt;

    REQUIRE(VALID_RWLOCK(rwl));

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_PREUNLOCK, "preunlock"), rwl, type);
#endif

    if (type == isc_rwlocktype_read) {
        prev_cnt = isc_atomic_xadd(&rwl->cnt_and_flag, -READER_INCR);

        /*
         * If we're the last reader and any writers are waiting, wake
         * them up.  We need to wake up all of them to ensure the
         * FIFO order.
         */
        if (prev_cnt == READER_INCR &&
            rwl->write_completions != rwl->write_requests) {
            LOCK(&rwl->lock);
            BROADCAST(&rwl->writeable);
            UNLOCK(&rwl->lock);
        }
    } else {
        isc_boolean_t wakeup_writers = ISC_TRUE;

        /*
         * Reset the flag, and (implicitly) tell other writers
         * we are done.
         */
        (void)isc_atomic_xadd(&rwl->cnt_and_flag, -WRITER_ACTIVE);
        (void)isc_atomic_xadd(&rwl->write_completions, 1);

        if (rwl->write_granted >= rwl->write_quota ||
            rwl->write_requests == rwl->write_completions ||
            (rwl->cnt_and_flag & ~WRITER_ACTIVE) != 0) {
            /*
             * We have passed the write quota, no writer is
             * waiting, or some readers are almost ready, pending
             * possible writers.  Note that the last case can
             * happen even if write_requests != write_completions
             * (which means a new writer in the queue), so we need
             * to catch the case explicitly.
             */
            LOCK(&rwl->lock);
            if (rwl->readers_waiting > 0) {
                wakeup_writers = ISC_FALSE;
                BROADCAST(&rwl->readable);
            }
            UNLOCK(&rwl->lock);
        }

        if (rwl->write_requests != rwl->write_completions &&
            wakeup_writers) {
            LOCK(&rwl->lock);
            BROADCAST(&rwl->writeable);
            UNLOCK(&rwl->lock);
        }
    }

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_POSTUNLOCK, "postunlock"),
           rwl, type);
#endif

    return (ISC_R_SUCCESS);
}

#else /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */

static isc_result_t
doit(isc_rwlock_t *rwl, isc_rwlocktype_t type, isc_boolean_t nonblock) {
    isc_boolean_t skip = ISC_FALSE;
    isc_boolean_t done = ISC_FALSE;
    isc_result_t result = ISC_R_SUCCESS;

    REQUIRE(VALID_RWLOCK(rwl));

    LOCK(&rwl->lock);

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_PRELOCK, "prelock"), rwl, type);
#endif

    if (type == isc_rwlocktype_read) {
        if (rwl->readers_waiting != 0)
            skip = ISC_TRUE;
        while (!done) {
            if (!skip &&
                ((rwl->active == 0 ||
                  (rwl->type == isc_rwlocktype_read &&
                   (rwl->writers_waiting == 0 ||
                rwl->granted < rwl->read_quota)))))
            {
                rwl->type = isc_rwlocktype_read;
                rwl->active++;
                rwl->granted++;
                done = ISC_TRUE;
            } else if (nonblock) {
                result = ISC_R_LOCKBUSY;
                done = ISC_TRUE;
            } else {
                skip = ISC_FALSE;
                rwl->readers_waiting++;
                WAIT(&rwl->readable, &rwl->lock);
                rwl->readers_waiting--;
            }
        }
    } else {
        if (rwl->writers_waiting != 0)
            skip = ISC_TRUE;
        while (!done) {
            if (!skip && rwl->active == 0) {
                rwl->type = isc_rwlocktype_write;
                rwl->active = 1;
                rwl->granted++;
                done = ISC_TRUE;
            } else if (nonblock) {
                result = ISC_R_LOCKBUSY;
                done = ISC_TRUE;
            } else {
                skip = ISC_FALSE;
                rwl->writers_waiting++;
                WAIT(&rwl->writeable, &rwl->lock);
                rwl->writers_waiting--;
            }
        }
    }

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_POSTLOCK, "postlock"), rwl, type);
#endif

    UNLOCK(&rwl->lock);

    return (result);
}

isc_result_t
isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    return (doit(rwl, type, ISC_FALSE));
}

isc_result_t
isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    return (doit(rwl, type, ISC_TRUE));
}

isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
    isc_result_t result = ISC_R_SUCCESS;

    REQUIRE(VALID_RWLOCK(rwl));
    LOCK(&rwl->lock);
    REQUIRE(rwl->type == isc_rwlocktype_read);
    REQUIRE(rwl->active != 0);

    /* If we are the only reader then succeed. */
    if (rwl->active == 1) {
        rwl->original = (rwl->original == isc_rwlocktype_none) ?
                isc_rwlocktype_read : isc_rwlocktype_none;
        rwl->type = isc_rwlocktype_write;
    } else
        result = ISC_R_LOCKBUSY;

    UNLOCK(&rwl->lock);
    return (result);
}

void
isc_rwlock_downgrade(isc_rwlock_t *rwl) {

    REQUIRE(VALID_RWLOCK(rwl));
    LOCK(&rwl->lock);
    REQUIRE(rwl->type == isc_rwlocktype_write);
    REQUIRE(rwl->active == 1);

    rwl->type = isc_rwlocktype_read;
    rwl->original = (rwl->original == isc_rwlocktype_none) ?
            isc_rwlocktype_write : isc_rwlocktype_none;
    /*
     * Resume processing any read request that were blocked when
     * we upgraded.
     */
    if (rwl->original == isc_rwlocktype_none &&
        (rwl->writers_waiting == 0 || rwl->granted < rwl->read_quota) &&
        rwl->readers_waiting > 0)
        BROADCAST(&rwl->readable);

    UNLOCK(&rwl->lock);
}

isc_result_t
isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {

    REQUIRE(VALID_RWLOCK(rwl));
    LOCK(&rwl->lock);
    REQUIRE(rwl->type == type);

    UNUSED(type);

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_PREUNLOCK, "preunlock"), rwl, type);
#endif

    INSIST(rwl->active > 0);
    rwl->active--;
    if (rwl->active == 0) {
        if (rwl->original != isc_rwlocktype_none) {
            rwl->type = rwl->original;
            rwl->original = isc_rwlocktype_none;
        }
        if (rwl->type == isc_rwlocktype_read) {
            rwl->granted = 0;
            if (rwl->writers_waiting > 0) {
                rwl->type = isc_rwlocktype_write;
                SIGNAL(&rwl->writeable);
            } else if (rwl->readers_waiting > 0) {
                /* Does this case ever happen? */
                BROADCAST(&rwl->readable);
            }
        } else {
            if (rwl->readers_waiting > 0) {
                if (rwl->writers_waiting > 0 &&
                    rwl->granted < rwl->write_quota) {
                    SIGNAL(&rwl->writeable);
                } else {
                    rwl->granted = 0;
                    rwl->type = isc_rwlocktype_read;
                    BROADCAST(&rwl->readable);
                }
            } else if (rwl->writers_waiting > 0) {
                rwl->granted = 0;
                SIGNAL(&rwl->writeable);
            } else {
                rwl->granted = 0;
            }
        }
    }
    INSIST(rwl->original == isc_rwlocktype_none);

#ifdef ISC_RWLOCK_TRACE
    print_lock(isc_msgcat_get(isc_msgcat, ISC_MSGSET_RWLOCK,
                  ISC_MSG_POSTUNLOCK, "postunlock"),
           rwl, type);
#endif

    UNLOCK(&rwl->lock);

    return (ISC_R_SUCCESS);
}

#endif /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */
#else /* ISC_PLATFORM_USETHREADS */

isc_result_t
isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
        unsigned int write_quota)
{
    REQUIRE(rwl != NULL);

    UNUSED(read_quota);
    UNUSED(write_quota);

    rwl->type = isc_rwlocktype_read;
    rwl->active = 0;
    rwl->magic = RWLOCK_MAGIC;

    return (ISC_R_SUCCESS);
}

isc_result_t
isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    REQUIRE(VALID_RWLOCK(rwl));

    if (type == isc_rwlocktype_read) {
        if (rwl->type != isc_rwlocktype_read && rwl->active != 0)
            return (ISC_R_LOCKBUSY);
        rwl->type = isc_rwlocktype_read;
        rwl->active++;
    } else {
        if (rwl->active != 0)
            return (ISC_R_LOCKBUSY);
        rwl->type = isc_rwlocktype_write;
        rwl->active = 1;
    }
    return (ISC_R_SUCCESS);
}

isc_result_t
isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    return (isc_rwlock_lock(rwl, type));
}

isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl) {
    isc_result_t result = ISC_R_SUCCESS;

    REQUIRE(VALID_RWLOCK(rwl));
    REQUIRE(rwl->type == isc_rwlocktype_read);
    REQUIRE(rwl->active != 0);

    /* If we are the only reader then succeed. */
    if (rwl->active == 1)
        rwl->type = isc_rwlocktype_write;
    else
        result = ISC_R_LOCKBUSY;
    return (result);
}

void
isc_rwlock_downgrade(isc_rwlock_t *rwl) {

    REQUIRE(VALID_RWLOCK(rwl));
    REQUIRE(rwl->type == isc_rwlocktype_write);
    REQUIRE(rwl->active == 1);

    rwl->type = isc_rwlocktype_read;
}

isc_result_t
isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
    REQUIRE(VALID_RWLOCK(rwl));
    REQUIRE(rwl->type == type);

    UNUSED(type);

    INSIST(rwl->active > 0);
    rwl->active--;

    return (ISC_R_SUCCESS);
}

void
isc_rwlock_destroy(isc_rwlock_t *rwl) {
    REQUIRE(rwl != NULL);
    REQUIRE(rwl->active == 0);
    rwl->magic = 0;
}

#endif /* ISC_PLATFORM_USETHREADS */