rwlock.h

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/*
 * Copyright (C) 2004-2007  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: rwlock.h,v 1.28 2007/06/19 23:47:18 tbox Exp $ */

#ifndef ISC_RWLOCK_H
#define ISC_RWLOCK_H 1

/*! \file isc/rwlock.h */

#include <isc/condition.h>
#include <isc/lang.h>
#include <isc/platform.h>
#include <isc/types.h>

ISC_LANG_BEGINDECLS

typedef enum {
        isc_rwlocktype_none = 0,
        isc_rwlocktype_read,
        isc_rwlocktype_write
} isc_rwlocktype_t;

#ifdef ISC_PLATFORM_USETHREADS
#if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
#define ISC_RWLOCK_USEATOMIC 1
#endif

struct isc_rwlock {
        /* Unlocked. */
        unsigned int            magic;
        isc_mutex_t             lock;

#if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
        /*
         * When some atomic instructions with hardware assistance are
         * available, rwlock will use those so that concurrent readers do not
         * interfere with each other through mutex as long as no writers
         * appear, massively reducing the lock overhead in the typical case.
         *
         * The basic algorithm of this approach is the "simple
         * writer-preference lock" shown in the following URL:
         * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html
         * but our implementation does not rely on the spin lock unlike the
         * original algorithm to be more portable as a user space application.
         */

        /* Read or modified atomically. */
        isc_int32_t             write_requests;
        isc_int32_t             write_completions;
        isc_int32_t             cnt_and_flag;

        /* Locked by lock. */
        isc_condition_t         readable;
        isc_condition_t         writeable;
        unsigned int            readers_waiting;

        /* Locked by rwlock itself. */
        unsigned int            write_granted;

        /* Unlocked. */
        unsigned int            write_quota;

#else  /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */

        /*%< Locked by lock. */
        isc_condition_t         readable;
        isc_condition_t         writeable;
        isc_rwlocktype_t        type;

        /*% The number of threads that have the lock. */
        unsigned int            active;

        /*%
         * The number of lock grants made since the lock was last switched
         * from reading to writing or vice versa; used in determining
         * when the quota is reached and it is time to switch.
         */
        unsigned int            granted;
        
        unsigned int            readers_waiting;
        unsigned int            writers_waiting;
        unsigned int            read_quota;
        unsigned int            write_quota;
        isc_rwlocktype_t        original;
#endif  /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */
};
#else /* ISC_PLATFORM_USETHREADS */
struct isc_rwlock {
        unsigned int            magic;
        isc_rwlocktype_t        type;
        unsigned int            active;
};
#endif /* ISC_PLATFORM_USETHREADS */


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

isc_result_t
isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type);

isc_result_t
isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type);

isc_result_t
isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type);

isc_result_t
isc_rwlock_tryupgrade(isc_rwlock_t *rwl);

void
isc_rwlock_downgrade(isc_rwlock_t *rwl);

void
isc_rwlock_destroy(isc_rwlock_t *rwl);

ISC_LANG_ENDDECLS

#endif /* ISC_RWLOCK_H */

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