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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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