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/*
* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
*
* SPDX-License-Identifier: MPL-2.0
*
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, you can obtain one at https://mozilla.org/MPL/2.0/.
*
* See the COPYRIGHT file distributed with this work for additional
* information regarding copyright ownership.
*/
#pragma once
#include <inttypes.h>
/*! \file isc/rwlock.h */
#include <isc/atomic.h>
#include <isc/condition.h>
#include <isc/lang.h>
#include <isc/types.h>
ISC_LANG_BEGINDECLS
typedef enum {
isc_rwlocktype_none = 0,
isc_rwlocktype_read,
isc_rwlocktype_write
} isc_rwlocktype_t;
#if USE_PTHREAD_RWLOCK
#include <pthread.h>
struct isc_rwlock {
pthread_rwlock_t rwlock;
atomic_bool downgrade;
};
#else /* USE_PTHREAD_RWLOCK */
struct isc_rwlock {
/* Unlocked. */
unsigned int magic;
isc_mutex_t lock;
atomic_int_fast32_t spins;
/*
* 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. */
atomic_int_fast32_t write_requests;
atomic_int_fast32_t write_completions;
atomic_int_fast32_t cnt_and_flag;
/* Locked by lock. */
isc_condition_t readable;
isc_condition_t writeable;
unsigned int readers_waiting;
/* Locked by rwlock itself. */
atomic_uint_fast32_t write_granted;
/* Unlocked. */
unsigned int write_quota;
};
#endif /* USE_PTHREAD_RWLOCK */
void
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
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