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/*
* Copyright (C) Internet Systems Consortium, Inc. ("ISC")
*
* 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 http://mozilla.org/MPL/2.0/.
*
* See the COPYRIGHT file distributed with this work for additional
* information regarding copyright ownership.
*/
#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>
#if defined(ISC_PLATFORM_HAVESTDATOMIC)
#include <stdint.h>
#include <stdatomic.h>
#endif
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_HAVESTDATOMIC) && defined(ATOMIC_INT_LOCK_FREE)) || (defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG))
#define ISC_RWLOCK_USEATOMIC 1
#if (defined(ISC_PLATFORM_HAVESTDATOMIC) && defined(ATOMIC_INT_LOCK_FREE))
#define ISC_RWLOCK_USESTDATOMIC 1
#endif
#endif
struct isc_rwlock {
/* Unlocked. */
unsigned int magic;
isc_mutex_t lock;
isc_int32_t spins;
#if defined(ISC_RWLOCK_USEATOMIC)
/*
* 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. */
#if defined(ISC_RWLOCK_USESTDATOMIC)
atomic_int_fast32_t write_requests;
atomic_int_fast32_t write_completions;
atomic_int_fast32_t cnt_and_flag;
#else
isc_int32_t write_requests;
isc_int32_t write_completions;
isc_int32_t cnt_and_flag;
#endif
/* 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_RWLOCK_USEATOMIC */
/*%< 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_RWLOCK_USEATOMIC */
};
#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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