/*

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* Copyright (C) Internet Systems Consortium, Inc. ("ISC")

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*

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* This Source Code Form is subject to the terms of the Mozilla Public

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* License, v. 2.0. If a copy of the MPL was not distributed with this

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* file, you can obtain one at https://mozilla.org/MPL/2.0/.

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*

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* See the COPYRIGHT file distributed with this work for additional

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* information regarding copyright ownership.

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*/

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#ifndef ISC_RWLOCK_H

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#define ISC_RWLOCK_H 1

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#include <inttypes.h>

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

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#include <isc/condition.h>

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#include <isc/lang.h>

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#include <isc/platform.h>

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#include <isc/types.h>

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#if defined(ISC_PLATFORM_HAVESTDATOMIC)

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#if defined(__cplusplus)

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#include <isc/stdatomic.h>

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#else

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#include <stdatomic.h>

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#endif

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#endif

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ISC_LANG_BEGINDECLS

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typedef enum {

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isc_rwlocktype_none = 0,

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isc_rwlocktype_read,

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isc_rwlocktype_write

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} isc_rwlocktype_t;

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#ifdef ISC_PLATFORM_USETHREADS

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# if defined(ISC_PLATFORM_HAVESTDATOMIC)

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# define ISC_RWLOCK_USEATOMIC 1

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# define ISC_RWLOCK_USESTDATOMIC 1

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# else /* defined(ISC_PLATFORM_HAVESTDATOMIC) */

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

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# define ISC_RWLOCK_USEATOMIC 1

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# endif

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# endif /* defined(ISC_PLATFORM_HAVESTDATOMIC) */

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struct isc_rwlock {

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/* Unlocked. */

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unsigned int magic;

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isc_mutex_t lock;

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#if defined(ISC_RWLOCK_USEATOMIC)

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/*

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* When some atomic instructions with hardware assistance are

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* available, rwlock will use those so that concurrent readers do not

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* interfere with each other through mutex as long as no writers

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* appear, massively reducing the lock overhead in the typical case.

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*

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* The basic algorithm of this approach is the "simple

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* writer-preference lock" shown in the following URL:

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* http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html

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* but our implementation does not rely on the spin lock unlike the

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* original algorithm to be more portable as a user space application.

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*/

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/* Read or modified atomically. */

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#if defined(ISC_RWLOCK_USESTDATOMIC)

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atomic_int_fast32_t spins;

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atomic_int_fast32_t write_requests;

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atomic_int_fast32_t write_completions;

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atomic_int_fast32_t cnt_and_flag;

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atomic_int_fast32_t write_granted;

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#else

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int32_t spins;

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int32_t write_requests;

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int32_t write_completions;

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int32_t cnt_and_flag;

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int32_t write_granted;

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#endif

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/* Locked by lock. */

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isc_condition_t readable;

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isc_condition_t writeable;

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unsigned int readers_waiting;

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/* Unlocked. */

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unsigned int write_quota;

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#else /* ISC_RWLOCK_USEATOMIC */

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/*%< Locked by lock. */

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isc_condition_t readable;

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isc_condition_t writeable;

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isc_rwlocktype_t type;

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/*% The number of threads that have the lock. */

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unsigned int active;

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/*%

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* The number of lock grants made since the lock was last switched

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* from reading to writing or vice versa; used in determining

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* when the quota is reached and it is time to switch.

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*/

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unsigned int granted;

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unsigned int spins;

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unsigned int readers_waiting;

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unsigned int writers_waiting;

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unsigned int read_quota;

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unsigned int write_quota;

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isc_rwlocktype_t original;

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#endif /* ISC_RWLOCK_USEATOMIC */

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};

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#else /* ISC_PLATFORM_USETHREADS */

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struct isc_rwlock {

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unsigned int magic;

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isc_rwlocktype_t type;

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unsigned int active;

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};

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#endif /* ISC_PLATFORM_USETHREADS */

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isc_result_t

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isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,

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unsigned int write_quota);

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isc_result_t

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isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type);

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isc_result_t

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isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type);

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isc_result_t

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isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type);

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isc_result_t

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isc_rwlock_tryupgrade(isc_rwlock_t *rwl);

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void

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isc_rwlock_downgrade(isc_rwlock_t *rwl);

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void

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isc_rwlock_destroy(isc_rwlock_t *rwl);

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ISC_LANG_ENDDECLS

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#endif /* ISC_RWLOCK_H */

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