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rwlock.h
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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 */