rwlock.h revision 0c27b3fe77ac1d5094ba3521e8142d9e7973133f
/*
* Copyright (C) 1998-2001, 2003-2007, 2016 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/.
*/
/* $Id: rwlock.h,v 1.28 2007/06/19 23:47:18 tbox Exp $ */
#ifndef ISC_RWLOCK_H
#define ISC_RWLOCK_H 1
#include <isc/condition.h>
#include <isc/platform.h>
typedef enum {
isc_rwlocktype_none = 0,
#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;
#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:
* 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. */
/* Locked by lock. */
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. */
/*% 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;
#endif /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */
};
#else /* ISC_PLATFORM_USETHREADS */
struct isc_rwlock {
unsigned int magic;
unsigned int active;
};
#endif /* ISC_PLATFORM_USETHREADS */
unsigned int write_quota);
void
void
#endif /* ISC_RWLOCK_H */