contrib/bind-9.5.2/lib/isc/include/isc/rwlock.h

   1 /*
   2  * Copyright (C) 2004-2007  Internet Systems Consortium, Inc. ("ISC")
   3  * Copyright (C) 1998-2001, 2003  Internet Software Consortium.
   4  *
   5  * Permission to use, copy, modify, and/or distribute this software for any
   6  * purpose with or without fee is hereby granted, provided that the above
   7  * copyright notice and this permission notice appear in all copies.
   8  *
   9  * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
  10  * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
  11  * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
  12  * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
  13  * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
  14  * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
  15  * PERFORMANCE OF THIS SOFTWARE.
  16  */
  17
  18 /* $Id: rwlock.h,v 1.28 2007/06/19 23:47:18 tbox Exp $ */
  19
  20 #ifndef ISC_RWLOCK_H
  21 #define ISC_RWLOCK_H 1
  22
  23 /*! \file isc/rwlock.h */
  24
  25 #include <isc/condition.h>
  26 #include <isc/lang.h>
  27 #include <isc/platform.h>
  28 #include <isc/types.h>
  29
  30 ISC_LANG_BEGINDECLS
  31
  32 typedef enum {
  33         isc_rwlocktype_none = 0,
  34         isc_rwlocktype_read,
  35         isc_rwlocktype_write
  36 } isc_rwlocktype_t;
  37
  38 #ifdef ISC_PLATFORM_USETHREADS
  39 #if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
  40 #define ISC_RWLOCK_USEATOMIC 1
  41 #endif
  42
  43 struct isc_rwlock {
  44         /* Unlocked. */
  45         unsigned int            magic;
  46         isc_mutex_t             lock;
  47
  48 #if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
  49         /*
  50          * When some atomic instructions with hardware assistance are
  51          * available, rwlock will use those so that concurrent readers do not
  52          * interfere with each other through mutex as long as no writers
  53          * appear, massively reducing the lock overhead in the typical case.
  54          *
  55          * The basic algorithm of this approach is the "simple
  56          * writer-preference lock" shown in the following URL:
  57          * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/rw.html
  58          * but our implementation does not rely on the spin lock unlike the
  59          * original algorithm to be more portable as a user space application.
  60          */
  61
  62         /* Read or modified atomically. */
  63         isc_int32_t             write_requests;
  64         isc_int32_t             write_completions;
  65         isc_int32_t             cnt_and_flag;
  66
  67         /* Locked by lock. */
  68         isc_condition_t         readable;
  69         isc_condition_t         writeable;
  70         unsigned int            readers_waiting;
  71
  72         /* Locked by rwlock itself. */
  73         unsigned int            write_granted;
  74
  75         /* Unlocked. */
  76         unsigned int            write_quota;
  77
  78 #else  /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */
  79
  80         /*%< Locked by lock. */
  81         isc_condition_t         readable;
  82         isc_condition_t         writeable;
  83         isc_rwlocktype_t        type;
  84
  85         /*% The number of threads that have the lock. */
  86         unsigned int            active;
  87
  88         /*%
  89          * The number of lock grants made since the lock was last switched
  90          * from reading to writing or vice versa; used in determining
  91          * when the quota is reached and it is time to switch.
  92          */
  93         unsigned int            granted;
  94
  95         unsigned int            readers_waiting;
  96         unsigned int            writers_waiting;
  97         unsigned int            read_quota;
  98         unsigned int            write_quota;
  99         isc_rwlocktype_t        original;
 100 #endif  /* ISC_PLATFORM_HAVEXADD && ISC_PLATFORM_HAVECMPXCHG */
 101 };
 102 #else /* ISC_PLATFORM_USETHREADS */
 103 struct isc_rwlock {
 104         unsigned int            magic;
 105         isc_rwlocktype_t        type;
 106         unsigned int            active;
 107 };
 108 #endif /* ISC_PLATFORM_USETHREADS */
 109
 110
 111 isc_result_t
 112 isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
 113                 unsigned int write_quota);
 114
 115 isc_result_t
 116 isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
 117
 118 isc_result_t
 119 isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
 120
 121 isc_result_t
 122 isc_rwlock_unlock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
 123
 124 isc_result_t
 125 isc_rwlock_tryupgrade(isc_rwlock_t *rwl);
 126
 127 void
 128 isc_rwlock_downgrade(isc_rwlock_t *rwl);
 129
 130 void
 131 isc_rwlock_destroy(isc_rwlock_t *rwl);
 132
 133 ISC_LANG_ENDDECLS
 134
 135 #endif /* ISC_RWLOCK_H */