usr/src/uts/common/fs/zfs/zrlock.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21 /*
  22  * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
  23  */
  24
  25 /*
  26  * A Zero Reference Lock (ZRL) is a reference count that can lock out new
  27  * references only when the count is zero and only without waiting if the count
  28  * is not already zero. It is similar to a read-write lock in that it allows
  29  * multiple readers and only a single writer, but it does not allow a writer to
  30  * block while waiting for readers to exit, and therefore the question of
  31  * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
  32  * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
  33  * is perfectly safe for the same reader to acquire the same lock multiple
  34  * times. The fact that a ZRL is reentrant for readers (through multiple calls
  35  * to zrl_add()) makes it convenient for determining whether something is
  36  * actively referenced without the fuss of flagging lock ownership across
  37  * function calls.
  38  */
  39 #include <sys/zrlock.h>
  40
  41 /*
  42  * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
  43  * treated as zero references.
  44  */
  45 #define ZRL_LOCKED      ((uint32_t)-1)
  46 #define ZRL_DESTROYED   -2
  47
  48 void
  49 zrl_init(zrlock_t *zrl)
  50 {
  51         mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
  52         zrl->zr_refcount = 0;
  53         cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
  54 #ifdef  ZFS_DEBUG
  55         zrl->zr_owner = NULL;
  56         zrl->zr_caller = NULL;
  57 #endif
  58 }
  59
  60 void
  61 zrl_destroy(zrlock_t *zrl)
  62 {
  63         ASSERT(zrl->zr_refcount == 0);
  64
  65         mutex_destroy(&zrl->zr_mtx);
  66         zrl->zr_refcount = ZRL_DESTROYED;
  67         cv_destroy(&zrl->zr_cv);
  68 }
  69
  70 void
  71 #ifdef  ZFS_DEBUG
  72 zrl_add_debug(zrlock_t *zrl, const char *zc)
  73 #else
  74 zrl_add(zrlock_t *zrl)
  75 #endif
  76 {
  77         uint32_t n = (uint32_t)zrl->zr_refcount;
  78
  79         while (n != ZRL_LOCKED) {
  80                 uint32_t cas = atomic_cas_32(
  81                     (uint32_t *)&zrl->zr_refcount, n, n + 1);
  82                 if (cas == n) {
  83                         ASSERT((int32_t)n >= 0);
  84 #ifdef  ZFS_DEBUG
  85                         if (zrl->zr_owner == curthread) {
  86                                 DTRACE_PROBE2(zrlock__reentry,
  87                                     zrlock_t *, zrl, uint32_t, n);
  88                         }
  89                         zrl->zr_owner = curthread;
  90                         zrl->zr_caller = zc;
  91 #endif
  92                         return;
  93                 }
  94                 n = cas;
  95         }
  96
  97         mutex_enter(&zrl->zr_mtx);
  98         while (zrl->zr_refcount == ZRL_LOCKED) {
  99                 cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
 100         }
 101         ASSERT(zrl->zr_refcount >= 0);
 102         zrl->zr_refcount++;
 103 #ifdef  ZFS_DEBUG
 104         zrl->zr_owner = curthread;
 105         zrl->zr_caller = zc;
 106 #endif
 107         mutex_exit(&zrl->zr_mtx);
 108 }
 109
 110 void
 111 zrl_remove(zrlock_t *zrl)
 112 {
 113         uint32_t n;
 114
 115         n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
 116         ASSERT((int32_t)n >= 0);
 117 #ifdef  ZFS_DEBUG
 118         if (zrl->zr_owner == curthread) {
 119                 zrl->zr_owner = NULL;
 120                 zrl->zr_caller = NULL;
 121         }
 122 #endif
 123 }
 124
 125 int
 126 zrl_tryenter(zrlock_t *zrl)
 127 {
 128         uint32_t n = (uint32_t)zrl->zr_refcount;
 129
 130         if (n == 0) {
 131                 uint32_t cas = atomic_cas_32(
 132                     (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
 133                 if (cas == 0) {
 134 #ifdef  ZFS_DEBUG
 135                         ASSERT(zrl->zr_owner == NULL);
 136                         zrl->zr_owner = curthread;
 137 #endif
 138                         return (1);
 139                 }
 140         }
 141
 142         ASSERT((int32_t)n > ZRL_DESTROYED);
 143
 144         return (0);
 145 }
 146
 147 void
 148 zrl_exit(zrlock_t *zrl)
 149 {
 150         ASSERT(zrl->zr_refcount == ZRL_LOCKED);
 151
 152         mutex_enter(&zrl->zr_mtx);
 153 #ifdef  ZFS_DEBUG
 154         ASSERT(zrl->zr_owner == curthread);
 155         zrl->zr_owner = NULL;
 156         membar_producer();      /* make sure the owner store happens first */
 157 #endif
 158         zrl->zr_refcount = 0;
 159         cv_broadcast(&zrl->zr_cv);
 160         mutex_exit(&zrl->zr_mtx);
 161 }
 162
 163 int
 164 zrl_refcount(zrlock_t *zrl)
 165 {
 166         ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
 167
 168         int n = (int)zrl->zr_refcount;
 169         return (n <= 0 ? 0 : n);
 170 }
 171
 172 int
 173 zrl_is_zero(zrlock_t *zrl)
 174 {
 175         ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
 176
 177         return (zrl->zr_refcount <= 0);
 178 }
 179
 180 int
 181 zrl_is_locked(zrlock_t *zrl)
 182 {
 183         ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
 184
 185         return (zrl->zr_refcount == ZRL_LOCKED);
 186 }
 187
 188 #ifdef  ZFS_DEBUG
 189 kthread_t *
 190 zrl_owner(zrlock_t *zrl)
 191 {
 192         return (zrl->zr_owner);
 193 }
 194 #endif