module/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 #include <sys/trace_zrlock.h>
  41
  42 /*
  43  * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
  44  * treated as zero references.
  45  */
  46 #define ZRL_LOCKED      ((uint32_t)-1)
  47 #define ZRL_DESTROYED   -2
  48
  49 void
  50 zrl_init(zrlock_t *zrl)
  51 {
  52         mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
  53         zrl->zr_refcount = 0;
  54         cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
  55 #ifdef  ZFS_DEBUG
  56         zrl->zr_owner = NULL;
  57         zrl->zr_caller = NULL;
  58 #endif
  59 }
  60
  61 void
  62 zrl_destroy(zrlock_t *zrl)
  63 {
  64         ASSERT(zrl->zr_refcount == 0);
  65
  66         mutex_destroy(&zrl->zr_mtx);
  67         zrl->zr_refcount = ZRL_DESTROYED;
  68         cv_destroy(&zrl->zr_cv);
  69 }
  70
  71 void
  72 #ifdef  ZFS_DEBUG
  73 zrl_add_debug(zrlock_t *zrl, const char *zc)
  74 #else
  75 zrl_add(zrlock_t *zrl)
  76 #endif
  77 {
  78         uint32_t n = (uint32_t)zrl->zr_refcount;
  79
  80         while (n != ZRL_LOCKED) {
  81                 uint32_t cas = atomic_cas_32(
  82                     (uint32_t *)&zrl->zr_refcount, n, n + 1);
  83                 if (cas == n) {
  84                         ASSERT((int32_t)n >= 0);
  85 #ifdef  ZFS_DEBUG
  86                         if (zrl->zr_owner == curthread) {
  87                                 DTRACE_PROBE2(zrlock__reentry,
  88                                     zrlock_t *, zrl, uint32_t, n);
  89                         }
  90                         zrl->zr_owner = curthread;
  91                         zrl->zr_caller = zc;
  92 #endif
  93                         return;
  94                 }
  95                 n = cas;
  96         }
  97
  98         mutex_enter(&zrl->zr_mtx);
  99         while (zrl->zr_refcount == ZRL_LOCKED) {
 100                 cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
 101         }
 102         ASSERT(zrl->zr_refcount >= 0);
 103         zrl->zr_refcount++;
 104 #ifdef  ZFS_DEBUG
 105         zrl->zr_owner = curthread;
 106         zrl->zr_caller = zc;
 107 #endif
 108         mutex_exit(&zrl->zr_mtx);
 109 }
 110
 111 void
 112 zrl_remove(zrlock_t *zrl)
 113 {
 114         uint32_t n;
 115
 116         n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
 117         ASSERT((int32_t)n >= 0);
 118 #ifdef  ZFS_DEBUG
 119         if (zrl->zr_owner == curthread) {
 120                 zrl->zr_owner = NULL;
 121                 zrl->zr_caller = NULL;
 122         }
 123 #endif
 124 }
 125
 126 int
 127 zrl_tryenter(zrlock_t *zrl)
 128 {
 129         uint32_t n = (uint32_t)zrl->zr_refcount;
 130
 131         if (n == 0) {
 132                 uint32_t cas = atomic_cas_32(
 133                     (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
 134                 if (cas == 0) {
 135 #ifdef  ZFS_DEBUG
 136                         ASSERT(zrl->zr_owner == NULL);
 137                         zrl->zr_owner = curthread;
 138 #endif
 139                         return (1);
 140                 }
 141         }
 142
 143         ASSERT((int32_t)n > ZRL_DESTROYED);
 144
 145         return (0);
 146 }
 147
 148 void
 149 zrl_exit(zrlock_t *zrl)
 150 {
 151         ASSERT(zrl->zr_refcount == ZRL_LOCKED);
 152
 153         mutex_enter(&zrl->zr_mtx);
 154 #ifdef  ZFS_DEBUG
 155         ASSERT(zrl->zr_owner == curthread);
 156         zrl->zr_owner = NULL;
 157         membar_producer();      /* make sure the owner store happens first */
 158 #endif
 159         zrl->zr_refcount = 0;
 160         cv_broadcast(&zrl->zr_cv);
 161         mutex_exit(&zrl->zr_mtx);
 162 }
 163
 164 int
 165 zrl_refcount(zrlock_t *zrl)
 166 {
 167         int n;
 168
 169         ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
 170
 171         n = (int)zrl->zr_refcount;
 172         return (n <= 0 ? 0 : n);
 173 }
 174
 175 int
 176 zrl_is_zero(zrlock_t *zrl)
 177 {
 178         ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
 179
 180         return (zrl->zr_refcount <= 0);
 181 }
 182
 183 int
 184 zrl_is_locked(zrlock_t *zrl)
 185 {
 186         ASSERT(zrl->zr_refcount > ZRL_DESTROYED);
 187
 188         return (zrl->zr_refcount == ZRL_LOCKED);
 189 }
 190
 191 #ifdef  ZFS_DEBUG
 192 kthread_t *
 193 zrl_owner(zrlock_t *zrl)
 194 {
 195         return (zrl->zr_owner);
 196 }
 197 #endif
 198
 199 #if defined(_KERNEL) && defined(HAVE_SPL)
 200
 201 #ifdef ZFS_DEBUG
 202 EXPORT_SYMBOL(zrl_add_debug);
 203 #else
 204 EXPORT_SYMBOL(zrl_add);
 205 #endif
 206 EXPORT_SYMBOL(zrl_remove);
 207
 208 #endif