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  * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
  24  * Copyright 2016 The MathWorks, Inc. All rights reserved.
  25  */
  26
  27 /*
  28  * A Zero Reference Lock (ZRL) is a reference count that can lock out new
  29  * references only when the count is zero and only without waiting if the count
  30  * is not already zero. It is similar to a read-write lock in that it allows
  31  * multiple readers and only a single writer, but it does not allow a writer to
  32  * block while waiting for readers to exit, and therefore the question of
  33  * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
  34  * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
  35  * is perfectly safe for the same reader to acquire the same lock multiple
  36  * times. The fact that a ZRL is reentrant for readers (through multiple calls
  37  * to zrl_add()) makes it convenient for determining whether something is
  38  * actively referenced without the fuss of flagging lock ownership across
  39  * function calls.
  40  */
  41 #include <sys/zrlock.h>
  42
  43 /*
  44  * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
  45  * treated as zero references.
  46  */
  47 #define ZRL_LOCKED      -1
  48 #define ZRL_DESTROYED   -2
  49
  50 void
  51 zrl_init(zrlock_t *zrl)
  52 {
  53         mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
  54         zrl->zr_refcount = 0;
  55         cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
  56 #ifdef  ZFS_DEBUG
  57         zrl->zr_owner = NULL;
  58         zrl->zr_caller = NULL;
  59 #endif
  60 }
  61
  62 void
  63 zrl_destroy(zrlock_t *zrl)
  64 {
  65         ASSERT0(zrl->zr_refcount);
  66
  67         mutex_destroy(&zrl->zr_mtx);
  68         zrl->zr_refcount = ZRL_DESTROYED;
  69         cv_destroy(&zrl->zr_cv);
  70 }
  71
  72 void
  73 zrl_add_impl(zrlock_t *zrl, const char *zc)
  74 {
  75         for (;;) {
  76                 uint32_t n = (uint32_t)zrl->zr_refcount;
  77                 while (n != ZRL_LOCKED) {
  78                         uint32_t cas = atomic_cas_32(
  79                             (uint32_t *)&zrl->zr_refcount, n, n + 1);
  80                         if (cas == n) {
  81                                 ASSERT3S((int32_t)n, >=, 0);
  82 #ifdef  ZFS_DEBUG
  83                                 if (zrl->zr_owner == curthread) {
  84                                         DTRACE_PROBE2(zrlock__reentry,
  85                                             zrlock_t *, zrl, uint32_t, n);
  86                                 }
  87                                 zrl->zr_owner = curthread;
  88                                 zrl->zr_caller = zc;
  89 #endif
  90                                 return;
  91                         }
  92                         n = cas;
  93                 }
  94
  95                 mutex_enter(&zrl->zr_mtx);
  96                 while (zrl->zr_refcount == ZRL_LOCKED) {
  97                         cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
  98                 }
  99                 mutex_exit(&zrl->zr_mtx);
 100         }
 101 }
 102
 103 void
 104 zrl_remove(zrlock_t *zrl)
 105 {
 106         uint32_t n;
 107
 108 #ifdef  ZFS_DEBUG
 109         if (zrl->zr_owner == curthread) {
 110                 zrl->zr_owner = NULL;
 111                 zrl->zr_caller = NULL;
 112         }
 113 #endif
 114         n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
 115         ASSERT3S((int32_t)n, >=, 0);
 116 }
 117
 118 int
 119 zrl_tryenter(zrlock_t *zrl)
 120 {
 121         uint32_t n = (uint32_t)zrl->zr_refcount;
 122
 123         if (n == 0) {
 124                 uint32_t cas = atomic_cas_32(
 125                     (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
 126                 if (cas == 0) {
 127 #ifdef  ZFS_DEBUG
 128                         ASSERT3P(zrl->zr_owner, ==, NULL);
 129                         zrl->zr_owner = curthread;
 130 #endif
 131                         return (1);
 132                 }
 133         }
 134
 135         ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
 136
 137         return (0);
 138 }
 139
 140 void
 141 zrl_exit(zrlock_t *zrl)
 142 {
 143         ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
 144
 145         mutex_enter(&zrl->zr_mtx);
 146 #ifdef  ZFS_DEBUG
 147         ASSERT3P(zrl->zr_owner, ==, curthread);
 148         zrl->zr_owner = NULL;
 149         membar_producer();      /* make sure the owner store happens first */
 150 #endif
 151         zrl->zr_refcount = 0;
 152         cv_broadcast(&zrl->zr_cv);
 153         mutex_exit(&zrl->zr_mtx);
 154 }
 155
 156 int
 157 zrl_refcount(zrlock_t *zrl)
 158 {
 159         ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
 160
 161         int n = (int)zrl->zr_refcount;
 162         return (n <= 0 ? 0 : n);
 163 }
 164
 165 int
 166 zrl_is_zero(zrlock_t *zrl)
 167 {
 168         ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
 169
 170         return (zrl->zr_refcount <= 0);
 171 }
 172
 173 int
 174 zrl_is_locked(zrlock_t *zrl)
 175 {
 176         ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
 177
 178         return (zrl->zr_refcount == ZRL_LOCKED);
 179 }
 180
 181 #ifdef  ZFS_DEBUG
 182 kthread_t *
 183 zrl_owner(zrlock_t *zrl)
 184 {
 185         return (zrl->zr_owner);
 186 }
 187 #endif