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 188