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 by Delphix. All rights reserved. 24 */ 25 26 /* 27 * A Zero Reference Lock (ZRL) is a reference count that can lock out new 28 * references only when the count is zero and only without waiting if the count 29 * is not already zero. It is similar to a read-write lock in that it allows 30 * multiple readers and only a single writer, but it does not allow a writer to 31 * block while waiting for readers to exit, and therefore the question of 32 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of 33 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it 34 * is perfectly safe for the same reader to acquire the same lock multiple 35 * times. The fact that a ZRL is reentrant for readers (through multiple calls 36 * to zrl_add()) makes it convenient for determining whether something is 37 * actively referenced without the fuss of flagging lock ownership across 38 * function calls. 39 */ 40 #include <sys/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 -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 ASSERT0(zrl->zr_refcount); 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 ASSERT3S((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 ASSERT3S(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 #ifdef ZFS_DEBUG 117 if (zrl->zr_owner == curthread) { 118 zrl->zr_owner = NULL; 119 zrl->zr_caller = NULL; 120 } 121 #endif 122 n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount); 123 ASSERT3S((int32_t)n, >=, 0); 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 ASSERT3P(zrl->zr_owner, ==, NULL); 137 zrl->zr_owner = curthread; 138 #endif 139 return (1); 140 } 141 } 142 143 ASSERT3S((int32_t)n, >, ZRL_DESTROYED); 144 145 return (0); 146 } 147 148 void 149 zrl_exit(zrlock_t *zrl) 150 { 151 ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED); 152 153 mutex_enter(&zrl->zr_mtx); 154 #ifdef ZFS_DEBUG 155 ASSERT3P(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 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 168 169 int n = (int)zrl->zr_refcount; 170 return (n <= 0 ? 0 : n); 171 } 172 173 int 174 zrl_is_zero(zrlock_t *zrl) 175 { 176 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 177 178 return (zrl->zr_refcount <= 0); 179 } 180 181 int 182 zrl_is_locked(zrlock_t *zrl) 183 { 184 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 185 186 return (zrl->zr_refcount == ZRL_LOCKED); 187 } 188 189 #ifdef ZFS_DEBUG 190 kthread_t * 191 zrl_owner(zrlock_t *zrl) 192 { 193 return (zrl->zr_owner); 194 } 195 #endif 196