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 */ 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 zrl_add_impl(zrlock_t *zrl, const char *zc) 73 { 74 uint32_t n = (uint32_t)zrl->zr_refcount; 75 76 while (n != ZRL_LOCKED) { 77 uint32_t cas = atomic_cas_32( 78 (uint32_t *)&zrl->zr_refcount, n, n + 1); 79 if (cas == n) { 80 ASSERT3S((int32_t)n, >=, 0); 81 #ifdef ZFS_DEBUG 82 if (zrl->zr_owner == curthread) { 83 DTRACE_PROBE2(zrlock__reentry, 84 zrlock_t *, zrl, uint32_t, n); 85 } 86 zrl->zr_owner = curthread; 87 zrl->zr_caller = zc; 88 #endif 89 return; 90 } 91 n = cas; 92 } 93 94 mutex_enter(&zrl->zr_mtx); 95 while (zrl->zr_refcount == ZRL_LOCKED) { 96 cv_wait(&zrl->zr_cv, &zrl->zr_mtx); 97 } 98 ASSERT3S(zrl->zr_refcount, >=, 0); 99 zrl->zr_refcount++; 100 #ifdef ZFS_DEBUG 101 zrl->zr_owner = curthread; 102 zrl->zr_caller = zc; 103 #endif 104 mutex_exit(&zrl->zr_mtx); 105 } 106 107 void 108 zrl_remove(zrlock_t *zrl) 109 { 110 uint32_t n; 111 112 #ifdef ZFS_DEBUG 113 if (zrl->zr_owner == curthread) { 114 zrl->zr_owner = NULL; 115 zrl->zr_caller = NULL; 116 } 117 #endif 118 n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount); 119 ASSERT3S((int32_t)n, >=, 0); 120 } 121 122 int 123 zrl_tryenter(zrlock_t *zrl) 124 { 125 uint32_t n = (uint32_t)zrl->zr_refcount; 126 127 if (n == 0) { 128 uint32_t cas = atomic_cas_32( 129 (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED); 130 if (cas == 0) { 131 #ifdef ZFS_DEBUG 132 ASSERT3P(zrl->zr_owner, ==, NULL); 133 zrl->zr_owner = curthread; 134 #endif 135 return (1); 136 } 137 } 138 139 ASSERT3S((int32_t)n, >, ZRL_DESTROYED); 140 141 return (0); 142 } 143 144 void 145 zrl_exit(zrlock_t *zrl) 146 { 147 ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED); 148 149 mutex_enter(&zrl->zr_mtx); 150 #ifdef ZFS_DEBUG 151 ASSERT3P(zrl->zr_owner, ==, curthread); 152 zrl->zr_owner = NULL; 153 membar_producer(); /* make sure the owner store happens first */ 154 #endif 155 zrl->zr_refcount = 0; 156 cv_broadcast(&zrl->zr_cv); 157 mutex_exit(&zrl->zr_mtx); 158 } 159 160 int 161 zrl_refcount(zrlock_t *zrl) 162 { 163 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 164 165 int n = (int)zrl->zr_refcount; 166 return (n <= 0 ? 0 : n); 167 } 168 169 int 170 zrl_is_zero(zrlock_t *zrl) 171 { 172 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 173 174 return (zrl->zr_refcount <= 0); 175 } 176 177 int 178 zrl_is_locked(zrlock_t *zrl) 179 { 180 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 181 182 return (zrl->zr_refcount == ZRL_LOCKED); 183 } 184 185 #ifdef ZFS_DEBUG 186 kthread_t * 187 zrl_owner(zrlock_t *zrl) 188 { 189 return (zrl->zr_owner); 190 } 191 #endif 192