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