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 #include <sys/trace_zfs.h> 43 44 /* 45 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is 46 * treated as zero references. 47 */ 48 #define ZRL_LOCKED -1 49 #define ZRL_DESTROYED -2 50 51 void 52 zrl_init(zrlock_t *zrl) 53 { 54 mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL); 55 zrl->zr_refcount = 0; 56 cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL); 57 #ifdef ZFS_DEBUG 58 zrl->zr_owner = NULL; 59 zrl->zr_caller = NULL; 60 #endif 61 } 62 63 void 64 zrl_destroy(zrlock_t *zrl) 65 { 66 ASSERT0(zrl->zr_refcount); 67 68 mutex_destroy(&zrl->zr_mtx); 69 zrl->zr_refcount = ZRL_DESTROYED; 70 cv_destroy(&zrl->zr_cv); 71 } 72 73 void 74 zrl_add_impl(zrlock_t *zrl, const char *zc) 75 { 76 for (;;) { 77 uint32_t n = (uint32_t)zrl->zr_refcount; 78 while (n != ZRL_LOCKED) { 79 uint32_t cas = atomic_cas_32( 80 (uint32_t *)&zrl->zr_refcount, n, n + 1); 81 if (cas == n) { 82 ASSERT3S((int32_t)n, >=, 0); 83 #ifdef ZFS_DEBUG 84 if (zrl->zr_owner == curthread) { 85 DTRACE_PROBE3(zrlock__reentry, 86 zrlock_t *, zrl, 87 kthread_t *, curthread, 88 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 mutex_exit(&zrl->zr_mtx); 103 } 104 } 105 106 void 107 zrl_remove(zrlock_t *zrl) 108 { 109 uint32_t n; 110 111 #ifdef ZFS_DEBUG 112 if (zrl->zr_owner == curthread) { 113 zrl->zr_owner = NULL; 114 zrl->zr_caller = NULL; 115 } 116 #endif 117 n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount); 118 ASSERT3S((int32_t)n, >=, 0); 119 } 120 121 int 122 zrl_tryenter(zrlock_t *zrl) 123 { 124 uint32_t n = (uint32_t)zrl->zr_refcount; 125 126 if (n == 0) { 127 uint32_t cas = atomic_cas_32( 128 (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED); 129 if (cas == 0) { 130 #ifdef ZFS_DEBUG 131 ASSERT3P(zrl->zr_owner, ==, NULL); 132 zrl->zr_owner = curthread; 133 #endif 134 return (1); 135 } 136 } 137 138 ASSERT3S((int32_t)n, >, ZRL_DESTROYED); 139 140 return (0); 141 } 142 143 void 144 zrl_exit(zrlock_t *zrl) 145 { 146 ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED); 147 148 mutex_enter(&zrl->zr_mtx); 149 #ifdef ZFS_DEBUG 150 ASSERT3P(zrl->zr_owner, ==, curthread); 151 zrl->zr_owner = NULL; 152 membar_producer(); /* make sure the owner store happens first */ 153 #endif 154 zrl->zr_refcount = 0; 155 cv_broadcast(&zrl->zr_cv); 156 mutex_exit(&zrl->zr_mtx); 157 } 158 159 int 160 zrl_is_zero(zrlock_t *zrl) 161 { 162 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 163 164 return (zrl->zr_refcount <= 0); 165 } 166 167 int 168 zrl_is_locked(zrlock_t *zrl) 169 { 170 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 171 172 return (zrl->zr_refcount == ZRL_LOCKED); 173 } 174 175 #ifdef ZFS_DEBUG 176 kthread_t * 177 zrl_owner(zrlock_t *zrl) 178 { 179 return (zrl->zr_owner); 180 } 181 #endif 182 183 #if defined(_KERNEL) 184 185 EXPORT_SYMBOL(zrl_add_impl); 186 EXPORT_SYMBOL(zrl_remove); 187 188 #endif 189