1 // SPDX-License-Identifier: CDDL-1.0 2 /* 3 * CDDL HEADER START 4 * 5 * The contents of this file are subject to the terms of the 6 * Common Development and Distribution License (the "License"). 7 * You may not use this file except in compliance with the License. 8 * 9 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE 10 * or https://opensource.org/licenses/CDDL-1.0. 11 * See the License for the specific language governing permissions 12 * and limitations under the License. 13 * 14 * When distributing Covered Code, include this CDDL HEADER in each 15 * file and include the License file at usr/src/OPENSOLARIS.LICENSE. 16 * If applicable, add the following below this CDDL HEADER, with the 17 * fields enclosed by brackets "[]" replaced with your own identifying 18 * information: Portions Copyright [yyyy] [name of copyright owner] 19 * 20 * CDDL HEADER END 21 */ 22 /* 23 * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved. 24 * Copyright (c) 2014, 2015 by Delphix. All rights reserved. 25 * Copyright 2016 The MathWorks, Inc. All rights reserved. 26 */ 27 28 /* 29 * A Zero Reference Lock (ZRL) is a reference count that can lock out new 30 * references only when the count is zero and only without waiting if the count 31 * is not already zero. It is similar to a read-write lock in that it allows 32 * multiple readers and only a single writer, but it does not allow a writer to 33 * block while waiting for readers to exit, and therefore the question of 34 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of 35 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it 36 * is perfectly safe for the same reader to acquire the same lock multiple 37 * times. The fact that a ZRL is reentrant for readers (through multiple calls 38 * to zrl_add()) makes it convenient for determining whether something is 39 * actively referenced without the fuss of flagging lock ownership across 40 * function calls. 41 */ 42 #include <sys/zrlock.h> 43 #include <sys/trace_zfs.h> 44 45 /* 46 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is 47 * treated as zero references. 48 */ 49 #define ZRL_LOCKED -1 50 #define ZRL_DESTROYED -2 51 52 void 53 zrl_init(zrlock_t *zrl) 54 { 55 mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL); 56 zrl->zr_refcount = 0; 57 cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL); 58 #ifdef ZFS_DEBUG 59 zrl->zr_owner = NULL; 60 zrl->zr_caller = NULL; 61 #endif 62 } 63 64 void 65 zrl_destroy(zrlock_t *zrl) 66 { 67 ASSERT0(zrl->zr_refcount); 68 69 mutex_destroy(&zrl->zr_mtx); 70 zrl->zr_refcount = ZRL_DESTROYED; 71 cv_destroy(&zrl->zr_cv); 72 } 73 74 void 75 zrl_add_impl(zrlock_t *zrl, const char *zc) 76 { 77 for (;;) { 78 uint32_t n = (uint32_t)zrl->zr_refcount; 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 ASSERT3S((int32_t)n, >=, 0); 84 #ifdef ZFS_DEBUG 85 if (zrl->zr_owner == curthread) { 86 DTRACE_PROBE3(zrlock__reentry, 87 zrlock_t *, zrl, 88 kthread_t *, curthread, 89 uint32_t, n); 90 } 91 zrl->zr_owner = curthread; 92 zrl->zr_caller = zc; 93 #endif 94 return; 95 } 96 n = cas; 97 } 98 99 mutex_enter(&zrl->zr_mtx); 100 while (zrl->zr_refcount == ZRL_LOCKED) { 101 cv_wait(&zrl->zr_cv, &zrl->zr_mtx); 102 } 103 mutex_exit(&zrl->zr_mtx); 104 } 105 } 106 107 void 108 zrl_remove(zrlock_t *zrl) 109 { 110 #ifdef ZFS_DEBUG 111 if (zrl->zr_owner == curthread) { 112 zrl->zr_owner = NULL; 113 zrl->zr_caller = NULL; 114 } 115 int32_t n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount); 116 ASSERT3S(n, >=, 0); 117 #else 118 atomic_dec_32((uint32_t *)&zrl->zr_refcount); 119 #endif 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 ASSERT0P(zrl->zr_owner); 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_is_zero(zrlock_t *zrl) 162 { 163 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 164 165 return (zrl->zr_refcount <= 0); 166 } 167 168 int 169 zrl_is_locked(zrlock_t *zrl) 170 { 171 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED); 172 173 return (zrl->zr_refcount == ZRL_LOCKED); 174 } 175 176 #ifdef ZFS_DEBUG 177 kthread_t * 178 zrl_owner(zrlock_t *zrl) 179 { 180 return (zrl->zr_owner); 181 } 182 #endif 183 184 #if defined(_KERNEL) 185 186 EXPORT_SYMBOL(zrl_add_impl); 187 EXPORT_SYMBOL(zrl_remove); 188 189 #endif 190