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 2009 Sun Microsystems, Inc. All rights reserved. 23 * Use is subject to license terms. 24 */ 25 /* 26 * Copyright (c) 2012 by Delphix. All rights reserved. 27 */ 28 29 #include <sys/refcount.h> 30 #include <sys/rrwlock.h> 31 32 /* 33 * This file contains the implementation of a re-entrant read 34 * reader/writer lock (aka "rrwlock"). 35 * 36 * This is a normal reader/writer lock with the additional feature 37 * of allowing threads who have already obtained a read lock to 38 * re-enter another read lock (re-entrant read) - even if there are 39 * waiting writers. 40 * 41 * Callers who have not obtained a read lock give waiting writers priority. 42 * 43 * The rrwlock_t lock does not allow re-entrant writers, nor does it 44 * allow a re-entrant mix of reads and writes (that is, it does not 45 * allow a caller who has already obtained a read lock to be able to 46 * then grab a write lock without first dropping all read locks, and 47 * vice versa). 48 * 49 * The rrwlock_t uses tsd (thread specific data) to keep a list of 50 * nodes (rrw_node_t), where each node keeps track of which specific 51 * lock (rrw_node_t::rn_rrl) the thread has grabbed. Since re-entering 52 * should be rare, a thread that grabs multiple reads on the same rrwlock_t 53 * will store multiple rrw_node_ts of the same 'rrn_rrl'. Nodes on the 54 * tsd list can represent a different rrwlock_t. This allows a thread 55 * to enter multiple and unique rrwlock_ts for read locks at the same time. 56 * 57 * Since using tsd exposes some overhead, the rrwlock_t only needs to 58 * keep tsd data when writers are waiting. If no writers are waiting, then 59 * a reader just bumps the anonymous read count (rr_anon_rcount) - no tsd 60 * is needed. Once a writer attempts to grab the lock, readers then 61 * keep tsd data and bump the linked readers count (rr_linked_rcount). 62 * 63 * If there are waiting writers and there are anonymous readers, then a 64 * reader doesn't know if it is a re-entrant lock. But since it may be one, 65 * we allow the read to proceed (otherwise it could deadlock). Since once 66 * waiting writers are active, readers no longer bump the anonymous count, 67 * the anonymous readers will eventually flush themselves out. At this point, 68 * readers will be able to tell if they are a re-entrant lock (have a 69 * rrw_node_t entry for the lock) or not. If they are a re-entrant lock, then 70 * we must let the proceed. If they are not, then the reader blocks for the 71 * waiting writers. Hence, we do not starve writers. 72 */ 73 74 /* global key for TSD */ 75 uint_t rrw_tsd_key; 76 77 typedef struct rrw_node { 78 struct rrw_node *rn_next; 79 rrwlock_t *rn_rrl; 80 void *rn_tag; 81 } rrw_node_t; 82 83 static rrw_node_t * 84 rrn_find(rrwlock_t *rrl) 85 { 86 rrw_node_t *rn; 87 88 if (refcount_count(&rrl->rr_linked_rcount) == 0) 89 return (NULL); 90 91 for (rn = tsd_get(rrw_tsd_key); rn != NULL; rn = rn->rn_next) { 92 if (rn->rn_rrl == rrl) 93 return (rn); 94 } 95 return (NULL); 96 } 97 98 /* 99 * Add a node to the head of the singly linked list. 100 */ 101 static void 102 rrn_add(rrwlock_t *rrl, void *tag) 103 { 104 rrw_node_t *rn; 105 106 rn = kmem_alloc(sizeof (*rn), KM_SLEEP); 107 rn->rn_rrl = rrl; 108 rn->rn_next = tsd_get(rrw_tsd_key); 109 rn->rn_tag = tag; 110 VERIFY(tsd_set(rrw_tsd_key, rn) == 0); 111 } 112 113 /* 114 * If a node is found for 'rrl', then remove the node from this 115 * thread's list and return TRUE; otherwise return FALSE. 116 */ 117 static boolean_t 118 rrn_find_and_remove(rrwlock_t *rrl, void *tag) 119 { 120 rrw_node_t *rn; 121 rrw_node_t *prev = NULL; 122 123 if (refcount_count(&rrl->rr_linked_rcount) == 0) 124 return (B_FALSE); 125 126 for (rn = tsd_get(rrw_tsd_key); rn != NULL; rn = rn->rn_next) { 127 if (rn->rn_rrl == rrl && rn->rn_tag == tag) { 128 if (prev) 129 prev->rn_next = rn->rn_next; 130 else 131 VERIFY(tsd_set(rrw_tsd_key, rn->rn_next) == 0); 132 kmem_free(rn, sizeof (*rn)); 133 return (B_TRUE); 134 } 135 prev = rn; 136 } 137 return (B_FALSE); 138 } 139 140 void 141 rrw_init(rrwlock_t *rrl, boolean_t track_all) 142 { 143 mutex_init(&rrl->rr_lock, NULL, MUTEX_DEFAULT, NULL); 144 cv_init(&rrl->rr_cv, NULL, CV_DEFAULT, NULL); 145 rrl->rr_writer = NULL; 146 refcount_create(&rrl->rr_anon_rcount); 147 refcount_create(&rrl->rr_linked_rcount); 148 rrl->rr_writer_wanted = B_FALSE; 149 rrl->rr_track_all = track_all; 150 } 151 152 void 153 rrw_destroy(rrwlock_t *rrl) 154 { 155 mutex_destroy(&rrl->rr_lock); 156 cv_destroy(&rrl->rr_cv); 157 ASSERT(rrl->rr_writer == NULL); 158 refcount_destroy(&rrl->rr_anon_rcount); 159 refcount_destroy(&rrl->rr_linked_rcount); 160 } 161 162 void 163 rrw_enter_read(rrwlock_t *rrl, void *tag) 164 { 165 mutex_enter(&rrl->rr_lock); 166 #if !defined(DEBUG) && defined(_KERNEL) 167 if (rrl->rr_writer == NULL && !rrl->rr_writer_wanted && 168 !rrl->rr_track_all) { 169 rrl->rr_anon_rcount.rc_count++; 170 mutex_exit(&rrl->rr_lock); 171 return; 172 } 173 DTRACE_PROBE(zfs__rrwfastpath__rdmiss); 174 #endif 175 ASSERT(rrl->rr_writer != curthread); 176 ASSERT(refcount_count(&rrl->rr_anon_rcount) >= 0); 177 178 while (rrl->rr_writer != NULL || (rrl->rr_writer_wanted && 179 refcount_is_zero(&rrl->rr_anon_rcount) && 180 rrn_find(rrl) == NULL)) 181 cv_wait(&rrl->rr_cv, &rrl->rr_lock); 182 183 if (rrl->rr_writer_wanted || rrl->rr_track_all) { 184 /* may or may not be a re-entrant enter */ 185 rrn_add(rrl, tag); 186 (void) refcount_add(&rrl->rr_linked_rcount, tag); 187 } else { 188 (void) refcount_add(&rrl->rr_anon_rcount, tag); 189 } 190 ASSERT(rrl->rr_writer == NULL); 191 mutex_exit(&rrl->rr_lock); 192 } 193 194 void 195 rrw_enter_write(rrwlock_t *rrl) 196 { 197 mutex_enter(&rrl->rr_lock); 198 ASSERT(rrl->rr_writer != curthread); 199 200 while (refcount_count(&rrl->rr_anon_rcount) > 0 || 201 refcount_count(&rrl->rr_linked_rcount) > 0 || 202 rrl->rr_writer != NULL) { 203 rrl->rr_writer_wanted = B_TRUE; 204 cv_wait(&rrl->rr_cv, &rrl->rr_lock); 205 } 206 rrl->rr_writer_wanted = B_FALSE; 207 rrl->rr_writer = curthread; 208 mutex_exit(&rrl->rr_lock); 209 } 210 211 void 212 rrw_enter(rrwlock_t *rrl, krw_t rw, void *tag) 213 { 214 if (rw == RW_READER) 215 rrw_enter_read(rrl, tag); 216 else 217 rrw_enter_write(rrl); 218 } 219 220 void 221 rrw_exit(rrwlock_t *rrl, void *tag) 222 { 223 mutex_enter(&rrl->rr_lock); 224 #if !defined(DEBUG) && defined(_KERNEL) 225 if (!rrl->rr_writer && rrl->rr_linked_rcount.rc_count == 0) { 226 rrl->rr_anon_rcount.rc_count--; 227 if (rrl->rr_anon_rcount.rc_count == 0) 228 cv_broadcast(&rrl->rr_cv); 229 mutex_exit(&rrl->rr_lock); 230 return; 231 } 232 DTRACE_PROBE(zfs__rrwfastpath__exitmiss); 233 #endif 234 ASSERT(!refcount_is_zero(&rrl->rr_anon_rcount) || 235 !refcount_is_zero(&rrl->rr_linked_rcount) || 236 rrl->rr_writer != NULL); 237 238 if (rrl->rr_writer == NULL) { 239 int64_t count; 240 if (rrn_find_and_remove(rrl, tag)) { 241 count = refcount_remove(&rrl->rr_linked_rcount, tag); 242 } else { 243 ASSERT(!rrl->rr_track_all); 244 count = refcount_remove(&rrl->rr_anon_rcount, tag); 245 } 246 if (count == 0) 247 cv_broadcast(&rrl->rr_cv); 248 } else { 249 ASSERT(rrl->rr_writer == curthread); 250 ASSERT(refcount_is_zero(&rrl->rr_anon_rcount) && 251 refcount_is_zero(&rrl->rr_linked_rcount)); 252 rrl->rr_writer = NULL; 253 cv_broadcast(&rrl->rr_cv); 254 } 255 mutex_exit(&rrl->rr_lock); 256 } 257 258 /* 259 * If the lock was created with track_all, rrw_held(RW_READER) will return 260 * B_TRUE iff the current thread has the lock for reader. Otherwise it may 261 * return B_TRUE if any thread has the lock for reader. 262 */ 263 boolean_t 264 rrw_held(rrwlock_t *rrl, krw_t rw) 265 { 266 boolean_t held; 267 268 mutex_enter(&rrl->rr_lock); 269 if (rw == RW_WRITER) { 270 held = (rrl->rr_writer == curthread); 271 } else { 272 held = (!refcount_is_zero(&rrl->rr_anon_rcount) || 273 rrn_find(rrl) != NULL); 274 } 275 mutex_exit(&rrl->rr_lock); 276 277 return (held); 278 } 279 280 void 281 rrw_tsd_destroy(void *arg) 282 { 283 rrw_node_t *rn = arg; 284 if (rn != NULL) { 285 panic("thread %p terminating with rrw lock %p held", 286 (void *)curthread, (void *)rn->rn_rrl); 287 } 288 } 289