module/zfs/rrwlock.c

   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 (zfs_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 (zfs_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         zfs_refcount_create(&rrl->rr_anon_rcount);
 147         zfs_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         zfs_refcount_destroy(&rrl->rr_anon_rcount);
 159         zfs_refcount_destroy(&rrl->rr_linked_rcount);
 160 }
 161
 162 static void
 163 rrw_enter_read_impl(rrwlock_t *rrl, boolean_t prio, 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(zfs_refcount_count(&rrl->rr_anon_rcount) >= 0);
 177
 178         while (rrl->rr_writer != NULL || (rrl->rr_writer_wanted &&
 179             zfs_refcount_is_zero(&rrl->rr_anon_rcount) && !prio &&
 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) zfs_refcount_add(&rrl->rr_linked_rcount, tag);
 187         } else {
 188                 (void) zfs_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_read(rrwlock_t *rrl, void *tag)
 196 {
 197         rrw_enter_read_impl(rrl, B_FALSE, tag);
 198 }
 199
 200 /*
 201  * take a read lock even if there are pending write lock requests. if we want
 202  * to take a lock reentrantly, but from different threads (that have a
 203  * relationship to each other), the normal detection mechanism to overrule
 204  * the pending writer does not work, so we have to give an explicit hint here.
 205  */
 206 void
 207 rrw_enter_read_prio(rrwlock_t *rrl, void *tag)
 208 {
 209         rrw_enter_read_impl(rrl, B_TRUE, tag);
 210 }
 211
 212
 213 void
 214 rrw_enter_write(rrwlock_t *rrl)
 215 {
 216         mutex_enter(&rrl->rr_lock);
 217         ASSERT(rrl->rr_writer != curthread);
 218
 219         while (zfs_refcount_count(&rrl->rr_anon_rcount) > 0 ||
 220             zfs_refcount_count(&rrl->rr_linked_rcount) > 0 ||
 221             rrl->rr_writer != NULL) {
 222                 rrl->rr_writer_wanted = B_TRUE;
 223                 cv_wait(&rrl->rr_cv, &rrl->rr_lock);
 224         }
 225         rrl->rr_writer_wanted = B_FALSE;
 226         rrl->rr_writer = curthread;
 227         mutex_exit(&rrl->rr_lock);
 228 }
 229
 230 void
 231 rrw_enter(rrwlock_t *rrl, krw_t rw, void *tag)
 232 {
 233         if (rw == RW_READER)
 234                 rrw_enter_read(rrl, tag);
 235         else
 236                 rrw_enter_write(rrl);
 237 }
 238
 239 void
 240 rrw_exit(rrwlock_t *rrl, void *tag)
 241 {
 242         mutex_enter(&rrl->rr_lock);
 243 #if !defined(DEBUG) && defined(_KERNEL)
 244         if (!rrl->rr_writer && rrl->rr_linked_rcount.rc_count == 0) {
 245                 rrl->rr_anon_rcount.rc_count--;
 246                 if (rrl->rr_anon_rcount.rc_count == 0)
 247                         cv_broadcast(&rrl->rr_cv);
 248                 mutex_exit(&rrl->rr_lock);
 249                 return;
 250         }
 251         DTRACE_PROBE(zfs__rrwfastpath__exitmiss);
 252 #endif
 253         ASSERT(!zfs_refcount_is_zero(&rrl->rr_anon_rcount) ||
 254             !zfs_refcount_is_zero(&rrl->rr_linked_rcount) ||
 255             rrl->rr_writer != NULL);
 256
 257         if (rrl->rr_writer == NULL) {
 258                 int64_t count;
 259                 if (rrn_find_and_remove(rrl, tag)) {
 260                         count = zfs_refcount_remove(
 261                             &rrl->rr_linked_rcount, tag);
 262                 } else {
 263                         ASSERT(!rrl->rr_track_all);
 264                         count = zfs_refcount_remove(&rrl->rr_anon_rcount, tag);
 265                 }
 266                 if (count == 0)
 267                         cv_broadcast(&rrl->rr_cv);
 268         } else {
 269                 ASSERT(rrl->rr_writer == curthread);
 270                 ASSERT(zfs_refcount_is_zero(&rrl->rr_anon_rcount) &&
 271                     zfs_refcount_is_zero(&rrl->rr_linked_rcount));
 272                 rrl->rr_writer = NULL;
 273                 cv_broadcast(&rrl->rr_cv);
 274         }
 275         mutex_exit(&rrl->rr_lock);
 276 }
 277
 278 /*
 279  * If the lock was created with track_all, rrw_held(RW_READER) will return
 280  * B_TRUE iff the current thread has the lock for reader.  Otherwise it may
 281  * return B_TRUE if any thread has the lock for reader.
 282  */
 283 boolean_t
 284 rrw_held(rrwlock_t *rrl, krw_t rw)
 285 {
 286         boolean_t held;
 287
 288         mutex_enter(&rrl->rr_lock);
 289         if (rw == RW_WRITER) {
 290                 held = (rrl->rr_writer == curthread);
 291         } else {
 292                 held = (!zfs_refcount_is_zero(&rrl->rr_anon_rcount) ||
 293                     rrn_find(rrl) != NULL);
 294         }
 295         mutex_exit(&rrl->rr_lock);
 296
 297         return (held);
 298 }
 299
 300 void
 301 rrw_tsd_destroy(void *arg)
 302 {
 303         rrw_node_t *rn = arg;
 304         if (rn != NULL) {
 305                 panic("thread %p terminating with rrw lock %p held",
 306                     (void *)curthread, (void *)rn->rn_rrl);
 307         }
 308 }
 309
 310 /*
 311  * A reader-mostly lock implementation, tuning above reader-writer locks
 312  * for hightly parallel read acquisitions, while pessimizing writes.
 313  *
 314  * The idea is to split single busy lock into array of locks, so that
 315  * each reader can lock only one of them for read, depending on result
 316  * of simple hash function.  That proportionally reduces lock congestion.
 317  * Writer at the same time has to sequentially acquire write on all the locks.
 318  * That makes write acquisition proportionally slower, but in places where
 319  * it is used (filesystem unmount) performance is not critical.
 320  *
 321  * All the functions below are direct wrappers around functions above.
 322  */
 323 void
 324 rrm_init(rrmlock_t *rrl, boolean_t track_all)
 325 {
 326         int i;
 327
 328         for (i = 0; i < RRM_NUM_LOCKS; i++)
 329                 rrw_init(&rrl->locks[i], track_all);
 330 }
 331
 332 void
 333 rrm_destroy(rrmlock_t *rrl)
 334 {
 335         int i;
 336
 337         for (i = 0; i < RRM_NUM_LOCKS; i++)
 338                 rrw_destroy(&rrl->locks[i]);
 339 }
 340
 341 void
 342 rrm_enter(rrmlock_t *rrl, krw_t rw, void *tag)
 343 {
 344         if (rw == RW_READER)
 345                 rrm_enter_read(rrl, tag);
 346         else
 347                 rrm_enter_write(rrl);
 348 }
 349
 350 /*
 351  * This maps the current thread to a specific lock.  Note that the lock
 352  * must be released by the same thread that acquired it.  We do this
 353  * mapping by taking the thread pointer mod a prime number.  We examine
 354  * only the low 32 bits of the thread pointer, because 32-bit division
 355  * is faster than 64-bit division, and the high 32 bits have little
 356  * entropy anyway.
 357  */
 358 #define RRM_TD_LOCK()   (((uint32_t)(uintptr_t)(curthread)) % RRM_NUM_LOCKS)
 359
 360 void
 361 rrm_enter_read(rrmlock_t *rrl, void *tag)
 362 {
 363         rrw_enter_read(&rrl->locks[RRM_TD_LOCK()], tag);
 364 }
 365
 366 void
 367 rrm_enter_write(rrmlock_t *rrl)
 368 {
 369         int i;
 370
 371         for (i = 0; i < RRM_NUM_LOCKS; i++)
 372                 rrw_enter_write(&rrl->locks[i]);
 373 }
 374
 375 void
 376 rrm_exit(rrmlock_t *rrl, void *tag)
 377 {
 378         int i;
 379
 380         if (rrl->locks[0].rr_writer == curthread) {
 381                 for (i = 0; i < RRM_NUM_LOCKS; i++)
 382                         rrw_exit(&rrl->locks[i], tag);
 383         } else {
 384                 rrw_exit(&rrl->locks[RRM_TD_LOCK()], tag);
 385         }
 386 }
 387
 388 boolean_t
 389 rrm_held(rrmlock_t *rrl, krw_t rw)
 390 {
 391         if (rw == RW_WRITER) {
 392                 return (rrw_held(&rrl->locks[0], rw));
 393         } else {
 394                 return (rrw_held(&rrl->locks[RRM_TD_LOCK()], rw));
 395         }
 396 }