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 
 290 /*
 291  * A reader-mostly lock implementation, tuning above reader-writer locks
 292  * for hightly parallel read acquisitions, while pessimizing writes.
 293  *
 294  * The idea is to split single busy lock into array of locks, so that
 295  * each reader can lock only one of them for read, depending on result
 296  * of simple hash function.  That proportionally reduces lock congestion.
 297  * Writer same time has to sequentially aquire write on all the locks.
 298  * That makes write aquisition proportionally slower, but in places where
 299  * it is used (filesystem unmount) performance is not critical.
 300  *
 301  * All the functions below are direct wrappers around functions above.
 302  */
 303 void
 304 rrm_init(rrmlock_t *rrl, boolean_t track_all)
 305 {
 306         int i;
 307 
 308         for (i = 0; i < RRM_NUM_LOCKS; i++)
 309                 rrw_init(&rrl->locks[i], track_all);
 310 }
 311 
 312 void
 313 rrm_destroy(rrmlock_t *rrl)
 314 {
 315         int i;
 316 
 317         for (i = 0; i < RRM_NUM_LOCKS; i++)
 318                 rrw_destroy(&rrl->locks[i]);
 319 }
 320 
 321 void
 322 rrm_enter(rrmlock_t *rrl, krw_t rw, void *tag)
 323 {
 324         if (rw == RW_READER)
 325                 rrm_enter_read(rrl, tag);
 326         else
 327                 rrm_enter_write(rrl);
 328 }
 329 
 330 /*
 331  * This maps the current thread to a specific lock.  Note that the lock
 332  * must be released by the same thread that acquired it.  We do this
 333  * mapping by taking the thread pointer mod a prime number.  We examine
 334  * only the low 32 bits of the thread pointer, because 32-bit division
 335  * is faster than 64-bit division, and the high 32 bits have little
 336  * entropy anyway.
 337  */
 338 #define RRM_TD_LOCK()   (((uint32_t)(uintptr_t)(curthread)) % RRM_NUM_LOCKS)
 339 
 340 void
 341 rrm_enter_read(rrmlock_t *rrl, void *tag)
 342 {
 343         rrw_enter_read(&rrl->locks[RRM_TD_LOCK()], tag);
 344 }
 345 
 346 void
 347 rrm_enter_write(rrmlock_t *rrl)
 348 {
 349         int i;
 350 
 351         for (i = 0; i < RRM_NUM_LOCKS; i++)
 352                 rrw_enter_write(&rrl->locks[i]);
 353 }
 354 
 355 void
 356 rrm_exit(rrmlock_t *rrl, void *tag)
 357 {
 358         int i;
 359 
 360         if (rrl->locks[0].rr_writer == curthread) {
 361                 for (i = 0; i < RRM_NUM_LOCKS; i++)
 362                         rrw_exit(&rrl->locks[i], tag);
 363         } else {
 364                 rrw_exit(&rrl->locks[RRM_TD_LOCK()], tag);
 365         }
 366 }
 367 
 368 boolean_t
 369 rrm_held(rrmlock_t *rrl, krw_t rw)
 370 {
 371         if (rw == RW_WRITER) {
 372                 return (rrw_held(&rrl->locks[0], rw));
 373         } else {
 374                 return (rrw_held(&rrl->locks[RRM_TD_LOCK()], rw));
 375         }
 376 }