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