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.
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.
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]
22 * Copyright 2009 Sun Microsystems, Inc. All rights reserved.
23 * Use is subject to license terms.
26 * Copyright (c) 2012 by Delphix. All rights reserved.
29 #include <sys/refcount.h>
30 #include <sys/rrwlock.h>
33 * This file contains the implementation of a re-entrant read
34 * reader/writer lock (aka "rrwlock").
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
41 * Callers who have not obtained a read lock give waiting writers priority.
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
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.
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).
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.
74 /* global key for TSD */
77 typedef struct rrw_node {
78 struct rrw_node *rn_next;
84 rrn_find(rrwlock_t *rrl)
88 if (refcount_count(&rrl->rr_linked_rcount) == 0)
91 for (rn = tsd_get(rrw_tsd_key); rn != NULL; rn = rn->rn_next) {
92 if (rn->rn_rrl == rrl)
99 * Add a node to the head of the singly linked list.
102 rrn_add(rrwlock_t *rrl, void *tag)
106 rn = kmem_alloc(sizeof (*rn), KM_SLEEP);
108 rn->rn_next = tsd_get(rrw_tsd_key);
110 VERIFY(tsd_set(rrw_tsd_key, rn) == 0);
114 * If a node is found for 'rrl', then remove the node from this
115 * thread's list and return TRUE; otherwise return FALSE.
118 rrn_find_and_remove(rrwlock_t *rrl, void *tag)
121 rrw_node_t *prev = NULL;
123 if (refcount_count(&rrl->rr_linked_rcount) == 0)
126 for (rn = tsd_get(rrw_tsd_key); rn != NULL; rn = rn->rn_next) {
127 if (rn->rn_rrl == rrl && rn->rn_tag == tag) {
129 prev->rn_next = rn->rn_next;
131 VERIFY(tsd_set(rrw_tsd_key, rn->rn_next) == 0);
132 kmem_free(rn, sizeof (*rn));
141 rrw_init(rrwlock_t *rrl, boolean_t track_all)
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;
153 rrw_destroy(rrwlock_t *rrl)
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);
163 rrw_enter_read(rrwlock_t *rrl, void *tag)
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);
173 DTRACE_PROBE(zfs__rrwfastpath__rdmiss);
175 ASSERT(rrl->rr_writer != curthread);
176 ASSERT(refcount_count(&rrl->rr_anon_rcount) >= 0);
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);
183 if (rrl->rr_writer_wanted || rrl->rr_track_all) {
184 /* may or may not be a re-entrant enter */
186 (void) refcount_add(&rrl->rr_linked_rcount, tag);
188 (void) refcount_add(&rrl->rr_anon_rcount, tag);
190 ASSERT(rrl->rr_writer == NULL);
191 mutex_exit(&rrl->rr_lock);
195 rrw_enter_write(rrwlock_t *rrl)
197 mutex_enter(&rrl->rr_lock);
198 ASSERT(rrl->rr_writer != curthread);
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);
206 rrl->rr_writer_wanted = B_FALSE;
207 rrl->rr_writer = curthread;
208 mutex_exit(&rrl->rr_lock);
212 rrw_enter(rrwlock_t *rrl, krw_t rw, void *tag)
215 rrw_enter_read(rrl, tag);
217 rrw_enter_write(rrl);
221 rrw_exit(rrwlock_t *rrl, void *tag)
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);
232 DTRACE_PROBE(zfs__rrwfastpath__exitmiss);
234 ASSERT(!refcount_is_zero(&rrl->rr_anon_rcount) ||
235 !refcount_is_zero(&rrl->rr_linked_rcount) ||
236 rrl->rr_writer != NULL);
238 if (rrl->rr_writer == NULL) {
240 if (rrn_find_and_remove(rrl, tag)) {
241 count = refcount_remove(&rrl->rr_linked_rcount, tag);
243 ASSERT(!rrl->rr_track_all);
244 count = refcount_remove(&rrl->rr_anon_rcount, tag);
247 cv_broadcast(&rrl->rr_cv);
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);
255 mutex_exit(&rrl->rr_lock);
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.
264 rrw_held(rrwlock_t *rrl, krw_t rw)
268 mutex_enter(&rrl->rr_lock);
269 if (rw == RW_WRITER) {
270 held = (rrl->rr_writer == curthread);
272 held = (!refcount_is_zero(&rrl->rr_anon_rcount) ||
273 rrn_find(rrl) != NULL);
275 mutex_exit(&rrl->rr_lock);
281 rrw_tsd_destroy(void *arg)
283 rrw_node_t *rn = arg;
285 panic("thread %p terminating with rrw lock %p held",
286 (void *)curthread, (void *)rn->rn_rrl);