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 https://opensource.org/licenses/CDDL-1.0.
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 (c) 2010, Oracle and/or its affiliates. All rights reserved.
23 * Copyright (c) 2014, 2015 by Delphix. All rights reserved.
24 * Copyright 2016 The MathWorks, Inc. All rights reserved.
25 */
26
27 /*
28 * A Zero Reference Lock (ZRL) is a reference count that can lock out new
29 * references only when the count is zero and only without waiting if the count
30 * is not already zero. It is similar to a read-write lock in that it allows
31 * multiple readers and only a single writer, but it does not allow a writer to
32 * block while waiting for readers to exit, and therefore the question of
33 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
34 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
35 * is perfectly safe for the same reader to acquire the same lock multiple
36 * times. The fact that a ZRL is reentrant for readers (through multiple calls
37 * to zrl_add()) makes it convenient for determining whether something is
38 * actively referenced without the fuss of flagging lock ownership across
39 * function calls.
40 */
41 #include <sys/zrlock.h>
42 #include <sys/trace_zfs.h>
43
44 /*
45 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
46 * treated as zero references.
47 */
48 #define ZRL_LOCKED -1
49 #define ZRL_DESTROYED -2
50
51 void
zrl_init(zrlock_t * zrl)52 zrl_init(zrlock_t *zrl)
53 {
54 mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
55 zrl->zr_refcount = 0;
56 cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
57 #ifdef ZFS_DEBUG
58 zrl->zr_owner = NULL;
59 zrl->zr_caller = NULL;
60 #endif
61 }
62
63 void
zrl_destroy(zrlock_t * zrl)64 zrl_destroy(zrlock_t *zrl)
65 {
66 ASSERT0(zrl->zr_refcount);
67
68 mutex_destroy(&zrl->zr_mtx);
69 zrl->zr_refcount = ZRL_DESTROYED;
70 cv_destroy(&zrl->zr_cv);
71 }
72
73 void
zrl_add_impl(zrlock_t * zrl,const char * zc)74 zrl_add_impl(zrlock_t *zrl, const char *zc)
75 {
76 for (;;) {
77 uint32_t n = (uint32_t)zrl->zr_refcount;
78 while (n != ZRL_LOCKED) {
79 uint32_t cas = atomic_cas_32(
80 (uint32_t *)&zrl->zr_refcount, n, n + 1);
81 if (cas == n) {
82 ASSERT3S((int32_t)n, >=, 0);
83 #ifdef ZFS_DEBUG
84 if (zrl->zr_owner == curthread) {
85 DTRACE_PROBE3(zrlock__reentry,
86 zrlock_t *, zrl,
87 kthread_t *, curthread,
88 uint32_t, n);
89 }
90 zrl->zr_owner = curthread;
91 zrl->zr_caller = zc;
92 #endif
93 return;
94 }
95 n = cas;
96 }
97
98 mutex_enter(&zrl->zr_mtx);
99 while (zrl->zr_refcount == ZRL_LOCKED) {
100 cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
101 }
102 mutex_exit(&zrl->zr_mtx);
103 }
104 }
105
106 void
zrl_remove(zrlock_t * zrl)107 zrl_remove(zrlock_t *zrl)
108 {
109 #ifdef ZFS_DEBUG
110 if (zrl->zr_owner == curthread) {
111 zrl->zr_owner = NULL;
112 zrl->zr_caller = NULL;
113 }
114 int32_t n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
115 ASSERT3S(n, >=, 0);
116 #else
117 atomic_dec_32((uint32_t *)&zrl->zr_refcount);
118 #endif
119 }
120
121 int
zrl_tryenter(zrlock_t * zrl)122 zrl_tryenter(zrlock_t *zrl)
123 {
124 uint32_t n = (uint32_t)zrl->zr_refcount;
125
126 if (n == 0) {
127 uint32_t cas = atomic_cas_32(
128 (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
129 if (cas == 0) {
130 #ifdef ZFS_DEBUG
131 ASSERT3P(zrl->zr_owner, ==, NULL);
132 zrl->zr_owner = curthread;
133 #endif
134 return (1);
135 }
136 }
137
138 ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
139
140 return (0);
141 }
142
143 void
zrl_exit(zrlock_t * zrl)144 zrl_exit(zrlock_t *zrl)
145 {
146 ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
147
148 mutex_enter(&zrl->zr_mtx);
149 #ifdef ZFS_DEBUG
150 ASSERT3P(zrl->zr_owner, ==, curthread);
151 zrl->zr_owner = NULL;
152 membar_producer(); /* make sure the owner store happens first */
153 #endif
154 zrl->zr_refcount = 0;
155 cv_broadcast(&zrl->zr_cv);
156 mutex_exit(&zrl->zr_mtx);
157 }
158
159 int
zrl_is_zero(zrlock_t * zrl)160 zrl_is_zero(zrlock_t *zrl)
161 {
162 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
163
164 return (zrl->zr_refcount <= 0);
165 }
166
167 int
zrl_is_locked(zrlock_t * zrl)168 zrl_is_locked(zrlock_t *zrl)
169 {
170 ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
171
172 return (zrl->zr_refcount == ZRL_LOCKED);
173 }
174
175 #ifdef ZFS_DEBUG
176 kthread_t *
zrl_owner(zrlock_t * zrl)177 zrl_owner(zrlock_t *zrl)
178 {
179 return (zrl->zr_owner);
180 }
181 #endif
182
183 #if defined(_KERNEL)
184
185 EXPORT_SYMBOL(zrl_add_impl);
186 EXPORT_SYMBOL(zrl_remove);
187
188 #endif
189