xref: /illumos-gate/usr/src/uts/common/fs/zfs/zrlock.c (revision 260af64d)
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 (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 
43 /*
44  * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
45  * treated as zero references.
46  */
47 #define	ZRL_LOCKED	-1
48 #define	ZRL_DESTROYED	-2
49 
50 void
zrl_init(zrlock_t * zrl)51 zrl_init(zrlock_t *zrl)
52 {
53 	mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
54 	zrl->zr_refcount = 0;
55 	cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
56 #ifdef	ZFS_DEBUG
57 	zrl->zr_owner = NULL;
58 	zrl->zr_caller = NULL;
59 #endif
60 }
61 
62 void
zrl_destroy(zrlock_t * zrl)63 zrl_destroy(zrlock_t *zrl)
64 {
65 	ASSERT0(zrl->zr_refcount);
66 
67 	mutex_destroy(&zrl->zr_mtx);
68 	zrl->zr_refcount = ZRL_DESTROYED;
69 	cv_destroy(&zrl->zr_cv);
70 }
71 
72 void
zrl_add_impl(zrlock_t * zrl,const char * zc)73 zrl_add_impl(zrlock_t *zrl, const char *zc)
74 {
75 	for (;;) {
76 		uint32_t n = (uint32_t)zrl->zr_refcount;
77 		while (n != ZRL_LOCKED) {
78 			uint32_t cas = atomic_cas_32(
79 			    (uint32_t *)&zrl->zr_refcount, n, n + 1);
80 			if (cas == n) {
81 				ASSERT3S((int32_t)n, >=, 0);
82 #ifdef	ZFS_DEBUG
83 				if (zrl->zr_owner == curthread) {
84 					DTRACE_PROBE2(zrlock__reentry,
85 					    zrlock_t *, zrl, uint32_t, n);
86 				}
87 				zrl->zr_owner = curthread;
88 				zrl->zr_caller = zc;
89 #endif
90 				return;
91 			}
92 			n = cas;
93 		}
94 
95 		mutex_enter(&zrl->zr_mtx);
96 		while (zrl->zr_refcount == ZRL_LOCKED) {
97 			cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
98 		}
99 		mutex_exit(&zrl->zr_mtx);
100 	}
101 }
102 
103 void
zrl_remove(zrlock_t * zrl)104 zrl_remove(zrlock_t *zrl)
105 {
106 	uint32_t n;
107 
108 #ifdef	ZFS_DEBUG
109 	if (zrl->zr_owner == curthread) {
110 		zrl->zr_owner = NULL;
111 		zrl->zr_caller = NULL;
112 	}
113 #endif
114 	n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
115 	ASSERT3S((int32_t)n, >=, 0);
116 }
117 
118 int
zrl_tryenter(zrlock_t * zrl)119 zrl_tryenter(zrlock_t *zrl)
120 {
121 	uint32_t n = (uint32_t)zrl->zr_refcount;
122 
123 	if (n == 0) {
124 		uint32_t cas = atomic_cas_32(
125 		    (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
126 		if (cas == 0) {
127 #ifdef	ZFS_DEBUG
128 			ASSERT3P(zrl->zr_owner, ==, NULL);
129 			zrl->zr_owner = curthread;
130 #endif
131 			return (1);
132 		}
133 	}
134 
135 	ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
136 
137 	return (0);
138 }
139 
140 void
zrl_exit(zrlock_t * zrl)141 zrl_exit(zrlock_t *zrl)
142 {
143 	ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
144 
145 	mutex_enter(&zrl->zr_mtx);
146 #ifdef	ZFS_DEBUG
147 	ASSERT3P(zrl->zr_owner, ==, curthread);
148 	zrl->zr_owner = NULL;
149 	membar_producer();	/* make sure the owner store happens first */
150 #endif
151 	zrl->zr_refcount = 0;
152 	cv_broadcast(&zrl->zr_cv);
153 	mutex_exit(&zrl->zr_mtx);
154 }
155 
156 int
zrl_refcount(zrlock_t * zrl)157 zrl_refcount(zrlock_t *zrl)
158 {
159 	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
160 
161 	int n = (int)zrl->zr_refcount;
162 	return (n <= 0 ? 0 : n);
163 }
164 
165 int
zrl_is_zero(zrlock_t * zrl)166 zrl_is_zero(zrlock_t *zrl)
167 {
168 	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
169 
170 	return (zrl->zr_refcount <= 0);
171 }
172 
173 int
zrl_is_locked(zrlock_t * zrl)174 zrl_is_locked(zrlock_t *zrl)
175 {
176 	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
177 
178 	return (zrl->zr_refcount == ZRL_LOCKED);
179 }
180 
181 #ifdef	ZFS_DEBUG
182 kthread_t *
zrl_owner(zrlock_t * zrl)183 zrl_owner(zrlock_t *zrl)
184 {
185 	return (zrl->zr_owner);
186 }
187 #endif
188