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#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
51void
52zrl_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
63void
64zrl_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
73void
74zrl_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
106void
107zrl_remove(zrlock_t *zrl)
108{
109	uint32_t n;
110
111#ifdef	ZFS_DEBUG
112	if (zrl->zr_owner == curthread) {
113		zrl->zr_owner = NULL;
114		zrl->zr_caller = NULL;
115	}
116#endif
117	n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
118	ASSERT3S((int32_t)n, >=, 0);
119}
120
121int
122zrl_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
143void
144zrl_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
159int
160zrl_is_zero(zrlock_t *zrl)
161{
162	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
163
164	return (zrl->zr_refcount <= 0);
165}
166
167int
168zrl_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
176kthread_t *
177zrl_owner(zrlock_t *zrl)
178{
179	return (zrl->zr_owner);
180}
181#endif
182
183#if defined(_KERNEL)
184
185EXPORT_SYMBOL(zrl_add_impl);
186EXPORT_SYMBOL(zrl_remove);
187
188#endif
189