zrlock.c revision 290765 
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 */
25
26/*
27 * A Zero Reference Lock (ZRL) is a reference count that can lock out new
28 * references only when the count is zero and only without waiting if the count
29 * is not already zero. It is similar to a read-write lock in that it allows
30 * multiple readers and only a single writer, but it does not allow a writer to
31 * block while waiting for readers to exit, and therefore the question of
32 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
33 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
34 * is perfectly safe for the same reader to acquire the same lock multiple
35 * times. The fact that a ZRL is reentrant for readers (through multiple calls
36 * to zrl_add()) makes it convenient for determining whether something is
37 * actively referenced without the fuss of flagging lock ownership across
38 * function calls.
39 */
40#include <sys/zrlock.h>
41
42/*
43 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
44 * treated as zero references.
45 */
46#define	ZRL_LOCKED	-1
47#define	ZRL_DESTROYED	-2
48
49void
50zrl_init(zrlock_t *zrl)
51{
52	mutex_init(&zrl->zr_mtx, NULL, MUTEX_DEFAULT, NULL);
53	zrl->zr_refcount = 0;
54	cv_init(&zrl->zr_cv, NULL, CV_DEFAULT, NULL);
55#ifdef	ZFS_DEBUG
56	zrl->zr_owner = NULL;
57	zrl->zr_caller = NULL;
58#endif
59}
60
61void
62zrl_destroy(zrlock_t *zrl)
63{
64	ASSERT0(zrl->zr_refcount);
65
66	mutex_destroy(&zrl->zr_mtx);
67	zrl->zr_refcount = ZRL_DESTROYED;
68	cv_destroy(&zrl->zr_cv);
69}
70
71void
72zrl_add_impl(zrlock_t *zrl, const char *zc)
73{
74	uint32_t n = (uint32_t)zrl->zr_refcount;
75
76	while (n != ZRL_LOCKED) {
77		uint32_t cas = atomic_cas_32(
78		    (uint32_t *)&zrl->zr_refcount, n, n + 1);
79		if (cas == n) {
80			ASSERT3S((int32_t)n, >=, 0);
81#ifdef	ZFS_DEBUG
82			if (zrl->zr_owner == curthread) {
83				DTRACE_PROBE2(zrlock__reentry,
84				    zrlock_t *, zrl, uint32_t, n);
85			}
86			zrl->zr_owner = curthread;
87			zrl->zr_caller = zc;
88#endif
89			return;
90		}
91		n = cas;
92	}
93
94	mutex_enter(&zrl->zr_mtx);
95	while (zrl->zr_refcount == ZRL_LOCKED) {
96		cv_wait(&zrl->zr_cv, &zrl->zr_mtx);
97	}
98	ASSERT3S(zrl->zr_refcount, >=, 0);
99	zrl->zr_refcount++;
100#ifdef	ZFS_DEBUG
101	zrl->zr_owner = curthread;
102	zrl->zr_caller = zc;
103#endif
104	mutex_exit(&zrl->zr_mtx);
105}
106
107void
108zrl_remove(zrlock_t *zrl)
109{
110	uint32_t n;
111
112#ifdef	ZFS_DEBUG
113	if (zrl->zr_owner == curthread) {
114		zrl->zr_owner = NULL;
115		zrl->zr_caller = NULL;
116	}
117#endif
118	n = atomic_dec_32_nv((uint32_t *)&zrl->zr_refcount);
119	ASSERT3S((int32_t)n, >=, 0);
120}
121
122int
123zrl_tryenter(zrlock_t *zrl)
124{
125	uint32_t n = (uint32_t)zrl->zr_refcount;
126
127	if (n == 0) {
128		uint32_t cas = atomic_cas_32(
129		    (uint32_t *)&zrl->zr_refcount, 0, ZRL_LOCKED);
130		if (cas == 0) {
131#ifdef	ZFS_DEBUG
132			ASSERT3P(zrl->zr_owner, ==, NULL);
133			zrl->zr_owner = curthread;
134#endif
135			return (1);
136		}
137	}
138
139	ASSERT3S((int32_t)n, >, ZRL_DESTROYED);
140
141	return (0);
142}
143
144void
145zrl_exit(zrlock_t *zrl)
146{
147	ASSERT3S(zrl->zr_refcount, ==, ZRL_LOCKED);
148
149	mutex_enter(&zrl->zr_mtx);
150#ifdef	ZFS_DEBUG
151	ASSERT3P(zrl->zr_owner, ==, curthread);
152	zrl->zr_owner = NULL;
153	membar_producer();	/* make sure the owner store happens first */
154#endif
155	zrl->zr_refcount = 0;
156	cv_broadcast(&zrl->zr_cv);
157	mutex_exit(&zrl->zr_mtx);
158}
159
160int
161zrl_refcount(zrlock_t *zrl)
162{
163	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
164
165	int n = (int)zrl->zr_refcount;
166	return (n <= 0 ? 0 : n);
167}
168
169int
170zrl_is_zero(zrlock_t *zrl)
171{
172	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
173
174	return (zrl->zr_refcount <= 0);
175}
176
177int
178zrl_is_locked(zrlock_t *zrl)
179{
180	ASSERT3S(zrl->zr_refcount, >, ZRL_DESTROYED);
181
182	return (zrl->zr_refcount == ZRL_LOCKED);
183}
184
185#ifdef	ZFS_DEBUG
186kthread_t *
187zrl_owner(zrlock_t *zrl)
188{
189	return (zrl->zr_owner);
190}
191#endif
192