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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 (c) 2010, Oracle and/or its affiliates. All rights reserved.
26 * A Zero Reference Lock (ZRL) is a reference count that can lock out new
27 * references only when the count is zero and only without waiting if the count
28 * is not already zero. It is similar to a read-write lock in that it allows
29 * multiple readers and only a single writer, but it does not allow a writer to
30 * block while waiting for readers to exit, and therefore the question of
31 * reader/writer priority is moot (no WRWANT bit). Since the equivalent of
32 * rw_enter(&lock, RW_WRITER) is disallowed and only tryenter() is allowed, it
33 * is perfectly safe for the same reader to acquire the same lock multiple
34 * times. The fact that a ZRL is reentrant for readers (through multiple calls
35 * to zrl_add()) makes it convenient for determining whether something is
36 * actively referenced without the fuss of flagging lock ownership across
39 #include <sys/zrlock.h>
42 * A ZRL can be locked only while there are zero references, so ZRL_LOCKED is
43 * treated as zero references.
45 #define ZRL_LOCKED ((uint32_t)-1)
46 #define ZRL_DESTROYED -2
49 zrl_init(zrlock_t
*zrl
)
51 mutex_init(&zrl
->zr_mtx
, NULL
, MUTEX_DEFAULT
, NULL
);
53 cv_init(&zrl
->zr_cv
, NULL
, CV_DEFAULT
, NULL
);
56 zrl
->zr_caller
= NULL
;
61 zrl_destroy(zrlock_t
*zrl
)
63 ASSERT(zrl
->zr_refcount
== 0);
65 mutex_destroy(&zrl
->zr_mtx
);
66 zrl
->zr_refcount
= ZRL_DESTROYED
;
67 cv_destroy(&zrl
->zr_cv
);
72 zrl_add_debug(zrlock_t
*zrl
, const char *zc
)
74 zrl_add(zrlock_t
*zrl
)
77 uint32_t n
= (uint32_t)zrl
->zr_refcount
;
79 while (n
!= ZRL_LOCKED
) {
80 uint32_t cas
= atomic_cas_32(
81 (uint32_t *)&zrl
->zr_refcount
, n
, n
+ 1);
83 ASSERT((int32_t)n
>= 0);
85 if (zrl
->zr_owner
== curthread
) {
86 DTRACE_PROBE2(zrlock__reentry
,
87 zrlock_t
*, zrl
, uint32_t, n
);
89 zrl
->zr_owner
= curthread
;
97 mutex_enter(&zrl
->zr_mtx
);
98 while (zrl
->zr_refcount
== ZRL_LOCKED
) {
99 cv_wait(&zrl
->zr_cv
, &zrl
->zr_mtx
);
101 ASSERT(zrl
->zr_refcount
>= 0);
104 zrl
->zr_owner
= curthread
;
107 mutex_exit(&zrl
->zr_mtx
);
111 zrl_remove(zrlock_t
*zrl
)
115 n
= atomic_dec_32_nv((uint32_t *)&zrl
->zr_refcount
);
116 ASSERT((int32_t)n
>= 0);
118 if (zrl
->zr_owner
== curthread
) {
119 zrl
->zr_owner
= NULL
;
120 zrl
->zr_caller
= NULL
;
126 zrl_tryenter(zrlock_t
*zrl
)
128 uint32_t n
= (uint32_t)zrl
->zr_refcount
;
131 uint32_t cas
= atomic_cas_32(
132 (uint32_t *)&zrl
->zr_refcount
, 0, ZRL_LOCKED
);
135 ASSERT(zrl
->zr_owner
== NULL
);
136 zrl
->zr_owner
= curthread
;
142 ASSERT((int32_t)n
> ZRL_DESTROYED
);
148 zrl_exit(zrlock_t
*zrl
)
150 ASSERT(zrl
->zr_refcount
== ZRL_LOCKED
);
152 mutex_enter(&zrl
->zr_mtx
);
154 ASSERT(zrl
->zr_owner
== curthread
);
155 zrl
->zr_owner
= NULL
;
156 membar_producer(); /* make sure the owner store happens first */
158 zrl
->zr_refcount
= 0;
159 cv_broadcast(&zrl
->zr_cv
);
160 mutex_exit(&zrl
->zr_mtx
);
164 zrl_refcount(zrlock_t
*zrl
)
168 ASSERT(zrl
->zr_refcount
> ZRL_DESTROYED
);
170 n
= (int)zrl
->zr_refcount
;
171 return (n
<= 0 ? 0 : n
);
175 zrl_is_zero(zrlock_t
*zrl
)
177 ASSERT(zrl
->zr_refcount
> ZRL_DESTROYED
);
179 return (zrl
->zr_refcount
<= 0);
183 zrl_is_locked(zrlock_t
*zrl
)
185 ASSERT(zrl
->zr_refcount
> ZRL_DESTROYED
);
187 return (zrl
->zr_refcount
== ZRL_LOCKED
);
192 zrl_owner(zrlock_t
*zrl
)
194 return (zrl
->zr_owner
);
198 #if defined(_KERNEL) && defined(HAVE_SPL)
201 EXPORT_SYMBOL(zrl_add_debug
);
203 EXPORT_SYMBOL(zrl_add
);
205 EXPORT_SYMBOL(zrl_remove
);