1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2005 John Baldwin <jhb@FreeBSD.org>
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
14 *
15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
25 * SUCH DAMAGE.
26 */
27
28 #ifndef __SYS_REFCOUNT_H__
29 #define __SYS_REFCOUNT_H__
30
31 #include <sys/types.h>
32 #include <sys/kassert.h>
33 #if !defined(_KERNEL) && !defined(_STANDALONE)
34 #include <stdbool.h>
35 #endif
36
37 #include <machine/atomic.h>
38
39 #define REFCOUNT_SATURATED(val) (((val) & (1U << 31)) != 0)
40 #define REFCOUNT_SATURATION_VALUE (3U << 30)
41
42 /*
43 * Attempt to handle reference count overflow and underflow. Force the counter
44 * to stay at the saturation value so that a counter overflow cannot trigger
45 * destruction of the containing object and instead leads to a less harmful
46 * memory leak.
47 */
48 static __inline void
_refcount_update_saturated(volatile u_int * count)49 _refcount_update_saturated(volatile u_int *count)
50 {
51 #ifdef INVARIANTS
52 panic("refcount %p wraparound", count);
53 #else
54 atomic_store_int(count, REFCOUNT_SATURATION_VALUE);
55 #endif
56 }
57
58 static __inline void
refcount_init(volatile u_int * count,u_int value)59 refcount_init(volatile u_int *count, u_int value)
60 {
61 KASSERT(!REFCOUNT_SATURATED(value),
62 ("invalid initial refcount value %u", value));
63 atomic_store_int(count, value);
64 }
65
66 static __inline u_int
refcount_load(volatile const u_int * count)67 refcount_load(volatile const u_int *count)
68 {
69 return (atomic_load_int(count));
70 }
71
72 static __inline u_int
refcount_acquire(volatile u_int * count)73 refcount_acquire(volatile u_int *count)
74 {
75 u_int old;
76
77 old = atomic_fetchadd_int(count, 1);
78 if (__predict_false(REFCOUNT_SATURATED(old)))
79 _refcount_update_saturated(count);
80
81 return (old);
82 }
83
84 static __inline u_int
refcount_acquiren(volatile u_int * count,u_int n)85 refcount_acquiren(volatile u_int *count, u_int n)
86 {
87 u_int old;
88
89 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
90 ("refcount_acquiren: n=%u too large", n));
91 old = atomic_fetchadd_int(count, n);
92 if (__predict_false(REFCOUNT_SATURATED(old)))
93 _refcount_update_saturated(count);
94
95 return (old);
96 }
97
98 static __inline __result_use_check bool
refcount_acquire_checked(volatile u_int * count)99 refcount_acquire_checked(volatile u_int *count)
100 {
101 u_int old;
102
103 old = atomic_load_int(count);
104 for (;;) {
105 if (__predict_false(REFCOUNT_SATURATED(old + 1)))
106 return (false);
107 if (__predict_true(atomic_fcmpset_int(count, &old,
108 old + 1) == 1))
109 return (true);
110 }
111 }
112
113 /*
114 * This functions returns non-zero if the refcount was
115 * incremented. Else zero is returned.
116 */
117 static __inline __result_use_check bool
refcount_acquire_if_gt(volatile u_int * count,u_int n)118 refcount_acquire_if_gt(volatile u_int *count, u_int n)
119 {
120 u_int old;
121
122 old = atomic_load_int(count);
123 for (;;) {
124 if (old <= n)
125 return (false);
126 if (__predict_false(REFCOUNT_SATURATED(old)))
127 return (true);
128 if (atomic_fcmpset_int(count, &old, old + 1))
129 return (true);
130 }
131 }
132
133 static __inline __result_use_check bool
refcount_acquire_if_not_zero(volatile u_int * count)134 refcount_acquire_if_not_zero(volatile u_int *count)
135 {
136
137 return (refcount_acquire_if_gt(count, 0));
138 }
139
140 static __inline bool
refcount_releasen(volatile u_int * count,u_int n)141 refcount_releasen(volatile u_int *count, u_int n)
142 {
143 u_int old;
144
145 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
146 ("refcount_releasen: n=%u too large", n));
147
148 atomic_thread_fence_rel();
149 old = atomic_fetchadd_int(count, -n);
150 if (__predict_false(old < n || REFCOUNT_SATURATED(old))) {
151 _refcount_update_saturated(count);
152 return (false);
153 }
154 if (old > n)
155 return (false);
156
157 /*
158 * Last reference. Signal the user to call the destructor.
159 *
160 * Ensure that the destructor sees all updates. This synchronizes with
161 * release fences from all routines which drop the count.
162 */
163 atomic_thread_fence_acq();
164 return (true);
165 }
166
167 static __inline bool
refcount_release(volatile u_int * count)168 refcount_release(volatile u_int *count)
169 {
170
171 return (refcount_releasen(count, 1));
172 }
173
174 #define _refcount_release_if_cond(cond, name) \
175 static __inline __result_use_check bool \
176 _refcount_release_if_##name(volatile u_int *count, u_int n) \
177 { \
178 u_int old; \
179 \
180 KASSERT(n > 0, ("%s: zero increment", __func__)); \
181 old = atomic_load_int(count); \
182 for (;;) { \
183 if (!(cond)) \
184 return (false); \
185 if (__predict_false(REFCOUNT_SATURATED(old))) \
186 return (false); \
187 if (atomic_fcmpset_rel_int(count, &old, old - 1)) \
188 return (true); \
189 } \
190 }
191 _refcount_release_if_cond(old > n, gt)
192 _refcount_release_if_cond(old == n, eq)
193
194 static __inline __result_use_check bool
refcount_release_if_gt(volatile u_int * count,u_int n)195 refcount_release_if_gt(volatile u_int *count, u_int n)
196 {
197
198 return (_refcount_release_if_gt(count, n));
199 }
200
201 static __inline __result_use_check bool
refcount_release_if_last(volatile u_int * count)202 refcount_release_if_last(volatile u_int *count)
203 {
204
205 if (_refcount_release_if_eq(count, 1)) {
206 /* See the comment in refcount_releasen(). */
207 atomic_thread_fence_acq();
208 return (true);
209 }
210 return (false);
211 }
212
213 static __inline __result_use_check bool
refcount_release_if_not_last(volatile u_int * count)214 refcount_release_if_not_last(volatile u_int *count)
215 {
216
217 return (_refcount_release_if_gt(count, 1));
218 }
219
220 #endif /* !__SYS_REFCOUNT_H__ */
221