2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
4 * Copyright (c) 2005 John Baldwin <jhb@FreeBSD.org>
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
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.
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
30 #ifndef __SYS_REFCOUNT_H__
31 #define __SYS_REFCOUNT_H__
33 #include <machine/atomic.h>
36 #include <sys/systm.h>
39 #define KASSERT(exp, msg) /* */
42 #define REFCOUNT_SATURATED(val) (((val) & (1U << 31)) != 0)
43 #define REFCOUNT_SATURATION_VALUE (3U << 30)
46 * Attempt to handle reference count overflow and underflow. Force the counter
47 * to stay at the saturation value so that a counter overflow cannot trigger
48 * destruction of the containing object and instead leads to a less harmful
52 _refcount_update_saturated(volatile u_int *count)
55 panic("refcount %p wraparound", count);
57 atomic_store_int(count, REFCOUNT_SATURATION_VALUE);
62 refcount_init(volatile u_int *count, u_int value)
64 KASSERT(!REFCOUNT_SATURATED(value),
65 ("invalid initial refcount value %u", value));
70 refcount_acquire(volatile u_int *count)
74 old = atomic_fetchadd_int(count, 1);
75 if (__predict_false(REFCOUNT_SATURATED(old)))
76 _refcount_update_saturated(count);
79 static __inline __result_use_check bool
80 refcount_acquire_checked(volatile u_int *count)
84 for (lcount = *count;;) {
85 if (__predict_false(REFCOUNT_SATURATED(lcount + 1)))
87 if (__predict_true(atomic_fcmpset_int(count, &lcount,
94 refcount_release(volatile u_int *count)
98 atomic_thread_fence_rel();
99 old = atomic_fetchadd_int(count, -1);
100 if (__predict_false(old == 0 || REFCOUNT_SATURATED(old))) {
102 * Avoid multiple destructor invocations if underflow occurred.
103 * This is not perfect since the memory backing the containing
104 * object may already have been reallocated.
106 _refcount_update_saturated(count);
113 * Last reference. Signal the user to call the destructor.
115 * Ensure that the destructor sees all updates. The fence_rel
116 * at the start of the function synchronizes with this fence.
118 atomic_thread_fence_acq();
123 * This functions returns non-zero if the refcount was
124 * incremented. Else zero is returned.
126 static __inline __result_use_check bool
127 refcount_acquire_if_not_zero(volatile u_int *count)
135 if (__predict_false(REFCOUNT_SATURATED(old)))
137 if (atomic_fcmpset_int(count, &old, old + 1))
142 static __inline __result_use_check bool
143 refcount_release_if_not_last(volatile u_int *count)
151 if (__predict_false(REFCOUNT_SATURATED(old)))
153 if (atomic_fcmpset_int(count, &old, old - 1))
158 #endif /* ! __SYS_REFCOUNT_H__ */