sys/sys/refcount.h

   1 /*-
   2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
   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  * $FreeBSD$
  28  */
  29
  30 #ifndef __SYS_REFCOUNT_H__
  31 #define __SYS_REFCOUNT_H__
  32
  33 #include <machine/atomic.h>
  34
  35 #ifdef _KERNEL
  36 #include <sys/systm.h>
  37 #else
  38 #include <stdbool.h>
  39 #define KASSERT(exp, msg)       /* */
  40 #endif
  41
  42 #define REFCOUNT_SATURATED(val)         (((val) & (1U << 31)) != 0)
  43 #define REFCOUNT_SATURATION_VALUE       (3U << 30)
  44
  45 /*
  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
  49  * memory leak.
  50  */
  51 static __inline void
  52 _refcount_update_saturated(volatile u_int *count)
  53 {
  54 #ifdef INVARIANTS
  55         panic("refcount %p wraparound", count);
  56 #else
  57         atomic_store_int(count, REFCOUNT_SATURATION_VALUE);
  58 #endif
  59 }
  60
  61 static __inline void
  62 refcount_init(volatile u_int *count, u_int value)
  63 {
  64         KASSERT(!REFCOUNT_SATURATED(value),
  65             ("invalid initial refcount value %u", value));
  66         atomic_store_int(count, value);
  67 }
  68
  69 static __inline u_int
  70 refcount_acquire(volatile u_int *count)
  71 {
  72         u_int old;
  73
  74         old = atomic_fetchadd_int(count, 1);
  75         if (__predict_false(REFCOUNT_SATURATED(old)))
  76                 _refcount_update_saturated(count);
  77
  78         return (old);
  79 }
  80
  81 static __inline u_int
  82 refcount_acquiren(volatile u_int *count, u_int n)
  83 {
  84         u_int old;
  85
  86         KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
  87             ("refcount_acquiren: n=%u too large", n));
  88         old = atomic_fetchadd_int(count, n);
  89         if (__predict_false(REFCOUNT_SATURATED(old)))
  90                 _refcount_update_saturated(count);
  91
  92         return (old);
  93 }
  94
  95 static __inline __result_use_check bool
  96 refcount_acquire_checked(volatile u_int *count)
  97 {
  98         u_int old;
  99
 100         old = atomic_load_int(count);
 101         for (;;) {
 102                 if (__predict_false(REFCOUNT_SATURATED(old + 1)))
 103                         return (false);
 104                 if (__predict_true(atomic_fcmpset_int(count, &old,
 105                     old + 1) == 1))
 106                         return (true);
 107         }
 108 }
 109
 110 /*
 111  * This functions returns non-zero if the refcount was
 112  * incremented. Else zero is returned.
 113  */
 114 static __inline __result_use_check bool
 115 refcount_acquire_if_gt(volatile u_int *count, u_int n)
 116 {
 117         u_int old;
 118
 119         old = atomic_load_int(count);
 120         for (;;) {
 121                 if (old <= n)
 122                         return (false);
 123                 if (__predict_false(REFCOUNT_SATURATED(old)))
 124                         return (true);
 125                 if (atomic_fcmpset_int(count, &old, old + 1))
 126                         return (true);
 127         }
 128 }
 129
 130 static __inline __result_use_check bool
 131 refcount_acquire_if_not_zero(volatile u_int *count)
 132 {
 133
 134         return (refcount_acquire_if_gt(count, 0));
 135 }
 136
 137 static __inline bool
 138 refcount_releasen(volatile u_int *count, u_int n)
 139 {
 140         u_int old;
 141
 142         KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
 143             ("refcount_releasen: n=%u too large", n));
 144
 145         atomic_thread_fence_rel();
 146         old = atomic_fetchadd_int(count, -n);
 147         if (__predict_false(old < n || REFCOUNT_SATURATED(old))) {
 148                 _refcount_update_saturated(count);
 149                 return (false);
 150         }
 151         if (old > n)
 152                 return (false);
 153
 154         /*
 155          * Last reference.  Signal the user to call the destructor.
 156          *
 157          * Ensure that the destructor sees all updates. This synchronizes with
 158          * release fences from all routines which drop the count.
 159          */
 160         atomic_thread_fence_acq();
 161         return (true);
 162 }
 163
 164 static __inline bool
 165 refcount_release(volatile u_int *count)
 166 {
 167
 168         return (refcount_releasen(count, 1));
 169 }
 170
 171 static __inline __result_use_check bool
 172 refcount_release_if_gt(volatile u_int *count, u_int n)
 173 {
 174         u_int old;
 175
 176         KASSERT(n > 0,
 177             ("refcount_release_if_gt: Use refcount_release for final ref"));
 178         old = atomic_load_int(count);
 179         for (;;) {
 180                 if (old <= n)
 181                         return (false);
 182                 if (__predict_false(REFCOUNT_SATURATED(old)))
 183                         return (true);
 184                 /*
 185                  * Paired with acquire fence in refcount_releasen().
 186                  */
 187                 if (atomic_fcmpset_rel_int(count, &old, old - 1))
 188                         return (true);
 189         }
 190 }
 191
 192 static __inline __result_use_check bool
 193 refcount_release_if_not_last(volatile u_int *count)
 194 {
 195
 196         return (refcount_release_if_gt(count, 1));
 197 }
 198
 199 #endif /* !__SYS_REFCOUNT_H__ */