refcount.h

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    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 #if defined(_KERNEL) || defined(_STANDALONE)
   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_load(volatile u_int *count)
   71 {
   72         return (atomic_load_int(count));
   73 }
   74 
   75 static __inline u_int
   76 refcount_acquire(volatile u_int *count)
   77 {
   78         u_int old;
   79 
   80         old = atomic_fetchadd_int(count, 1);
   81         if (__predict_false(REFCOUNT_SATURATED(old)))
   82                 _refcount_update_saturated(count);
   83 
   84         return (old);
   85 }
   86 
   87 static __inline u_int
   88 refcount_acquiren(volatile u_int *count, u_int n)
   89 {
   90         u_int old;
   91 
   92         KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
   93             ("refcount_acquiren: n=%u too large", n));
   94         old = atomic_fetchadd_int(count, n);
   95         if (__predict_false(REFCOUNT_SATURATED(old)))
   96                 _refcount_update_saturated(count);
   97 
   98         return (old);
   99 }
  100 
  101 static __inline __result_use_check bool
  102 refcount_acquire_checked(volatile u_int *count)
  103 {
  104         u_int old;
  105 
  106         old = atomic_load_int(count);
  107         for (;;) {
  108                 if (__predict_false(REFCOUNT_SATURATED(old + 1)))
  109                         return (false);
  110                 if (__predict_true(atomic_fcmpset_int(count, &old,
  111                     old + 1) == 1))
  112                         return (true);
  113         }
  114 }
  115 
  116 /*
  117  * This functions returns non-zero if the refcount was
  118  * incremented. Else zero is returned.
  119  */
  120 static __inline __result_use_check bool
  121 refcount_acquire_if_gt(volatile u_int *count, u_int n)
  122 {
  123         u_int old;
  124 
  125         old = atomic_load_int(count);
  126         for (;;) {
  127                 if (old <= n)
  128                         return (false);
  129                 if (__predict_false(REFCOUNT_SATURATED(old)))
  130                         return (true);
  131                 if (atomic_fcmpset_int(count, &old, old + 1))
  132                         return (true);
  133         }
  134 }
  135 
  136 static __inline __result_use_check bool
  137 refcount_acquire_if_not_zero(volatile u_int *count)
  138 {
  139 
  140         return (refcount_acquire_if_gt(count, 0));
  141 }
  142 
  143 static __inline bool
  144 refcount_releasen(volatile u_int *count, u_int n)
  145 {
  146         u_int old;
  147 
  148         KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
  149             ("refcount_releasen: n=%u too large", n));
  150 
  151         atomic_thread_fence_rel();
  152         old = atomic_fetchadd_int(count, -n);
  153         if (__predict_false(old < n || REFCOUNT_SATURATED(old))) {
  154                 _refcount_update_saturated(count);
  155                 return (false);
  156         }
  157         if (old > n)
  158                 return (false);
  159 
  160         /*
  161          * Last reference.  Signal the user to call the destructor.
  162          *
  163          * Ensure that the destructor sees all updates. This synchronizes with
  164          * release fences from all routines which drop the count.
  165          */
  166         atomic_thread_fence_acq();
  167         return (true);
  168 }
  169 
  170 static __inline bool
  171 refcount_release(volatile u_int *count)
  172 {
  173 
  174         return (refcount_releasen(count, 1));
  175 }
  176 
  177 #define _refcount_release_if_cond(cond, name)                           \
  178 static __inline __result_use_check bool                                 \
  179 _refcount_release_if_##name(volatile u_int *count, u_int n)             \
  180 {                                                                       \
  181         u_int old;                                                      \
  182                                                                         \
  183         KASSERT(n > 0, ("%s: zero increment", __func__));               \
  184         old = atomic_load_int(count);                                   \
  185         for (;;) {                                                      \
  186                 if (!(cond))                                            \
  187                         return (false);                                 \
  188                 if (__predict_false(REFCOUNT_SATURATED(old)))           \
  189                         return (false);                                 \
  190                 if (atomic_fcmpset_rel_int(count, &old, old - 1))       \
  191                         return (true);                                  \
  192         }                                                               \
  193 }
  194 _refcount_release_if_cond(old > n, gt)
  195 _refcount_release_if_cond(old == n, eq)
  196 
  197 static __inline __result_use_check bool
  198 refcount_release_if_gt(volatile u_int *count, u_int n)
  199 {
  200 
  201         return (_refcount_release_if_gt(count, n));
  202 }
  203 
  204 static __inline __result_use_check bool
  205 refcount_release_if_last(volatile u_int *count)
  206 {
  207 
  208         if (_refcount_release_if_eq(count, 1)) {
  209                 /* See the comment in refcount_releasen(). */
  210                 atomic_thread_fence_acq();
  211                 return (true);
  212         }
  213         return (false);
  214 }
  215 
  216 static __inline __result_use_check bool
  217 refcount_release_if_not_last(volatile u_int *count)
  218 {
  219 
  220         return (_refcount_release_if_gt(count, 1));
  221 }
  222 
  223 #endif /* !__SYS_REFCOUNT_H__ */

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FreeBSD/Linux Kernel Cross Reference sys/sys/refcount.h

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sys/sys/refcount.h