The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/sys/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  * All rights reserved.
    6  *
    7  * Redistribution and use in source and binary forms, with or without
    8  * modification, are permitted provided that the following conditions
    9  * are met:
   10  * 1. Redistributions of source code must retain the above copyright
   11  *    notice, this list of conditions and the following disclaimer.
   12  * 2. Redistributions in binary form must reproduce the above copyright
   13  *    notice, this list of conditions and the following disclaimer in the
   14  *    documentation and/or other materials provided with the distribution.
   15  *
   16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   19  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   26  * SUCH DAMAGE.
   27  *
   28  * $FreeBSD: stable/12/sys/sys/refcount.h 351786 2019-09-03 19:52:28Z kib $
   29  */
   30 
   31 #ifndef __SYS_REFCOUNT_H__
   32 #define __SYS_REFCOUNT_H__
   33 
   34 #include <sys/limits.h>
   35 #include <machine/atomic.h>
   36 
   37 #ifdef _KERNEL
   38 #include <sys/systm.h>
   39 #else
   40 #include <stdbool.h>
   41 #define KASSERT(exp, msg)       /* */
   42 #endif
   43 
   44 static __inline void
   45 refcount_init(volatile u_int *count, u_int value)
   46 {
   47 
   48         *count = value;
   49 }
   50 
   51 static __inline void
   52 refcount_acquire(volatile u_int *count)
   53 {
   54 
   55         KASSERT(*count < UINT_MAX, ("refcount %p overflowed", count));
   56         atomic_add_int(count, 1);
   57 }
   58 
   59 static __inline __result_use_check bool
   60 refcount_acquire_checked(volatile u_int *count)
   61 {
   62         u_int lcount;
   63 
   64         for (lcount = *count;;) {
   65                 if (__predict_false(lcount + 1 < lcount))
   66                         return (false);
   67                 if (__predict_true(atomic_fcmpset_int(count, &lcount,
   68                     lcount + 1) == 1))
   69                         return (true);
   70         }
   71 }
   72 
   73 static __inline bool
   74 refcount_release(volatile u_int *count)
   75 {
   76         u_int old;
   77 
   78         atomic_thread_fence_rel();
   79         old = atomic_fetchadd_int(count, -1);
   80         KASSERT(old > 0, ("refcount %p is zero", count));
   81         if (old > 1)
   82                 return (false);
   83 
   84         /*
   85          * Last reference.  Signal the user to call the destructor.
   86          *
   87          * Ensure that the destructor sees all updates.  The fence_rel
   88          * at the start of the function synchronized with this fence.
   89          */
   90         atomic_thread_fence_acq();
   91         return (true);
   92 }
   93 
   94 /*
   95  * This functions returns non-zero if the refcount was
   96  * incremented. Else zero is returned.
   97  *
   98  * A temporary hack until refcount_* APIs are sorted out.
   99  */
  100 static __inline __result_use_check bool
  101 refcount_acquire_if_not_zero(volatile u_int *count)
  102 {
  103         u_int old;
  104 
  105         old = *count;
  106         for (;;) {
  107                 KASSERT(old < UINT_MAX, ("refcount %p overflowed", count));
  108                 if (old == 0)
  109                         return (false);
  110                 if (atomic_fcmpset_int(count, &old, old + 1))
  111                         return (true);
  112         }
  113 }
  114 
  115 static __inline __result_use_check bool
  116 refcount_release_if_not_last(volatile u_int *count)
  117 {
  118         u_int old;
  119 
  120         old = *count;
  121         for (;;) {
  122                 KASSERT(old > 0, ("refcount %p is zero", count));
  123                 if (old == 1)
  124                         return (false);
  125                 if (atomic_fcmpset_int(count, &old, old - 1))
  126                         return (true);
  127         }
  128 }
  129 
  130 #endif  /* ! __SYS_REFCOUNT_H__ */

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