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  *
    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: stable/12/sys/sys/refcount.h 370080 2021-07-05 12:49:30Z mjg $
   28  */
   29 
   30 #ifndef __SYS_REFCOUNT_H__
   31 #define __SYS_REFCOUNT_H__
   32 
   33 #include <sys/limits.h>
   34 #include <machine/atomic.h>
   35 
   36 #ifdef _KERNEL
   37 #include <sys/systm.h>
   38 #else
   39 #include <stdbool.h>
   40 #define KASSERT(exp, msg)       /* */
   41 #endif
   42 
   43 static __inline void
   44 refcount_init(volatile u_int *count, u_int value)
   45 {
   46 
   47         *count = value;
   48 }
   49 
   50 static __inline void
   51 refcount_acquire(volatile u_int *count)
   52 {
   53 
   54         KASSERT(*count < UINT_MAX, ("refcount %p overflowed", count));
   55         atomic_add_int(count, 1);
   56 }
   57 
   58 static __inline __result_use_check bool
   59 refcount_acquire_checked(volatile u_int *count)
   60 {
   61         u_int lcount;
   62 
   63         for (lcount = *count;;) {
   64                 if (__predict_false(lcount + 1 < lcount))
   65                         return (false);
   66                 if (__predict_true(atomic_fcmpset_int(count, &lcount,
   67                     lcount + 1) == 1))
   68                         return (true);
   69         }
   70 }
   71 
   72 static __inline bool
   73 refcount_releasen(volatile u_int *count, u_int n)
   74 {
   75         u_int old;
   76 
   77         atomic_thread_fence_rel();
   78         old = atomic_fetchadd_int(count, -n);
   79         KASSERT(old > 0, ("refcount %p is zero", count));
   80         if (old > n)
   81                 return (false);
   82 
   83         /*
   84          * Last reference.  Signal the user to call the destructor.
   85          *
   86          * Ensure that the destructor sees all updates. This synchronizes with
   87          * release fences from all routines which drop the count.
   88          */
   89         atomic_thread_fence_acq();
   90         return (true);
   91 }
   92 
   93 static __inline bool
   94 refcount_release(volatile u_int *count)
   95 {
   96 
   97         return (refcount_releasen(count, 1));
   98 }
   99 
  100 /*
  101  * This functions returns non-zero if the refcount was
  102  * incremented. Else zero is returned.
  103  *
  104  * A temporary hack until refcount_* APIs are sorted out.
  105  */
  106 static __inline __result_use_check bool
  107 refcount_acquire_if_not_zero(volatile u_int *count)
  108 {
  109         u_int old;
  110 
  111         old = *count;
  112         for (;;) {
  113                 KASSERT(old < UINT_MAX, ("refcount %p overflowed", count));
  114                 if (old == 0)
  115                         return (false);
  116                 if (atomic_fcmpset_int(count, &old, old + 1))
  117                         return (true);
  118         }
  119 }
  120 
  121 static __inline __result_use_check bool
  122 refcount_release_if_not_last(volatile u_int *count)
  123 {
  124         u_int old;
  125 
  126         old = *count;
  127         for (;;) {
  128                 KASSERT(old > 0, ("refcount %p is zero", count));
  129                 if (old == 1)
  130                         return (false);
  131                 if (atomic_fcmpset_int(count, &old, old - 1))
  132                         return (true);
  133         }
  134 }
  135 
  136 #endif  /* ! __SYS_REFCOUNT_H__ */

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