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 339860 2018-10-29 14:06:20Z hselasky $
   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 #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 int
   59 refcount_release(volatile u_int *count)
   60 {
   61         u_int old;
   62 
   63         atomic_thread_fence_rel();
   64         old = atomic_fetchadd_int(count, -1);
   65         KASSERT(old > 0, ("refcount %p is zero", count));
   66         if (old > 1)
   67                 return (0);
   68 
   69         /*
   70          * Last reference.  Signal the user to call the destructor.
   71          *
   72          * Ensure that the destructor sees all updates.  The fence_rel
   73          * at the start of the function synchronized with this fence.
   74          */
   75         atomic_thread_fence_acq();
   76         return (1);
   77 }
   78 
   79 /*
   80  * This functions returns non-zero if the refcount was
   81  * incremented. Else zero is returned.
   82  *
   83  * A temporary hack until refcount_* APIs are sorted out.
   84  */
   85 static __inline __result_use_check int
   86 refcount_acquire_if_not_zero(volatile u_int *count)
   87 {
   88         u_int old;
   89 
   90         old = *count;
   91         for (;;) {
   92                 KASSERT(old < UINT_MAX, ("refcount %p overflowed", count));
   93                 if (old == 0)
   94                         return (0);
   95                 if (atomic_fcmpset_int(count, &old, old + 1))
   96                         return (1);
   97         }
   98 }
   99 
  100 static __inline __result_use_check int
  101 refcount_release_if_not_last(volatile u_int *count)
  102 {
  103         u_int old;
  104 
  105         old = *count;
  106         for (;;) {
  107                 KASSERT(old > 0, ("refcount %p is zero", count));
  108                 if (old == 1)
  109                         return (0);
  110                 if (atomic_fcmpset_int(count, &old, old - 1))
  111                         return (1);
  112         }
  113 }
  114 
  115 #endif  /* ! __SYS_REFCOUNT_H__ */

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