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: head/sys/sys/refcount.h 344855 2019-03-06 22:11:45Z jhb $
   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 #define KASSERT(exp, msg)       /* */
   40 #endif
   41 
   42 static __inline void
   43 refcount_init(volatile u_int *count, u_int value)
   44 {
   45 
   46         *count = value;
   47 }
   48 
   49 static __inline void
   50 refcount_acquire(volatile u_int *count)
   51 {
   52 
   53         KASSERT(*count < UINT_MAX, ("refcount %p overflowed", count));
   54         atomic_add_int(count, 1);
   55 }
   56 
   57 static __inline int
   58 refcount_release(volatile u_int *count)
   59 {
   60         u_int old;
   61 
   62         atomic_thread_fence_rel();
   63         old = atomic_fetchadd_int(count, -1);
   64         KASSERT(old > 0, ("refcount %p is zero", count));
   65         if (old > 1)
   66                 return (0);
   67 
   68         /*
   69          * Last reference.  Signal the user to call the destructor.
   70          *
   71          * Ensure that the destructor sees all updates.  The fence_rel
   72          * at the start of the function synchronized with this fence.
   73          */
   74         atomic_thread_fence_acq();
   75         return (1);
   76 }
   77 
   78 /*
   79  * This functions returns non-zero if the refcount was
   80  * incremented. Else zero is returned.
   81  */
   82 static __inline __result_use_check int
   83 refcount_acquire_if_not_zero(volatile u_int *count)
   84 {
   85         u_int old;
   86 
   87         old = *count;
   88         for (;;) {
   89                 KASSERT(old < UINT_MAX, ("refcount %p overflowed", count));
   90                 if (old == 0)
   91                         return (0);
   92                 if (atomic_fcmpset_int(count, &old, old + 1))
   93                         return (1);
   94         }
   95 }
   96 
   97 static __inline __result_use_check int
   98 refcount_release_if_not_last(volatile u_int *count)
   99 {
  100         u_int old;
  101 
  102         old = *count;
  103         for (;;) {
  104                 KASSERT(old > 0, ("refcount %p is zero", count));
  105                 if (old == 1)
  106                         return (0);
  107                 if (atomic_fcmpset_int(count, &old, old - 1))
  108                         return (1);
  109         }
  110 }
  111 
  112 #endif  /* ! __SYS_REFCOUNT_H__ */

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