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

     /*-
      * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
      *
      * Copyright (c) 2005 John Baldwin <jhb@FreeBSD.org>
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
      * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     * $FreeBSD: head/sys/sys/refcount.h 344855 2019-03-06 22:11:45Z jhb $
     */
    
    #ifndef __SYS_REFCOUNT_H__
    #define __SYS_REFCOUNT_H__
    
    #include <sys/limits.h>
    #include <machine/atomic.h>
    
    #ifdef _KERNEL
    #include <sys/systm.h>
    #else
    #define KASSERT(exp, msg)       /* */
    #endif
    
    static __inline void
    refcount_init(volatile u_int *count, u_int value)
    {
    
            *count = value;
    }
    
    static __inline void
    refcount_acquire(volatile u_int *count)
    {
    
            KASSERT(*count < UINT_MAX, ("refcount %p overflowed", count));
            atomic_add_int(count, 1);
    }
    
    static __inline int
    refcount_release(volatile u_int *count)
    {
            u_int old;
    
            atomic_thread_fence_rel();
            old = atomic_fetchadd_int(count, -1);
            KASSERT(old > 0, ("refcount %p is zero", count));
            if (old > 1)
                    return (0);
    
            /*
             * Last reference.  Signal the user to call the destructor.
             *
             * Ensure that the destructor sees all updates.  The fence_rel
             * at the start of the function synchronized with this fence.
             */
            atomic_thread_fence_acq();
            return (1);
    }
    
    /*
     * This functions returns non-zero if the refcount was
     * incremented. Else zero is returned.
     */
    static __inline __result_use_check int
    refcount_acquire_if_not_zero(volatile u_int *count)
    {
            u_int old;
    
            old = *count;
            for (;;) {
                    KASSERT(old < UINT_MAX, ("refcount %p overflowed", count));
                    if (old == 0)
                            return (0);
                    if (atomic_fcmpset_int(count, &old, old + 1))
                            return (1);
            }
    }
    
    static __inline __result_use_check int
    refcount_release_if_not_last(volatile u_int *count)
    {
           u_int old;
   
           old = *count;
           for (;;) {
                   KASSERT(old > 0, ("refcount %p is zero", count));
                   if (old == 1)
                           return (0);
                   if (atomic_fcmpset_int(count, &old, old - 1))
                           return (1);
           }
   }
   
   #endif  /* ! __SYS_REFCOUNT_H__ */

Cache object: ae77ece433b21abf88142b9db3f3f133