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

     /*
      * Copyright (c) 2005 Jeffrey M. Hsu.  All rights reserved.
      *
      * This code is derived from software contributed to The DragonFly Project
      * by Jeffrey M. Hsu.
      *
      * 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.
     * 3. Neither the name of The DragonFly Project nor the names of its
     *    contributors may be used to endorse or promote products derived
     *    from this software without specific, prior written permission.
     *
     * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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
     * COPYRIGHT HOLDERS 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.
     *
     * $DragonFly: src/sys/sys/spinlock.h,v 1.5 2006/05/26 02:26:26 dillon Exp $
     */
    
    #ifndef _SYS_SPINLOCK_H_
    #define _SYS_SPINLOCK_H_
    
    /*
     * Note that the spinlock structure is retained whether we are SMP or not,
     * so structures using embedded spinlocks do not change size for SMP vs UP
     * builds.
     *
     * DragonFly spinlocks use a chasing counter.  A core desiring a spinlock
     * does a atomic_fetchadd_int() on countb and then waits for counta to
     * reach its value using MWAIT.  Releasing the spinlock involves an
     * atomic_add_int() on counta.  If no MWAIT is available the core can spin
     * waiting for the value to change which is still represented by a shared+ro
     * cache entry.
     */
    struct spinlock {
            int counta;
            int countb;
    };
    
    #define SPINLOCK_INITIALIZER(head)      { 0, 0 }
    
    #define SPINLOCK_SHARED                 0x80000000
    #define SPINLOCK_EXCLWAIT               0x00100000 /* high bits counter */
    
    #endif
    

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