FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_refcount.c

     /*
      * Copyright (c) 2011 The DragonFly Project.  All rights reserved.
      *
      * This code is derived from software contributed to The DragonFly Project
      * by Matthew Dillon <dillon@backplane.com>
      *
      * 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.
     */
    /*
     * Implement helper routines for the refcount inlines in sys/refcount.h.
     *
     * These helpers implement the refcount_release_wakeup() and refcount_wait()
     * APIs for the non-trivial or race case.  The trivial non-race case is
     * handled by the inline in sys/refcount.h
     */
    
    #include <sys/param.h>
    #include <sys/systm.h>
    #include <sys/kernel.h>
    #include <sys/sysctl.h>
    #include <sys/thread.h>
    #include <sys/refcount.h>
    
    #include <machine/cpufunc.h>
    
    #include <sys/thread2.h>
    
    /*
     * Helper function to wait for a reference count to become zero.
     * We set REFCNTF_WAITING and sleep if the reference count is not zero.
     *
     * In the case where REFCNTF_WAITING is already set the atomic op validates
     * that it is still set after the tsleep_interlock() call.
     *
     * Users of this waiting API must use refcount_release_wakeup() to release
     * refs instead of refcount_release().  refcount_release() will not wake
     * up waiters.
     */
    void
    _refcount_wait(volatile u_int *countp, const char *wstr)
    {
            u_int n;
            int base_ticks = ticks;
    
            for (;;) {
                    n = *countp;
                    cpu_ccfence();
                    if (n == 0)
                            break;
                    if ((int)(ticks - base_ticks) >= hz*60 - 1) {
                            kprintf("warning: refcount_wait %s: long wait\n",
                                    wstr);
                            base_ticks = ticks;
                    }
                    KKASSERT(n != REFCNTF_WAITING); /* impossible state */
                    tsleep_interlock(countp, 0);
                    if (atomic_cmpset_int(countp, n, n | REFCNTF_WAITING))
                            tsleep(countp, PINTERLOCKED, wstr, hz*10);
            }
    }
    
    /*
     * This helper function implements the release-with-wakeup API.  It is
     * executed for the non-trivial case or if the atomic op races.
     *
     * On the i->0 transition is REFCNTF_WAITING is set it will be cleared
     * and a wakeup() will be issued.
     *
     * On any other transition we simply subtract (i) and leave the
     * REFCNTF_WAITING flag intact.
     *
     * This function returns TRUE(1) on the last release, whether a wakeup
     * occured or not, and FALSE(0) otherwise.
     *
    * NOTE!  (i) cannot be 0
    */
   int
   _refcount_release_wakeup_n(volatile u_int *countp, u_int i)
   {
           u_int n;
   
           for (;;) {
                   n = *countp;
                   cpu_ccfence();
                   if (n == (REFCNTF_WAITING | i)) {
                           if (atomic_cmpset_int(countp, n, 0)) {
                                   wakeup(countp);
                                   n = i;
                                   break;
                           }
                   } else {
                           KKASSERT(n != REFCNTF_WAITING); /* illegal state */
                           if (atomic_cmpset_int(countp, n, n - i))
                                   break;
                   }
           }
           return (n == i);
   }

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