sys/kern/kern_spinlock.c

   1 /*
   2  * Copyright (c) 2005 Jeffrey M. Hsu.  All rights reserved.
   3  *
   4  * This code is derived from software contributed to The DragonFly Project
   5  * by Jeffrey M. Hsu.
   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  * 3. Neither the name of The DragonFly Project nor the names of its
  16  *    contributors may be used to endorse or promote products derived
  17  *    from this software without specific, prior written permission.
  18  *
  19  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  20  * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  21  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
  22  * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
  23  * COPYRIGHT HOLDERS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
  24  * INCIDENTAL, SPECIAL, EXEMPLARY OR CONSEQUENTIAL DAMAGES (INCLUDING,
  25  * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  26  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
  27  * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  28  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
  29  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  30  * SUCH DAMAGE.
  31  *
  32  * $DragonFly: src/sys/kern/kern_spinlock.c,v 1.2 2005/09/16 21:50:12 dillon Exp $
  33  */
  34
  35 #include <sys/param.h>
  36 #include <sys/types.h>
  37 #include <machine/atomic.h>
  38 #include <sys/spinlock.h>
  39 #include <sys/spinlock2.h>
  40
  41 #define BACKOFF_INITIAL 1
  42 #define BACKOFF_LIMIT   256
  43
  44 #ifdef SMP
  45
  46 void
  47 spin_lock_contested(struct spinlock *mtx)
  48 {
  49         int i, backoff = BACKOFF_INITIAL;
  50
  51         do {
  52                 /* exponential backoff to reduce contention */
  53                 for (i = 0; i < backoff; i++)
  54                         cpu_nop();
  55                 if (backoff < BACKOFF_LIMIT)
  56                         backoff <<= 1;
  57                 /* do non-bus-locked check first */
  58         } while (mtx->lock != 0 || atomic_swap_int(&mtx->lock, 1) != 0);
  59 }
  60
  61 #endif