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/seq.h

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    1 /*-
    2  * Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
    3  *
    4  * Redistribution and use in source and binary forms, with or without
    5  * modification, are permitted provided that the following conditions
    6  * are met:
    7  * 1. Redistributions of source code must retain the above copyright
    8  *    notice, this list of conditions and the following disclaimer.
    9  * 2. Redistributions in binary form must reproduce the above copyright
   10  *    notice, this list of conditions and the following disclaimer in the
   11  *    documentation and/or other materials provided with the distribution.
   12  *
   13  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   14  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   15  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   16  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   17  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   18  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   19  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   20  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   21  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   22  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   23  * SUCH DAMAGE.
   24  *
   25  * $FreeBSD: releng/10.4/sys/sys/seq.h 273109 2014-10-14 21:19:23Z mjg $
   26  */
   27 
   28 #ifndef _SYS_SEQ_H_
   29 #define _SYS_SEQ_H_
   30 
   31 #ifdef _KERNEL
   32 #include <sys/systm.h>
   33 #endif
   34 #include <sys/types.h>
   35 
   36 /*
   37  * seq_t may be included in structs visible to userspace
   38  */
   39 typedef uint32_t seq_t;
   40 
   41 #ifdef _KERNEL
   42 
   43 /*
   44  * Typical usage:
   45  *
   46  * writers:
   47  *      lock_exclusive(&obj->lock);
   48  *      seq_write_begin(&obj->seq);
   49  *      .....
   50  *      seq_write_end(&obj->seq);
   51  *      unlock_exclusive(&obj->unlock);
   52  *
   53  * readers:
   54  *      obj_t lobj;
   55  *      seq_t seq;
   56  *
   57  *      for (;;) {
   58  *              seq = seq_read(&gobj->seq);
   59  *              lobj = gobj;
   60  *              if (seq_consistent(&gobj->seq, seq))
   61  *                      break;
   62  *              cpu_spinwait();
   63  *      }
   64  *      foo(lobj);
   65  */             
   66 
   67 /* A hack to get MPASS macro */
   68 #include <sys/lock.h>
   69 
   70 #include <machine/cpu.h>
   71 
   72 /*
   73  * This is a temporary hack until memory barriers are cleaned up.
   74  *
   75  * atomic_load_acq_int at least on amd64 provides a full memory barrier,
   76  * in a way which affects perforance.
   77  *
   78  * Hack below covers all architectures and avoids most of the penalty at least
   79  * on amd64.
   80  */
   81 static __inline int
   82 atomic_load_acq_rmb_int(volatile u_int *p)
   83 {
   84         volatile u_int v;
   85 
   86         v = *p;
   87         atomic_load_acq_int(&v);
   88         return (v);
   89 }
   90 
   91 static __inline bool
   92 seq_in_modify(seq_t seqp)
   93 {
   94 
   95         return (seqp & 1);
   96 }
   97 
   98 static __inline void
   99 seq_write_begin(seq_t *seqp)
  100 {
  101 
  102         MPASS(!seq_in_modify(*seqp));
  103         atomic_add_acq_int(seqp, 1);
  104 }
  105 
  106 static __inline void
  107 seq_write_end(seq_t *seqp)
  108 {
  109 
  110         atomic_add_rel_int(seqp, 1);
  111         MPASS(!seq_in_modify(*seqp));
  112 }
  113 
  114 static __inline seq_t
  115 seq_read(seq_t *seqp)
  116 {
  117         seq_t ret;
  118 
  119         for (;;) {
  120                 ret = atomic_load_acq_rmb_int(seqp);
  121                 if (seq_in_modify(ret)) {
  122                         cpu_spinwait();
  123                         continue;
  124                 }
  125                 break;
  126         }
  127 
  128         return (ret);
  129 }
  130 
  131 static __inline seq_t
  132 seq_consistent(seq_t *seqp, seq_t oldseq)
  133 {
  134 
  135         return (atomic_load_acq_rmb_int(seqp) == oldseq);
  136 }
  137 
  138 static __inline seq_t
  139 seq_consistent_nomb(seq_t *seqp, seq_t oldseq)
  140 {
  141 
  142         return (*seqp == oldseq);
  143 }
  144 
  145 #endif  /* _KERNEL */
  146 #endif  /* _SYS_SEQ_H_ */

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