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/11.1/sys/sys/seq.h 312714 2017-01-24 19:39:24Z 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  *      }
   63  *      foo(lobj);
   64  */             
   65 
   66 /* A hack to get MPASS macro */
   67 #include <sys/lock.h>
   68 
   69 #include <machine/cpu.h>
   70 
   71 static __inline bool
   72 seq_in_modify(seq_t seqp)
   73 {
   74 
   75         return (seqp & 1);
   76 }
   77 
   78 static __inline void
   79 seq_write_begin(seq_t *seqp)
   80 {
   81 
   82         MPASS(!seq_in_modify(*seqp));
   83         *seqp += 1;
   84         atomic_thread_fence_rel();
   85 }
   86 
   87 static __inline void
   88 seq_write_end(seq_t *seqp)
   89 {
   90 
   91         atomic_store_rel_int(seqp, *seqp + 1);
   92         MPASS(!seq_in_modify(*seqp));
   93 }
   94 
   95 static __inline seq_t
   96 seq_read(const seq_t *seqp)
   97 {
   98         seq_t ret;
   99 
  100         for (;;) {
  101                 ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
  102                 if (seq_in_modify(ret)) {
  103                         cpu_spinwait();
  104                         continue;
  105                 }
  106                 break;
  107         }
  108 
  109         return (ret);
  110 }
  111 
  112 static __inline seq_t
  113 seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
  114 {
  115 
  116         return (*seqp == oldseq);
  117 }
  118 
  119 static __inline seq_t
  120 seq_consistent(const seq_t *seqp, seq_t oldseq)
  121 {
  122 
  123         atomic_thread_fence_acq();
  124         return (seq_consistent_nomb(seqp, oldseq));
  125 }
  126 
  127 #endif  /* _KERNEL */
  128 #endif  /* _SYS_SEQ_H_ */

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