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/11.0/sys/sys/seq.h 285294 2015-07-08 22:21:25Z 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 static __inline bool
   73 seq_in_modify(seq_t seqp)
   74 {
   75 
   76         return (seqp & 1);
   77 }
   78 
   79 static __inline void
   80 seq_write_begin(seq_t *seqp)
   81 {
   82 
   83         MPASS(!seq_in_modify(*seqp));
   84         *seqp += 1;
   85         atomic_thread_fence_rel();
   86 }
   87 
   88 static __inline void
   89 seq_write_end(seq_t *seqp)
   90 {
   91 
   92         atomic_store_rel_int(seqp, *seqp + 1);
   93         MPASS(!seq_in_modify(*seqp));
   94 }
   95 
   96 static __inline seq_t
   97 seq_read(const seq_t *seqp)
   98 {
   99         seq_t ret;
  100 
  101         for (;;) {
  102                 ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
  103                 if (seq_in_modify(ret)) {
  104                         cpu_spinwait();
  105                         continue;
  106                 }
  107                 break;
  108         }
  109 
  110         return (ret);
  111 }
  112 
  113 static __inline seq_t
  114 seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
  115 {
  116 
  117         return (*seqp == oldseq);
  118 }
  119 
  120 static __inline seq_t
  121 seq_consistent(const seq_t *seqp, seq_t oldseq)
  122 {
  123 
  124         atomic_thread_fence_acq();
  125         return (seq_consistent_nomb(seqp, oldseq));
  126 }
  127 
  128 #endif  /* _KERNEL */
  129 #endif  /* _SYS_SEQ_H_ */

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