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Re: Volatile happens before question

From markspace <-@.>
Newsgroups comp.lang.java.programmer
Subject Re: Volatile happens before question
Date 2012-01-17 11:24 -0800
Organization A noiseless patient Spider
Message-ID <jf4hsl$tte$1@dont-email.me> (permalink)
References <09848313-2372-4c23-8f52-fa84c612c100@u32g2000yqe.googlegroups.com> <1e7b4r38kmn2f$.1kwqeem7f8lxb.dlg@40tude.net> <40835b59-f9af-4895-bb47-2f7dcf475e3a@k6g2000vbz.googlegroups.com>

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On 1/17/2012 8:54 AM, raphfrk@gmail.com wrote:

> However, if it places a sync actions later than another sync action,
> that still doesn't mean that the later one "happens after" the first
> one?


Specifically, no.

It wasn't clear to me that was the question you where asking. (Sorry 
about that!)  Your program was a little hard to read.  But the answer is 
that there is are no semantics for multi-threaded reads and rights if 
there is no happens-before relationship.

What you have there is called a data race.  Those are completely broken. 
  The reason is that hardware involved will optimize reads and writes 
aggressively.  (The same problem would occur even if you wrote this code 
in assembly language.  This is not a Java problem, it's a system 
problem.  The JLS actually just tells you how modern CPUs work.)

In this case, by creating a data race and reading b before you create 
the happens-before relationship, Thread 2 could see any value for b, 
even values that are neither 1 nor 0.  It's a random number generator. 
(And the more complete story is even worse.  I'm still looking for some 
sources to back that up though.)

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Thread

Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 04:04 -0800
  Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 08:17 -0800
    Re: Volatile happens before question markspace <-@.> - 2012-01-17 08:50 -0800
      Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 09:00 -0800
    Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 08:54 -0800
      Re: Volatile happens before question markspace <-@.> - 2012-01-17 11:24 -0800
    Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-17 14:49 -0800
      Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 15:49 -0800
      Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 17:14 -0800
        Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-17 18:17 -0800
          Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 16:45 -0800
  Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 08:18 -0800
    Re: Volatile happens before question markspace <-@.> - 2012-01-17 08:33 -0800
      Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 08:49 -0800
        Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 09:21 -0800
          Re: Volatile happens before question markspace <-@.> - 2012-01-17 10:10 -0800
            Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-17 10:28 -0800
              Re: Volatile happens before question markspace <-@.> - 2012-01-17 10:41 -0800
                Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-17 11:06 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-17 12:09 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-17 12:12 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-17 12:38 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-17 13:35 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-17 14:41 -0800
                Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-17 14:55 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-17 15:27 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 17:37 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 17:42 -0800
            Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 15:46 -0800
              Re: Volatile happens before question markspace <-@.> - 2012-01-17 16:14 -0800
                Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 20:01 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 20:37 -0800
                Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-17 21:52 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-17 23:47 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 00:08 -0800
                Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-18 09:04 -0800
                Re: Volatile happens before question Steven Simpson <ss@domain.invalid> - 2012-01-18 11:22 +0000
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 06:24 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 07:32 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 08:34 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-18 10:12 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 11:15 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-18 11:59 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 12:24 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-18 13:16 -0800
                Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-18 15:00 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-18 16:12 -0800
                Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-18 16:34 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-18 17:17 -0800
                Re: Volatile happens before question Patricia Shanahan <pats@acm.org> - 2012-01-18 19:03 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-18 15:30 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-18 16:10 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 08:18 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-19 09:40 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 10:10 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-19 10:39 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 12:13 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-19 13:31 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-19 16:14 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-19 21:16 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-20 03:23 -0800
                Re: Volatile happens before question "raphfrk@gmail.com" <raphfrk@gmail.com> - 2012-01-20 06:46 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-20 08:56 -0800
                Re: Volatile happens before question Lew <noone@lewscanon.com> - 2012-01-20 11:12 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 17:06 -0800
                Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-19 12:46 -0800
                Re: Volatile happens before question markspace <-@.> - 2012-01-19 13:38 -0800
                Re: Volatile happens before question Peter Duniho <NpOeStPeAdM@NnOwSlPiAnMk.com> - 2012-01-18 07:32 -0800
                Re: Volatile happens before question Lew <noone@lewscanon.com> - 2012-01-18 00:06 -0800
                Re: Volatile happens before question Knute Johnson <nospam@knutejohnson.com> - 2012-01-18 09:23 -0800
                Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 17:07 -0800
                Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 17:12 -0800
                Re: Volatile happens before question Joshua Maurice <joshuamaurice@gmail.com> - 2012-01-19 19:22 -0800
  Re: Volatile happens before question Daniel Pitts <newsgroup.nospam@virtualinfinity.net> - 2012-01-17 10:24 -0800

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