Groups > comp.lang.java.programmer > #11475

Re: Volatile happens before question

From	"raphfrk@gmail.com" <raphfrk@gmail.com>
Newsgroups	comp.lang.java.programmer
Subject	Re: Volatile happens before question
Date	2012-01-18 12:24 -0800
Organization	http://groups.google.com
Message-ID	<652cb878-fb2f-4361-8f70-61bc493cebf6@r16g2000yqi.googlegroups.com> (permalink)
References	(14 earlier) <s96bklu5m72u.h0brrnp2d8fq$.dlg@40tude.net> <f8b028b6-9f5b-4838-bae3-9a1d1b0716d3@l1g2000vbc.googlegroups.com> <jf7239$k2b$1@dont-email.me> <726da9ce-57f4-4136-b50b-56a032aca196@f1g2000yqi.googlegroups.com> <jf78bd$s4v$1@dont-email.me>

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On Jan 18, 7:59 pm, markspace <-@.> wrote:
> Looking at his code a bit more carefully, I think it has problems, but not the same one.

I would be interested in his basic locking principle.

I think it has the same problem.  He is assuming that if he .gets() an
even number, then no write lock has happened.

However, that isn't guaranteed.

> The bit that confuses me is that he "protects" his map with a write counter:
>
>      writeCounter.getAndIncrement();
>      map.put(key, value);
>      writeCounter.getAndIncrement();
>
> And then allows the reader(s) into the critical section whenever the
> writeCounter is even (why not just decrement the counter after a write
> access?).

This is to handle the case where the reader reads the counter before
and after a write.  It needs to be able to tell something changed.

>      while (((save = writeCounter.get()) & 1) == 1);
>      value = map.get(key);
>
> So if a reader starts "first" and passes the first test
> (writeCounter==0), and then starts to access the map, and then a writer
> comes in and also begins to access the map... yeah you have two threads
> accessing the map at the same time.  This doesn't seem to work at all.

True, he is assuming that a .get() on an unstable map won't do
anything dangerous, like cause an infinite loop.

Also, a .get() might be setup so that it makes some housekeeping
changes to the map.

>
> (Hint: ConcurrentHashMap
>
> <http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Concurr...>)

I was just wondering about his basic method.

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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