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Re: reading the JLS (17.4.5)

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On 12/21/2011 12:37 AM, Andreas Leitgeb wrote:
> Lew<lewbloch@gmail.com>  wrote:
>> Andreas Leitgeb wrote:
>>> Given some (simplified) sample code:
>>>    class Test {
>>>       /*non-vol*/ int n1;
>>>       volatile    int v1;
>>>
>>>       void t1() {
>>>         n1 = 1;
>>>         v1 = n1; // v1 uses n1
>>>       }
>>>
>>>       void t2() {
>>>         int r1=v1, r2=r1*n1; // r2 uses r1
>>>         assert ! (r1 == 1&&  r2 != 1) : "huh?";
>>>       }
>>>    }
>>> Two threads T1 and T2 may at some point run t1() and t2()
>>> respectively, then I should expect, per transitivity of
>> You have to establish a /happens-before/ between the invocations
>> of t1() an t2().
>> If two threads begin to execute the methods, you cannot guarantee
>> that t1() will /happen-before/ t2(), so the read of 'v1' in the
>> latter could result in the default value.
>
> Perhaps you found the/a sore spot of my (mis?)understanding.
>
> *iff* a *volatile* read gets to see the result of a *volatile*
> write, then doesn't that say anything about that the write must
> have "happened-before" the read?
>
> In my example, I didn't mean to imply, that r1 would always
> turn out to be 1. But *if* it does, then could anything be
> assumed about r2 ?
>
> If the write to v1 (but not to n1) in t1() were in a
> synchronized(this)-block, and so were the read of v1
> (but not n1) in t2(), then would anything be implied
> about r2 *if* observing r1 == 1 in T2?
>
> In practice, I think that establishing a happens-before
> based on observation of a read's value is quite essential,
> but I may of course be wrong here. I don't even see how one
> could determine the order in which synchronized blocks for
> a common monitor are actually passed through by different
> threads, other than by observing reads.
>

I think we need to distinguish between two things. I'm going to copy
some notation from Lew. /happens-before/, printed in italics in the JLS,
is not an observation about what really happened, but a requirement of
the form "X must appear to happen before Y", with a definition of what
that means in terms e.g. of the results of read operations.

That is entirely distinct from the actual order of operations. If a read
sees the result of a write, that write must have really happened before
(no slashes, no italics) the read, but that does not prove the existence
of a /happens-before/ relationship between them.

However, the volatile reads and writes are synchronization actions, so
they are ordered by the synchronization order, and whichever comes first
in the synchronization order /happens-before/ the other. I agree with
your assertion.

Patricia

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reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-18 12:10 +0000
  Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-18 06:57 -0800
    Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-20 17:54 +0000
      Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-20 10:50 -0800
        Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-20 12:12 -0800
          Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-21 08:54 +0000
            Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-21 10:56 -0800
              Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-21 12:02 -0800
  Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-18 09:21 -0800
    Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-20 18:35 +0000
      Re: reading the JLS (17.4.5) Lew <lewbloch@gmail.com> - 2011-12-20 19:08 -0800
        Re: reading the JLS (17.4.5) Andreas Leitgeb <avl@gamma.logic.tuwien.ac.at> - 2011-12-21 08:37 +0000
          Re: reading the JLS (17.4.5) Patricia Shanahan <pats@acm.org> - 2011-12-21 10:46 -0800
          Re: reading the JLS (17.4.5) markspace <-@.> - 2011-12-21 12:09 -0800

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