[concurrency-interest] Relativity of guarantees provided by volatile

Yuval Shavit yshavit at akiban.com
Fri Aug 17 19:10:15 EDT 2012


The more I think about Marko's problem, the more it bugs me. I don't think
it's that the three writes can be reduced to the last one -- it's that
there's no requirement for this to ever be seen. That is, given threads t1
and t2, such that by clock time:

   (A) t1: write to volatileField
   (B) t2:  read from volatileField

The JLS states that if A happens-before B, then B must see A. It defines
hb(A, B) as synchronized-with(A, B), which is true if t2 is subsequent to
t1. But "subsequent" isn't defined as clock time. It's left undefined in
JLS 17, except for twice in 17.4.4 in which it's defined as "according to
the synchronization order" -- which seems like a tautology!

In other words, I think it comes down to the definition of "subsequent,"
which is undefined. There's nothing preventing a JVM from deciding that
even though A happened before B in clock time, A is subsequent to B.

On Fri, Aug 17, 2012 at 6:58 PM, David Holmes <davidcholmes at aapt.net.au>wrote:

> Hi Marko,
>
> I think the "surprise" is only in the way you formulated this. Said another
> way a write takes a finite amount of time from when the instruction starts
> to execute to when the store is actually available for a read to see.
> (Similarly a read takes a finite amount of time.) So depending on those two
> times a read and write that happen "around the same time" may appear to
> have
> occurred in either order. But when you program with threads you never know
> the relative interleavings (or should never assume) so it makes no
> difference how the program perceives the order compared to how some
> external
> observer might perceive it.
>
> As for your optimization to "chunk" volatile writes, I don't see a problem
> here if you are basically asking if given:
>
> w = 1;  // w is volatile
> w = 2;
> w = 3;
>
> that this could be reduced to the last write alone? I see no reason why
> not.
> Without some additional coordination between a reader thread and the writer
> thread, reading w==3 is a legitimate outcome. If you are thinking about how
> the hardware might chunk things then that is a different matter. We have to
> use the hardware in a way that complies with the memory model - if the
> hardware can't comply then you can't run Java on it.
>
> David Holmes
> ------------
>
> > -----Original Message-----
> > From: concurrency-interest-bounces at cs.oswego.edu
> > [mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Marko
> > Topolnik
> > Sent: Saturday, 18 August 2012 7:24 AM
> > To: concurrency-interest at cs.oswego.edu
> > Subject: [concurrency-interest] Relativity of guarantees provided by
> > volatile
> >
> >
> > Consider the following synchronization order of a program
> > execution involving a total of two threads, R and W:
> >
> > - thread R begins;
> >
> > - thread R reads a volatile int sharedVar several times. Each
> > time it reads the value 0;
> >
> > - thread R completes;
> >
> > - thread W begins;
> >
> > - thread W writes the sharedVar several times. Each time it
> > writes the value 1;
> >
> > - thread W completes.
> >
> > Now consider the wall-clock timing of the events:
> >
> > - thread R reads 0 at t = {1, 4, 7, 10};
> > - thread W writes 1 at t = {0, 3, 6, 9}.
> >
> > As far as the Java Memory Model is concerned, there is no
> > contradiction between the synchronization order and the
> > wall-clock times, as the JMM is wall-clock agnostic. However, I
> > have yet to meet a single Java professional who wouldn't at least
> > be very surprised to hear that the specification allows this.
> >
> > I understand that the SMP architecture that dominates the world
> > of computing today practically never takes these liberties and
> > makes the volatile writes visible almost instantaneously. This
> > may change at any time, however, especially with the advent of
> > massively parrallel architectures that seem to be the future. For
> > example, an optimization technique may choose to chunk many
> > volatile writes together and make them visible in a single bulk
> > operation. This can be safely done as long as there are no
> > intervening read-y actions (targets of the synchronizes-with
> > edges as defined by JLS/JSE7 17.4.4).
> >
> > Now, my questions are:
> >
> > 1. Is there a loophole in my reasoning?
> >
> > 2. If there is no loophole, is there anything to worry about,
> > given that practically 100% developers out there consider as
> > guaranteed something that isn't?
> >
> >
> > -Marko
> >
> >
> >
> >
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> >
>
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