[concurrency-interest] Relativity of guarantees provided byvolatile

David Holmes davidcholmes at aapt.net.au
Fri Aug 17 19:03:48 EDT 2012


Nathan,

Is there a synchronization order here that indicates that the read occurs
after the write? "subsequent" is not defined by wall-clock time as
externally observed.

And to batch responses :) Hotspot's implementation requires that all writes
will become visible without need for explicit barriers/fences to force that.

David
  -----Original Message-----
  From: concurrency-interest-bounces at cs.oswego.edu
[mailto:concurrency-interest-bounces at cs.oswego.edu]On Behalf Of Nathan
Reynolds
  Sent: Saturday, 18 August 2012 8:47 AM
  To: concurrency-interest at cs.oswego.edu
  Subject: Re: [concurrency-interest] Relativity of guarantees provided
byvolatile


  > I have yet to meet a single Java professional who wouldn't at least be
very surprised to hear that the specification allows this.

  Add me to the list of surprised.

  Here's an excerpt from the specification.

  A write to a volatile variable (§8.3.1.4) v synchronizes-with all
subsequent reads of v by any thread (where subsequent is defined according
to the synchronization order).

  http://docs.oracle.com/javase/specs/jls/se5.0/html/memory.html#17.4.4

  Doesn't this mean that R will always read 1 since the read occurs after
the write?


  Nathan Reynolds | Consulting Member of Technical Staff | 602.333.9091
  Oracle PSR Engineering | Server Technology

  On 8/17/2012 2:24 PM, Marko Topolnik wrote:

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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