[concurrency-interest] Relativity of guarantees provided by volatile

Marko Topolnik mtopolnik at inge-mark.hr
Fri Aug 17 17:24:07 EDT 2012


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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