RFR: 8166842: String.hashCode() has a non-benign data race

[David Holmes]

On 28/09/2016 10:44 PM, Peter Levart wrote:
> Hi,
>
> According to discussion here:
>
> http://cs.oswego.edu/pipermail/concurrency-interest/2016-September/015414.html
>
>
> it seems compact strings introduced (at least theoretical) non-benign
> data race into String.hasCode() method.
>
> Here is a proposed patch:
>
> http://cr.openjdk.java.net/~plevart/jdk9-dev/8166842_String.hashCode/webrev.01/

I'm far from convinced that the bug exists - theoretical or otherwise - 
but the "fix" is harmless.

When we were working on JSR-133 one of the conceptual models is that 
every write to a variable goes into the set of values that a read may 
potentially return (so no out-of-thin-air for example). happens-before 
establishes constraints on which value can legally be returned - the 
most recent. An additional property was that once a value was returned, 
a later read could not return an earlier value - in essence once a read 
returns a given value, all earlier written values are removed from the 
set of potential values that can be read.

Your bug requires that the code act as-if written:

int temp = hash;
if (temp == 0) {
    hash = ...
}
return temp;

and I do not believe that is allowed.

David

>
> For the bug:
>
>     https://bugs.openjdk.java.net/browse/JDK-8166842
>
>
>
> JDK 8 did not have this problem, so no back-porting necessary.
>
> Regards, Peter
>