On 09/28/2016 03:05 PM, David Holmes wrote:
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.
That would be a nice property, yes.
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
Well, I can't find anything like that in JMM description. Could you
point me to it? Above example only reads once from hash. The code in
question is this:
if (hash == 0) { // 1st read
hash = ...
}
return hash; // 2nd read
And the bug requires the code to act like:
int temp2 = hash; // 2nd read
int temp1 = hash; // 1st read
if (temp1 == 0) {
return (hash = ...);
}
return temp2;
Is this allowed?
Regards, Peter
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
