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From "Boris (JIRA)" <j...@apache.org>
Subject [jira] Created: (HARMONY-6404) possible data-reordering in some hashCode-methods (e.g. String or URL)
Date Thu, 10 Dec 2009 20:30:18 GMT
possible data-reordering in some hashCode-methods (e.g. String or URL)
----------------------------------------------------------------------

                 Key: HARMONY-6404
                 URL: https://issues.apache.org/jira/browse/HARMONY-6404
             Project: Harmony
          Issue Type: Bug
            Reporter: Boris


First I have to say that I don't know if the Java Memory Model is relevant for Harmony, so
please reject the bug if this is not the case.

The current implementation of several of Harmony's classes that try to cache hashCode in an
optimized way are affected by a reordering-bug that can occur because of the way the hashCode
is cached and retrieved.
The problem is that the method contains no memory barriers, so that the VM may reorder the
data-reads at its own will. With an unlucky reordering the method could return 0 although
the actual hashCode is NOT 0.

Or to speak in code: This actual code:
        if (hashCode == 0) {
            // calculate hash
            hashCode = hash;
        }
        return hashCode;
could be reordered to something like
        return hashCode;
        if (hashCode == 0) {
            // calculate hash
            hashCode = hash;
        }
(which is of course no valid Java-code, but that is what the VM might do internally).

One common solution is to assign the field but then return the temp-value (in this case the
variable "hash") and NOT the field itself. So that the read can not be reordered. (This way
might be even faster because it may be one memory-read less)
Another solution would be to make hashCode volatile or to not cache the hashCode, but these
have a performance cost.

I'll attach a patch I wrote. I could not get harmony to compile, so these are untested.
BTW: This fix also fixes a more likely bug in BigInteger and BigDecimal: Callers of hashCode
might have seen half-constructed hashCodes there.

(This bug was found via the entry LANG-481 see there for some more details)

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