On 11/Dec/2009 14:32, Egor Pasko wrote:
> On the 0x684 day of Apache Harmony Tim Ellison wrote:
>> On 11/Dec/2009 04:09, Vijay Menon wrote:
>>> Perhaps I'm missing some context, but I don't see any problem. I don't
>>> believe that this:
>>>
>>> if (hashCode == 0) {
>>> // calculate hash
>>> hashCode = hash;
>>> }
>>> return hashCode;
>>>
>>> can ever return 0 (assuming hash is non-zero), regardless of memory fences.
>>> The JMM won't allow visible reordering in a single threaded program.
>> I agree. In the multi-threaded case there can be a data race on the
>> hashCode, with the effect that the same hashCode value is unnecessarily,
>> but harmlessly, recalculated.
>
> Vijay, Tim, you are not 100% correct here.
>
> 1. there should be an extra restriction that the part "calculate hash"
> does not touch the hashCode field. Without that restriction more
> trivial races can happen as discussed in LANG-481.
>
> So we should assume this code:
>
> if (this.hashCode == 0) {
> int hash;
> if (this.hashCode == 0) {
> // Calculate using 'hash' only, not this.hashCode.
> this.hashCode = hash;
> }
> return this.hashCode;
> }
Yes, I guess I figured that was assumed :-)
Of course, there are lots of things you could put into the
"// Calculate..." section that would be unsafe. We should stick with
showing the non-abbreviated implementation to avoid ambiguity:
public int hashCode() {
if (hashCode == 0) {
if (count == 0) {
return 0;
}
int hash = 0;
for (int i = offset; i < count + offset; i++) {
hash = value[i] + ((hash << 5) - hash);
}
hashCode = hash;
}
return hashCode;
}
> where 'this.*' is always a memory reference while 'hash' is a thread private
> var.
>
> 2. DRLVM JIT indeed does not privatize field access to threads. It
> always loads fields from memory in original order. Hence this
> potential bug does not affect DRLVM .. now. But potentially it can
> start optimizing things this way because current behavior prevents
> a bunch of high level optimizations.
>
> 3. Jeremy Manson, being an expert in Java Memory Model claims [1] that
> such reordering is allowed theoretically. I.e. like this:
>
> int hash = this.hashCode;
> if (this.hashCode == 0) {
> hash = this.hashCode = // calculate hash
> }
> return hash;
>
> This is a correct single-threaded code. What happened here is a
> lengthy thread privatization of this.hashCode (again, does not happen
> in DRLVM). That is incorrect in multithreaded environment and needs
> extra synchronization/volatile/etc.
>
> 4. I do not know why a JIT would want to do that, i.e. two sequential
> reads from the same memory location. Sounds like a bit synthetic example.
...at which point a bunch of code probably would go wrong! So hopefully
it remains only a theoretical possibility.
Regards,
Tim
> [1] http://jeremymanson.blogspot.com/2008/12/benign-data-races-in-java.html
>
