RFR: 8198540: Dynalink leaks memory when generating type converters [v5]

Peter Levart peter.levart at gmail.com
Fri Jan 8 13:13:45 UTC 2021

On 1/8/21 2:03 PM, Peter Levart wrote:
> On Fri, 8 Jan 2021 12:23:36 GMT, Attila Szegedi <attila at openjdk.org> wrote:
>> IIUC, your changes mostly all flow from the decision to declare the fields as non-volatile; if they were still declared as volatile then it'd be impossible to observe null in them, I think (correct me if I'm wrong; it seems like you thought through this quite thoroughly) as then I don't see how could a volatile read happen before the initial volatile writes as the writes are part of the ClassValues constructor invocation and the reference to the ClassValues object is unavailable externally before the constructor completes. In any case, your approach definitely avoids any of these concerns so I'm inclined to go with it.
> It depends entirely on the guarantees of ClassValue and not on whether the fields are volatile or not. If ClassValue publishes the BiClassValues object via data race then even if the fields in BiClassValues are volatile and initialized in constructor, the publishing write in ClassValue could get reordered past

correction: past -> before

>   the volatile writes of the fields, so you could observe the fields uninitialized.
> I can't find in the spec of ClassValue any guarantees of ordering, but I guess the implementation does guarantee safe publication. So if you want to rely on ClassValue guaranteeing safe publication, you can pre-initialized the fields in constructor and code can assume they are never null even if they are not volatile.
> -------------
> PR: https://git.openjdk.java.net/jdk/pull/1918

To explain: Normal writes that appear in program order before a volatile 
write can not be observed to appear later than the volatile write. But 
normal writes that appear in program order after a volatile write can be 
observed to appear before the volatile write.

Regards, Peter

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