RFR: 8338566: Missing membar in ciEnv::get_or_create_exception before publishing handle

[Dean Long]

On Wed, 11 Sep 2024 14:17:30 GMT, Tobias Hartmann <thartmann at openjdk.org> wrote:

> Similar to [JDK-8251923](https://bugs.openjdk.org/browse/JDK-8251923), we need a store-store barrier before publishing a handle because otherwise another thread could observe the handle before it's fully initialized and read null from it. This affects architectures with a weak memory model like AArch64.
> 
> Unfortunately, this only happened twice in our testing and I was never able to reproduce it.
> 
> Thanks,
> Tobias

I agree, the ciEnv::*Exception_instance() methods look like a problem.  They are using shared static jobjects without synchronization.  If there is a race, one compiler thread can overwrite the existing handle that another compiler thread used to resolve the oop.  I think we need synchronization or compare-and-swap when assigning to the static handle to prevent a leak.  We may be OK without the load-acquire on the read side, if we rely on the data dependency like interpreter/generated code does.

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PR Comment: https://git.openjdk.org/jdk/pull/20950#issuecomment-2345118335