RFR: 8308341: JNI_GetCreatedJavaVMs returns a partially initialized JVM [v2]

[David Holmes]

On Tue, 30 May 2023 14:39:21 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>>> I have one question though: why wouldn't it be enough to move `vm_created = 1` from where we had it before, down to where we now have `vm_created = COMPLETE` ?
>> 
>> Because we need to prevent two threads from concurrently loading and initializing a VM in the same process.
>
>> > I have one question though: why wouldn't it be enough to move `vm_created = 1` from where we had it before, down to where we now have `vm_created = COMPLETE` ?
>> 
>> Because we need to prevent two threads from concurrently loading and initializing a VM in the same process.
> 
> Wouldn't it be less complex to use a lock or some "init once" mechanism (pthread_once) instead of a 3 stage atomic field? Do we really need to know whether the process is in the middle of initialization for any reason other than whether it's actually done?
> 
> Just to clarify - I'm OK with the fix you have proposed, I was just curious if you considered any other alternatives.

Thanks for the review @gerard-ziemski 

> Just to clarify - I'm OK with the fix you have proposed, I was just curious if you considered any other alternatives.

Atomics are the simplest cross-platform solution available at this stage of VM init (which is "nothing is initialized yet") - even Atomic use is limited to Atomic::xchg as we can't require any stubs. We have no VM locks available and anything external to the VM (like pthread_once) would require as OS-specific solution in shared code.

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PR Comment: https://git.openjdk.org/jdk/pull/14139#issuecomment-1569223117