RFR: 8265753: Remove manual JavaThread transitions to blocked [v5]

[David Holmes]

On Mon, 24 May 2021 02:08:43 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Ah, you mean the pre-exiting missing of a storestore in TBIVM? Fixed.
>> But we have more of those.
>> 
>> Note that on TSO it's not needed, since both are volatile and will not be re-ordered by compiler.
>> Yes, the platform which actually do care about avoiding release for performance have it (because it's needed).
>> So yes it should always be there, good RFE!
>
> Okay I will file a RFE to get the memory ordering semantics of set_thread_state cleaned up so we don't have any missing barriers where needed, nor redundant barriers.
> 
> Note that the expectation is that we write the code for the loosest possible memory model with all barriers expressed in the code, and it is then up to the implementation of those barriers to reduce it to nothing if not needed on a given platform.
> 
> Thanks,
> David

I have filed JDK-8267585, but I realize now what you are saying. The existing code handles the barrier inconsistently. In most places it is only inside set_thread_state, for those platforms that need it. Elsewhere it is explicit in the code that calls set_thread_state - which leads to redundant barriers on the platforms that need them. I mistakenly flagged the implicit cases as a bug based on the existence of the explicit cases - but that is not the case. Sorry for the confusion.

The additional storestore barriers you added before the call to set_thread_state can be removed, and I will tidy this up using the new RFE.

Thanks,
David

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PR: https://git.openjdk.java.net/jdk/pull/3875