RFR: 8373366: HandshakeState should disallow suspend ops for disabler threads [v4]

[Serguei Spitsyn]

On Wed, 17 Dec 2025 08:14:41 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> An asynchronous handshake operation (`ThreadSelfSuspensionHandshakeClosure`) can be installed when the target thread is not in a `MountUnmountDisabler` scope. But the target thread can enter such scope by the time the operation is self-processed by the target thread.
>> 
>> This is fixed by a small tweak in the function
>> `HandshakeOperation* HandshakeState::get_op_for_self(bool allow_suspend, bool check_async_exception)`.
>> The tweak is to skip a `HandshakeOperation` if `_handshakee->is_vthread_transition_disabler() == true`, so the same temporary suspension disabling mechanism would be used as for `_handshakee->is_disable_suspend() == true`.
>> 
>> All other changes are to move the `is_vthread_transition_disabler()` out of DEBUG to product.
>> 
>> Testing:
>>  - In progress: mach5 tiers 1-6
>
> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review: replace fix with just one assert

Replaced the fix with one simple assert. It did not fire in mach5 tiers 1-6. Will do more test runs though.
The suggestion is to keep this assert until we discover it ever failing. If it happens then we could return to the original fix.

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PR Comment: https://git.openjdk.org/jdk/pull/28740#issuecomment-3664195008