RFR: 8373839: Disable JVM TI suspension during JNI critical regions

[David Holmes]

On Thu, 18 Dec 2025 01:24:42 GMT, David Holmes <dholmes at openjdk.org> wrote:

> To ensure JNI critical access to a raw array can't interfere with actions of the debugger, we disable JVM TI suspension whilst JNI critical access is active, as originally suggested by @fisk. We assume the debugger is being operated correctly (ie the thread using the raw array will be suspended), and that the critical section is short so as to not delay debugging too long. 
> 
> The mechanism for this already exists courtesy of the virtual thread support.
> 
> Testing:
>  - tiers 1 - 6 sanity

FTR I don't think "returning a copy" is an option - as I tried to point out in the other JBS issue. It would have to be based on there being a debugger attached, but with dynamic attach for the debugger you'd have to wait for an existing critical region to complete when attaching. If we have to wait for something then probably better just to wait for the thread in the critical region that we are trying to suspend. But doing a busy-wait (`while (t->in_critical()) SpinYield()`) in `SuspendThreadHandshakeClosure` seems a bit hackish. I had hoped the "disable suspend" mechanism would be simple and elegant.

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PR Comment: https://git.openjdk.org/jdk/pull/28884#issuecomment-3673153689