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

[David Holmes]

On Thu, 18 Dec 2025 13:32:53 GMT, Fredrik Bredberg <fbredberg 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
>
> src/hotspot/share/runtime/javaThread.hpp line 966:
> 
>> 964:   void exit_critical() {
>> 965:     assert(Thread::current() == this, "this must be current thread");
>> 966:     if (_jni_active_critical-- == 1) {
> 
> Suggestion:
> 
>     if (--_jni_active_critical == 0) {
> 
> I know my suggested line really does the same as yours, but this way I feel it's easier to see that we re-enable suspension when the number of active critical regions are down to 0. I.e. I prefer pre-decrement and compare to 0, vs. post-decrement and compare to 1.

I prefer to view the transitions on the current count as being zero and one, rather than the post-count being one and zero.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28884#discussion_r2632659327