RFR: 8364343: Virtual Thread transition management needs to be independent of JVM TI [v3]

[Patricio Chilano Mateo]

On Thu, 20 Nov 2025 22:17:53 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Patricio Chilano Mateo has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Add Alan's comment in VirtualThread
>
> src/hotspot/share/classfile/javaClasses.cpp line 1757:
> 
>> 1755:   jint* addr = java_thread->field_addr<jint>(_VTMS_transition_disable_count_offset);
>> 1756:   int val = AtomicAccess::load(addr);
>> 1757:   AtomicAccess::store(addr, val + 1);
> 
> Suggestion:
> 
>   AtomicAccess::inc(addr);

Same here.

> src/hotspot/share/classfile/javaClasses.cpp line 1764:
> 
>> 1762:   jint* addr = java_thread->field_addr<jint>(_VTMS_transition_disable_count_offset);
>> 1763:   int val = AtomicAccess::load(addr);
>> 1764:   AtomicAccess::store(addr, val - 1);
> 
> Suggestion:
> 
>   AtomicAccess::dec(addr);

I’d prefer to leave it as a plain store to avoid the unnecessary extra fence.

> src/hotspot/share/opto/runtime.hpp line 740:
> 
>> 738:     return vthread_transition_Type();
>> 739:   }
>> 740: 
> 
> I do not know C2 but this looks really strange - 4 different functions all return the same thing. ???

We need to define them because the `GEN_C2_STUB` macro will look for the type of the C function based on its name (`C2_STUB_TYPEFUNC(name)`), otherwise we get a compilation failure. The four C functions have the same type though so they all return `_vthread_transition_Type`.

> src/hotspot/share/runtime/handshake.cpp line 374:
> 
>> 372:     JavaThread* target = java_lang_Thread::thread(carrier_thread);
>> 373:     assert(target != nullptr, "");
>> 374:     // Technically there is need for a ThreadsListHandle since the target
> 
> Suggestion:
> 
>     // Technically there is no need for a ThreadsListHandle since the target
> 
> ?

Yes, fixed.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28361#discussion_r2561198741
PR Review Comment: https://git.openjdk.org/jdk/pull/28361#discussion_r2561198549
PR Review Comment: https://git.openjdk.org/jdk/pull/28361#discussion_r2561200538
PR Review Comment: https://git.openjdk.org/jdk/pull/28361#discussion_r2561202212