RFR: 8369238: Allow virtual thread preemption on some common class initialization paths [v2]

[Andrew Haley]

On Tue, 14 Oct 2025 20:23:33 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> If a thread tries to initialize a class that is already being initialized by another thread, it will block until notified. Since at this blocking point there are native frames on the stack, a virtual thread cannot be unmounted and is pinned to its carrier. Besides harming scalability, this can, in some pathological cases, lead to a deadlock, for example, if the thread executing the class initialization method is blocked waiting for some unmounted virtual thread to run, but all carriers are blocked waiting for that class to be initialized.
>> 
>> As of JDK-8338383, virtual threads blocked in the VM on `ObjectMonitor` operations can be unmounted. Since synchronization on class initialization is implemented using `ObjectLocker`, we can reuse the same mechanism to unmount virtual threads on these cases too.
>> 
>> This patch adds support for unmounting virtual threads on some of the most common class initialization paths, specifically when calling `InterpreterRuntime::_new` (`new` bytecode), and `InterpreterRuntime::resolve_from_cache` for `invokestatic`, `getstatic` or `putstatic` bytecodes. In the future we might consider extending this mechanism to include initialization calls originating from native methods such as `Class.forName0`.
>> 
>> ### Summary of implementation
>> 
>> The ObjectLocker class was modified to not pin the continuation if we are coming from a preemptable path, which will be the case when calling `InstanceKlass::initialize_impl` from new method `InstanceKlass::initialize_preemptable`. This means that for these cases, a virtual thread can now be unmounted either when contending for the init_lock in the `ObjectLocker` constructor, or in the call to `wait_uninterruptibly`. Also, since the call to initialize a class includes a previous call to `link_class` which also uses `ObjectLocker` to protect concurrent calls from multiple threads, we will allow preemption there too.
>> 
>> If preempted, we will throw a pre-allocated exception which will get propagated with the `TRAPS/CHECK` macros all the way back to the VM entry point. The exception will be cleared and on return back to Java the virtual thread will go through the preempt stub and unmount. When running again, at the end of the thaw call we will identify this preemption case and redo the original VM call (either `InterpreterRuntime::_new` or `InterpreterRuntime::resolve_from_cache`). 
>> 
>> ### Notes
>> 
>> `InterpreterRuntime::call_VM_preemptable` used previously only for `InterpreterRuntime::mon...
>
> Patricio Chilano Mateo has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:
> 
>  - Merge branch 'master' into JDK-8369238
>  - RISC-V support
>  - Fix whitespaces
>  - v1

src/hotspot/cpu/aarch64/continuationFreezeThaw_aarch64.inline.hpp line 209:

> 207:     // the last_sp saved in the frame (remove possible alignment added while
> 208:     // thawing, see ThawBase::finish_thaw()). We also need to clear the last_sp
> 209:     // saved in the frame as it is not expected to be set in case we preempt again.

A bit stronger?
Suggestion:

    // saved in the frame because it must be clear if we freeze again.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27802#discussion_r2435147223