RFR: 8369250: Assess and remedy any unsafe usage of the Semaphore used by NonJavaThread::List

[Stefan Karlsson]

On Tue, 7 Oct 2025 06:13:51 GMT, David Holmes <dholmes at openjdk.org> wrote:

> The `NonJavaThread::List` has a statically allocated instance which contains a `SingleWriterSynchronizer`, which contains a `Semaphore`. The `VMThread` can try to remove itself from the list after the static destructors have executed, leading to an assertion failure for the `Semaphore` but potentially it could crash as the `List` itself gets deallocated..  Simplest fix is to change the `List` instance to a `DeferredStatic` such that it is never destroyed.
> 
> Testing:
>  - tiers 1-3 (sanity)
> 
> Thanks.

src/hotspot/share/runtime/nonJavaThread.hpp line 49:

> 47:   }
> 48: 
> 49:   NonJavaThread* volatile _next;

Is there a reason why the code couldn't stay in the .cpp file?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27664#discussion_r2409545857