[foreign-memaccess+abi] RFR: 8270851: Logic for attaching/detaching native threads could be improved

[David Holmes]

On Fri, 16 Jul 2021 15:59:18 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> For a complete description of the issue, please refer to:
> 
> https://bugs.openjdk.java.net/browse/JDK-8270851
> 
> This patch makes the logic for detaching native threads more lazy - by only doing the detach when a native thread has completed. This is achieved by using some thread local storage, which is used to keep track of the Java thread associated to a given native thread. If, by the time the thread local storage is destroyed, we see that a Java thread has been attached, we do a detach operation. This trick effectively minimizes the number of Thread instances created when interacting with multi-threaded native code.
> 
> This patch also tweaks the logic for attaching native threads to the VM by using the "daemon" attach variant. That is, native threads registered against the VM (because of Panama upcalls) should not prevent the JVM to shut down in an orderly fashion (in cases where the native threads might outlive the JVM).

This change takes you from one extreme to another and so will introduce different problems. Changing to using daemon threads tries to avoid the obvious problem - that a non-terminating native thread causes the VM to never exit. But now the VM can exit in the middle of an upcall unless something else is keeping the VM alive. No implicit lifecycle management policy can suit all cases here.

David

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PR: https://git.openjdk.java.net/panama-foreign/pull/570