Integrated: 8322846: Running with -Djdk.tracePinnedThreads set can hang

[Alan Bateman]

On Tue, 2 Jan 2024 13:53:39 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> -Djdk.tracePinnedThreads is a debugging option that dates from early development in the loom repo to identify pinned threads. It has several issues and this tracing option will eventually be removed (use the JFR events instead). Several hangs have been reported when running with the system property set. The "hangs" stem from the onPinned callback executing while the virtual thread is in a transition state (typically parking). If the virtual parks while printing the stack trace then it works like a nested park where the thread state is never restored. Contention on the System.out can also lead to deadlock when there are platform and pinned virtual threads printing to System.out around the same time.
> 
> This PR brings over the changes from the loom repo to avoid these hangs. The changes mean the stack trace is only printed to System.out when the PrintStream lock can be acquired without blocking. It also restores the thread state after printing. An alternative to not printing traces would of course be to queue the traces so they are printed by another thread but this is just adding complexity for a debugging option that we want to go away.

This pull request has now been integrated.

Changeset: faa9c690
Author:    Alan Bateman <alanb at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/faa9c6909dda635eb008b9dada6e06fca47c17d6
Stats:     128 lines in 3 files changed: 100 ins; 12 del; 16 mod

8322846: Running with -Djdk.tracePinnedThreads set can hang

Reviewed-by: jpai

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PR: https://git.openjdk.org/jdk/pull/17221