Integrated: 8343177: JFR: Remove critical section for thread id assignment

[Markus Grönlund]

On Tue, 29 Oct 2024 10:28:45 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

> Greetings,
> 
> With Loom, JFR had to change the assignment of unique thread IDs to threads. For JavaThreads, a dependency exists on the threadObj before the thread ID can be determined and assigned. We currently have a lazy assignment scheme that assigns on first use. Several threads, such as thread iterators and sampling threads, can issue the first use. Hence, a critical section protects assignments.
> 
> However, a critical section at this location makes it challenging to build robust signal handlers, for example, because we cannot read the thread ID.
> 
> We can remove this critical section with careful rearrangements, ensuring threads are only assigned a thread ID in thread state _thread_new (a thread state that at least the JFR sampler and the JFR iterators exclude).
> 
> The problem child is JNI attaching threads, created directly into state _thread_in_vm and added to the threads list before the creation and assignment of the threadObj. We can also manage this case by being careful when we sample such a thread, alternatively allowing for a thread ID of 0 or taking account of the 'attaching via jni' property. 
> 
> Testing: jdk_jfr Tier 1- 3, Stress testing
> 
> Thanks
> Markus

This pull request has now been integrated.

Changeset: 5995786d
Author:    Markus Grönlund <mgronlun at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/5995786dbd69ed11dd1cacb2a3ac86e3e6f43ab7
Stats:     79 lines in 10 files changed: 46 ins; 13 del; 20 mod

8343177: JFR: Remove critical section for thread id assignment

Reviewed-by: dholmes

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PR: https://git.openjdk.org/jdk/pull/21756