RFR: 8343177: JFR: Remove critical section for thread id assignment [v3]

[David Holmes]

On Thu, 31 Oct 2024 12:15:05 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
>
> Markus Grönlund has updated the pull request incrementally with one additional commit since the last revision:
> 
>   rename primordial to main thread

Marked as reviewed by dholmes (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/21756#pullrequestreview-2410005294