RFR: 8269466: Factor out the common code for initializing and starting internal VM JavaThreads [v3]

[Coleen Phillimore]

On Thu, 1 Jul 2021 04:18:28 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Please see the JBS issue for more details, but basically we have 8 different kinds of internal VM JavaThreads (grouping the three types of CompilerThread together) that all basically duplicated the logic for initializing (preparing is the term we use for user-defined JavaThreads) and starting the new thread. This common code can be factored out into static helpers in JavaThread.
>> 
>> This change does not look at the way the java.lang.Thread instance is created - that will be handled by a separate RFE.
>> 
>> The semantics of the changes are not identical, but I don't believe there is any observable change in behaviour. The scope of holding the Threads_lock has been reduced, and we now create the JavaThread instances ("new XXXThread(...)") outside of the lock. As far as I can see nothing in the construction process needs to happen under the Threads_lock.
>> 
>> A few of the threads use a static `_instance` field to hold a reference to the create JavaThread. This proved very difficult to handle, as logically the field would need to be updated in the middle of the new factored-out method: after setting all the fields but before releasing the newly started thread. I eventually realized that in all but one case those `_instance` fields are never used and so could be deleted. The one case remaining does not need to be set as I just described, but can be set after the thread has started, as the new thread does not examine it (arguably its existence is unnecessary).
>> 
>> The trickiest changes related to the CompilerThreads, so they need particular scrutiny.
>> 
>> Testing: tiers 1-3
>> 
>> Thanks,
>> David
>
> David Holmes has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Rename vm_exit_on_thread_allocation_failure to vm_exit_on_osthread_failure
>  - Adjust comment
>  - Comments from PR review:
>    - remove unnecessary new_thread NULL checks
>    - adjust some comments
>    - fix whitespace

This is a nice cleanup!

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Marked as reviewed by coleenp (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/4629