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

[David Holmes]

> 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

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4629/files
  - new: https://git.openjdk.java.net/jdk/pull/4629/files/e547e0a5..8275b184

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4629&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4629&range=01-02

  Stats: 28 lines in 10 files changed: 1 ins; 0 del; 27 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4629.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4629/head:pull/4629

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