RFR: 8264123: add ThreadsList.is_valid() support

[Robbin Ehn]

On Mon, 29 Mar 2021 21:48:23 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

> ThreadsLists need an is_valid() function and checks in various
> places to help catch bugs where a ThreadsList is dangling.
> 
> Other minor changes:
> - change raw `_threads_hazard_ptr` access to used `get_threads_hazard_ptr()`.
> - `get_threads_hazard_ptr()` should be `const`.
> - fix a couple of old typos.
> 
> Update: forgot to mention that this fix was tested with Mach5 Tier[1-8].

Hi Dan,

I have seen some other verification code which also might be performance impacting, can we got over and measure what the cost is for different verification when you gotten your bake time here? So we can take an informed decision based on what the cost vs debuggability in product is.

Looks good, thanks.

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Marked as reviewed by rehn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3255