RFR: 8264123: add ThreadsList.is_valid() support

Wed Mar 31 16:08:22 UTC 2021

On Wed, 31 Mar 2021 14:22:46 GMT, Erik Österlund <eosterlund 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].
>
> Looks good to me!

@fisk - Thanks for the review!

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

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