RFR: 8264123: add ThreadsList.is_valid() support

[Daniel D.Daugherty]

On Tue, 30 Mar 2021 16:29:48 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> Hi Dan,
>> 
>> I'm not quite seeing how this single-field checks constitutes a useful validity check - sorry.
>> 
>> Any why is this all product code rather than debug?
>> 
>> Thanks,
>> David
>
> @dholmes-ora - Thanks for the review!
> 
>> I'm not quite seeing how this single-field checks constitutes a useful validity check - sorry.
> 
> Please see my reply about the ThreadsList destructor code that you missed.
> 
>> Any why is this all product code rather than debug?
> 
> Please see my reply about guarantee() versus assert().

https://github.com/openjdk/jdk/pull/3272 is now a dependent PR on this fix (woot!) so now
you can see the race that I was chasing with this validation code! There's a long analysis
note attached to "JDK-8264393 JDK-8258284 introduced dangling TLH race".

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