RFR 8242484: Rework thread deletion during VM termination

[coleen.phillimore at oracle.com]

This looks good to me also, although I'd like a reprisal of the comment 
before the delete thread line that says something ilke:

// We are here after VM_Exit::set_vm_exited() so we can't call
// thread->smr_delete() or we will block on the Threads_lock.


But why would it block? ... I don't see what thread has the 
Threads_lock.  Is this comment (that I want put back) accurate?

thanks,
Coleen

On 4/17/20 11:09 AM, Robbin Ehn wrote:
> Hi Patricio, looks good, thanks.
>
> /Robbin
>
> On 2020-04-14 22:07, Patricio Chilano wrote:
>> Hi,
>>
>> Please review this small follow up for 8240918 to always delete the 
>> JavaThread that calls Threads::destroy_vm().
>> In 8240918 I added a conditional check in Threads::destroy_vm() to 
>> prevent deleting a JavaThread when there are handshakers blocked in 
>> its handshake_turn_sem semaphore.  We can actually avoid this issue 
>> altogether if after removing the JavaThread from the active list and 
>> before triggering the terminating safepoint, we wait until the 
>> JavaThread is no longer protected by a ThreadsList reference.
>>
>> Tested in mach5 tiers 1-6.
>>
>> Bug:
>>     https://bugs.openjdk.java.net/browse/JDK-8242484
>>
>> Webrev:
>>     http://cr.openjdk.java.net/~pchilanomate/8242484/v1/webrev/
>>
>> Thanks,
>> Patricio