RFR: 8234009: ZGC: Move resurrection unblock to before the _unload.purge() call

[Per Liden]

Looks good!

/Per

On 11/21/19 2:28 PM, Stefan Karlsson wrote:
> Hi all,
> 
> Please review this patch to move the resurrection to before the 
> _unload.purge() call.
> 
> https://cr.openjdk.java.net/~stefank/8234009/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8234009
> 
> After JDK-8230661 we are guaranteed that no mutator holds a weak oop 
> containing a "dead" object when the ZResurrection::unblock() call happens.
> 
> Therefore, it doesn't matter if the load barrier that the thread 
> executes runs the code guarded by ZResurrection::is_blocked() or the 
> "non-blocked" part, when ZResurrection::unblock() changes the state. As 
> long as the ZResurrection::unblock() call happens after the handshake, 
> we are good to go.
> 
> Today, we perform the purging and deletion of metadata in 
> _unload.purge(), and call ZResurrection:unblock() after that. There's no 
> need to delay the unblocking to after the purge. We have the opportunity 
> to shrink the resurrection-blocked window by moving the 
> ZResurrection::unblock() call to before the call to _unload.purge(), but 
> still after the handshake.
> 
> Thanks,
> StefanK