Work-in-progress: 8236485: Epoch synchronization protocol for G1 concurrent refinement

[Man Cao]

Hi all,

I have written up a description and challenges for implementing an epoch
synchronization protocol. This protocol is necessary for removing the
StoreLoad fence in G1's post-write barrier (JDK-8226731)

Description: https://bugs.openjdk.java.net/browse/JDK-8236485
Work-in-progress webrev:
https://cr.openjdk.java.net/~manc/8236485/webrev_wip0/

There are two main challenges that I'm not sure how to resolve:
- Triggering a thread-local handshake is a blocking operation that can pass
a safepoint.
- There are native post-write barriers executed by threads in native/VM
state.

Discussions and suggestions are highly appreciated!

-Man