RFR 8246050: Improve scalability of MemoryScope

[Maurizio Cimadamore]

Hi,
during the review of [1] it emerged that the implementation of the 
memory scope abstraction (which is used to keep track of temporal scope 
of a memory segment) does not scale well in situations where there is a 
lot of contention on the acquire() method due to many threads working 
simultaneously on different chunks of the segment.

Peter has proposed an alternate implementation [2] which, instead of 
using CAS, it cleverly uses LongAdders.

While that implementation worked correctly, we managed to simplify it 
further, by realizing that what we needed here was an instance of a 
read-write lock: a thread that acquires a segment does a "read", while a 
thread closing a segment does a "write". By using optimistic reads with 
a StampedLock we were able to gain back scalability and maintain the 
code relatively readable.

Webrev:

http://cr.openjdk.java.net/~mcimadamore/8246050/webrev/

Cheers
Maurizio

[1] - 
https://mail.openjdk.java.net/pipermail/core-libs-dev/2020-April/066136.html
[2] - https://git.openjdk.java.net/panama-foreign/pull/142