RFR: 8254164: G1 only removes self forwarding pointers for last collection set increment [v2]

[Thomas Schatzl]

> Hi all,
> 
>   can I have reviews for this change that fixes a previously hard to reproduce crash that started showing up much more
>   frequently in conjunction with changes to young gen sizing (JDK-8244603)?
> 
> The issues is that the code to process regions where evacuation failed only processes the last increment. This leaves
> forwarded pointers in the mark word of some objects. Obviously other code does not like that, e.g. the crashes in
> JDK-8248438 which I plan to close as duplicate.  Only checking the last collection set increment for regions that
> failed evacuation is wrong in case there is an evacuation failure caused by reference processing in a region that has
> been evacuated in earlier evacuation increments. Reference processing (e.g. finalizers) can make it necessary to
> resurrect an otherwise unreachable object at the very end of the collection that can't be copied and is located in a
> region evacuated in an earlier increment.  This optimization to only look at the last increment for removal of self
> forwarding pointers has been introduced in JDK-8218668.  Until changes to young gen sizing in JDK-8244603 this crashes
> has been a very rare occurrence, but with that it has been common in some tier8 tests (KitchenSink8/24h, DaCapo24h)
> particularly with some additional targeted verification (enable verification only at the end of mixed gcs with optional
> evacuation). Without this fix both tests fail within 10 minutes to 2 hours. With the patch everything completes fine.
> Testing:
> - tier1-5
> - KitchenSink/Dacapo24h with verification code and JDK-8244603.
> 
> Note: since this change strictly does more work during evacuation failure handling I consider this amount of testing
> sufficient, i.e. with JDK-8244603. It is very hard to reproduce without JDK-8244603.

Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:

  sjohanss comment

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/556/files
  - new: https://git.openjdk.java.net/jdk/pull/556/files/b37d02eb..6a5e3d72

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=556&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=556&range=00-01

  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/556.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/556/head:pull/556

PR: https://git.openjdk.java.net/jdk/pull/556