RFR: 8256517: (ref) Reference.clear during reference processing may lose notification [v2]

[Kim Barrett]

> Please review this change to Reference.clear() to address several issues.
> 
> (JDK-8240696) For GCs using a SATB barrier, simply assigning the referent
> field to null may extend the lifetime of the referent value.
> 
> (JDK-8240696) For GCs with concurrent reference processing, clearing the
> referent field during reference processing may discard the expected
> notification.
> 
> Both of these are addressed by introducing a private native helper function
> for clearing the referent, rather than using an ordinary in-Java field
> assignment. Tests have been added for both of these issues. This required
> adding a new breakpoint in reference processing for ZGC.
> 
> Of course, finalization adds some complexity to the problem.  We deal with
> that by having FinalReference override clear.  The implementation is
> provided by a new package-private method in Reference.  (There are a number
> of alternatives, all of them clumsy; finalization is annoying that way.)
> 
> While dealing with FinalReference clearing it was noted that the recent
> JDK-8256106 and JDK-8256370 have some problems. FinalizerHistogram was not
> updated to call the new Reference.getInactive(), instead still calling get()
> on FinalReferences, with the JDK-8256106 problems. Fixing that showed the
> assertion for inactive FinalReference added by JDK-8256370 used the wrong
> test.
> 
> Rather than tracking down and changing all get() and clear() calls on final
> references and changing them to use getInactive and a new similar clear
> function, I've changed FinalReference to override get and clear, which call
> the helper functions in Reference. I've also renamed getInactive to be more
> explanatory and less convenient to call directly, and similarly named the
> helper for clear. This means that get/clear should never be called on an
> active FinalReference. That's already never done, and would have problems
> if it were.
> 
> Testing:
> mach5 tier1-6
> Local (linux-x64) tier1 using Shenandoah.
> New TestReferenceClearDuringMarking fails for G1 without these changes.
> New TestReferenceClearDuringReferenceProcessing fails for ZGC without these changes.

Kim Barrett has updated the pull request incrementally with two additional commits since the last revision:

 - new tests need ref in oldgen too
 - remove obsolete comment about races with clear and enqueue

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1376/files
  - new: https://git.openjdk.java.net/jdk/pull/1376/files/46e5b1f6..c19efd70

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1376&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1376&range=00-01

  Stats: 17 lines in 3 files changed: 7 ins; 6 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1376.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1376/head:pull/1376

PR: https://git.openjdk.java.net/jdk/pull/1376