RFR: 8328235: GenShen: Robustify ShenandoahGCSession and fix missing use [v19]

[Y. Srinivas Ramakrishna]

> ShenandoahGCSession is intended to create a scope where the ShenandoahHeap's _gc_cause and _gc_generation field reflect the current gc cycle. We now check that we do not overwrite existing non-default settings (respectively _no_gc and nullptr). The destructor of the scope/stack object also resets these fields to their default settings, ensuring intended uses. This uncovered a situation where the scope was not entered when it should have been, which we have now fixed.
> 
> A case of flickering of active_generation() was identified when used concurrently by mutators while it was being modified by the controller thread. To deal with this, we have carefully gone through the setting and use of the field, and found that an expedient fix for the race is to split the variable into two:
> - _gc_generation is set & cleared by the controller thread whenever it enters and exits a GC scope, and services concurrent gc cycles for young or old generations.
> - _active_generation is set to the value in _gc_generation at the start of each Shenandoah GC safepoint operation so that mutator threads and load barriers always see a consistent value between safepoints.
> 
> Asserts check the protocol for setting and clearing the variables.
> 
> An alternative approach is to not use a global variable for the _gc_generation indirected through the heap, but rather to pass it into the closures that do the work. This would work as well, but the changes would potentially touch more code. We would still have to have set the variable that is consulted by the load barriers, in a mutator-safe fashion at a safepoint, like we do today. This or other alternative approaches may be investigated in the future to potentially make this protocol more self-contained and robust rather than leaking as it does today into many places in the code.
> 
> *Testing*:
> - [x] code pipeline
> - [x] specjbb testing
> - [ ] specjbb performance
> - [x] jtreg:hotspot_gc and jtreg:hotspot:tier1 w/fastdebug
> - [x] GHA

Y. Srinivas Ramakrishna has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 49 commits:

 - Merge branch 'master' into active_generation
 - Merge branch 'master' into active_generation
 - Revert debugging detritus.
 - Merge branch 'master' into active_generation
 - Merge branch 'master' into active_generation
 - Merge branch 'master' into active_generation
 - Code called from LRB cannot rely on SGH::_gc_generation and must instead
   use the _active_generation field.
   
   Still need to aduit remaining _gc_generation uses for safety.
   
   Removed too strong assert introduced for debugging in previous commit.
 - Merge branch 'master' into active_generation
 - Some asserts to catch a tricky race. These assertions may be too strong
   in general but would help with debugging a rare crash.
 - Merge branch 'master' into active_generation
 - ... and 39 more: https://git.openjdk.org/shenandoah/compare/9d2712dd...b12e17cb

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Changes: https://git.openjdk.org/shenandoah/pull/407/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=407&range=18
  Stats: 166 lines in 16 files changed: 133 ins; 1 del; 32 mod
  Patch: https://git.openjdk.org/shenandoah/pull/407.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/407/head:pull/407

PR: https://git.openjdk.org/shenandoah/pull/407