RFR: 8293824: gc/whitebox/TestConcMarkCycleWB.java failed "RuntimeException: assertTrue: expected true, was false" [v2]

[Kim Barrett]

> Please review this change to WhiteBox and some tests involving G1 concurrent GCs.
> 
> Some tests currently use WhiteBox.g1StartConcMarkCycle() to trigger a
> concurrent GC.  Many of them follow it with a loop waiting for a concurrent
> cycle to not be in progress.  A few also preceed that call with a similar
> loop, since that call does nothing and returns false if a concurrent cycle is
> already in progress.  Those tests typically want to ensure there was a
> concurrent cycle that was started after some setup.
> 
> The failing test calls that function, asserting that it returned true, e.g. a
> new concurrent cycle was started.
> 
> There are various problems with this, due to races with concurrent cycles
> started automatically and possibly aborted (by full GCs) concurrent cycles,
> making some of these tests unreliable in some configurations.
> 
> For example, the test failure associated with this bug intermittently arises
> when running with `-Xcomp`, triggering a concurrent cycle before the explicit
> requests by the test, causing the explicit request to fail (because there is
> already one in progress), failing the assert.  Waiting for there not to be an
> in-progress cycle before the explicit request just narrows the race window.
> 
> We have a different mechanism for controlling concurrent cycles, the
> concurrent GC breakpoint mechanism.  By adding a counter specifically for such
> cycles, we can use GC breakpoints to ensure only the concurrent cycles the
> test wants are occurring, and can verify they completed successfully.
> 
> So we change tests using WhiteBox.g1StartConcMarkCycle() to instead use GC
> breakpoints, along with the new WhiteBox.g1CompletedConcurrentMarkCycles() to
> avoid racing request problems and to detect aborted cycles.  Since it is no
> longer used, WhiteBox.g1StartConcMarkCycle() is removed.
> 
> Testing:
> mach5 tier1-6

Kim Barrett has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - use new whitebox api for running conc gc
 - Merge branch 'master' into test-concmark
 - copyrights
 - remove WB.g1StartConcMarkCyle
 - update tests
 - add utility GC breakpoint functions
 - add G1ConcurrentMark::completed_mark_cycles() and whitebox access

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Changes: https://git.openjdk.org/jdk/pull/11435/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11435&range=01
  Stats: 281 lines in 22 files changed: 96 ins; 131 del; 54 mod
  Patch: https://git.openjdk.org/jdk/pull/11435.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11435/head:pull/11435

PR: https://git.openjdk.org/jdk/pull/11435