Commits on Apr 28, 2020

1. Collector STW flag …

   Generalize the existing flag that was being set only in Concurrent
   Global GC to being maintained by base Collector, so that can be used
   for any STW Collector.
   
   Specifically, it was needed in an unusual combination when Concurrent
   Scavenger was enabled, but Concurrent Mark disabled (so Global GC was
   not MM_ConcurrentGC, but MM_ParallelGlobalGC which would not set STW
   flag), and when CS would abort and Global Marking would miss to fixup
   forwarded slots (during STW phase).
   
   In future, the flag might be useful for other code in CS & Balanced to
   determine if GC threads running in STW or concurrent phase. But for that
   we have to properly set it for cases when Master Threads is explicit.
   This is better to do in a separate change, if/when it becomes important.
   For now, this flag is reliable only any STW GC with implicit Master
   Thread.
   
   This is a slightly adjusted variant of
   eclipse-omr#5112 that had a problem with setting
   this flag before completeExternalConcurrentCycle(). While in theory that
   would be ok, heap resizing code in ConcurrentGC would (somewhat
   incorrectly) rely on _stwCollectionInProgress being now true to conclude
   that resizing request was not coming from overlapping Scavenger
   (thinking that it's not possible that both Global and Scavenger could be
   STW at the same time). That code is just way too risky to change, hence
   we keep the old behavior where _stwCollectionInProgress for global GC is
   false while completeExternalConcurrentCycle is in progress, and set it
   to true marginally later, just after completeExternalConcurrentCycle is
   complete.
   
   Signed-off-by: Aleksandar Micic <amicic@ca.ibm.com>

   

   amicic committed Apr 28, 2020
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