6692906: CMS: parallel concurrent marking may be prone to hanging or stalling mutators for periods of time Summary: Inserted missing yield(check)s in closures used during the work-stealing phase of parallel concurrent marking, a missing synchronous yield-request in the cms perm gen allocation path, and a terminator-terminator for the offer_termination invocation that monitors the yield status of the concurrent marking task. Elaborated some documentation comments and made some task queue termination loop flags configurable at start-up to aid debugging in the field. Reviewed-by: jmasa, johnc, poonam

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