RFR (S): JDK-8046518: G1: Double calls to register_concurrent_cycle_end()

[Bengt Rutisson]

Hi all,

Can I have a review for this change?

http://cr.openjdk.java.net/~brutisso/8046518/webrev.00/

https://bugs.openjdk.java.net/browse/JDK-8046518

Background:
When we abort a concurrent cycle due to a Full GC in G1 we call 
ConcurrentMark::abort(). That will set _has_aborted flag and then call 
register_concurrent_cycle_end().

The concurrent marking thread will see the _has_aborted flag in its 
ConcurrentMarkThread::run() method, abort the execution and then call 
register_concurrent_cycle_end().

Currently this works since the code inside 
register_concurrent_cycle_end() is guarded by _concurrent_cycle_started 
which it then resets. So, the double calls will not necessarily result 
in too much extra work being done. But one of the things that 
register_concurrent_cycle_end() does is to call report_gc_end() on the 
concurrent GC tracer. That prevents further use of it for this GC. This 
means that inside the ConcurrentMarkThread::run() method we can not rely 
on the tracer.

Removing the call to register_concurrent_cycle_end() in 
ConcurrentMark::abort() and relying on the call in 
ConcurrentMarkThread::run() seems to be a reasonable approach.

Thanks,
Bengt
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