RFR: JDK-8129626: G1: set_in_progress() and clear_started() needs a barrier on non-TSO platforms

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change?

https://bugs.openjdk.java.net/browse/JDK-8129626
http://cr.openjdk.java.net/~brutisso/8129626/webrev.00/

We need to add a barrier between the calls to set_in_progress() and 
clear_started() to make sure that other threads sees the correct value 
when they use ConcurrentMarkThread::during_cycle().

Thanks to Per and Bertrand for helping out identifying and sorting this out.

 From the bug report:

ConcurrentMarkThread::during_cycle() is implemented as:

bool during_cycle() { return started() || in_progress(); }

So, it checks both ConcurrentMarkThread::_started and 
ConcurrentMarkThread::_in_progress and they are meant to overlap. That 
is, we should not set _started to false until after we have set 
_in_progress to true. This is done in sleepBeforeNextCycle():

void ConcurrentMarkThread::sleepBeforeNextCycle() {
   // We join here because we don't want to do the "shouldConcurrentMark()"
   // below while the world is otherwise stopped.
   assert(!in_progress(), "should have been cleared");

   MutexLockerEx x(CGC_lock, Mutex::_no_safepoint_check_flag);
   while (!started() && !_should_terminate) {
     CGC_lock->wait(Mutex::_no_safepoint_check_flag);
   }

   if (started()) {
     set_in_progress();
     clear_started();
   }
}

On non-TSO platforms there is a risk that the write to _in_progress 
(from set_in_progress()) is seen by other threads after the write to 
_started (in clear_started()). In that case there is a window when 
during_cycle() may return false even though we are in a concurrent cycle.


Thanks,
Bengt
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