CRR: 7034139: G1: assert(Thread::current()->is_ConcurrentGC_thread()) failed: only a conc GC thread can call this (S)

[Tony Printezis]

Hi,

Could I get a couple of people to look at this? (I'd like to push this 
this week if possible)

http://cr.openjdk.java.net/~tonyp/7034139/webrev.0/

The actual fix is reasonably small (leave / join the 
SuspendibleThreadSet only if we are in concurrent mode). Most of the 
changes are new infrastructure to cause a fixed number of overflows 
during marking (in non-product builds of course) to stress the overflow 
code. This was the only way I could reliably reproduce the failure. This 
did uncover a couple of extra issues which I also fixed:

- If we overflow during remark we should not actually deal with it 
during remark but we should abort the remark pause and restart a 
concurrent mark phase. For some reason we were not doing that. I fixed 
that (for this I had to ensure that the overflow flag is not cleared 
when we exit the do_marking_step() method).
- Because we were clearing the overflow, it was also possible that the 
workers would deadlock (for that to happen a worker had to finish 
handling one overflow and immediately raise another one, so it was 
highly unlikely to occur in prcatice; good to find it and eliminate it 
though).

I've already tested it, I'll run more tests overnight.

Tony