RFR(S): 8237837: Shenandoah: assert(mem == __null) failed: only one safepoint

[Roland Westrelin]

Hi Roman,

Thanks for reviewing this. 

> What sort of testing have you run to verify the change? Would be good to
> cover at least hotspot_gc_shenandoah, and the CTW tests if this is
> supposed to go into jdk14.

hotspot_gc_shenandoah passes. I ran a bunch of CTW testing too. 

> Do you expect any performance impact (esp a regression)?

No. It's a rare correctness issue.

> Do we know if it also affects 11u and/or 8u? If so, update the bug entry
> accordingly. Also, maybe also update the synopsis if you can think of a
> better one (up to you).

Yes, it affects 11 and 8. I added both of them to the affected version
in the bug.

Roland.