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

[Roman Kennke]

Hi Roland,

> 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.

Good.

>> 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.

Good. Please push the fix.

Thanks,
Roman