RFR (S) 8251336: Shenandoah: assert "only get here when SATB active" after JDK-8244997

[Roman Kennke]

On Tue, 2020-08-11 at 23:35 +1000, David Holmes wrote:
> Hi Coleen,
> 
> On 11/08/2020 9:40 pm, Coleen Phillimore wrote:
> > Summary: Release OopStorage oops outside a safepoint.
> > 
> > Tested with tier1-6.
> > 
> > open webrev at 
> > http://cr.openjdk.java.net/~coleenp/2020/8251336.01/webrev
> > bug link https://bugs.openjdk.java.net/browse/JDK-8251336
> > 
> > Thank you to Zhengyu for alerting me about this issue.
> 
> Yeah ... I was focusing on the locking aspect and completely
> overlooked 
> the fact that section of code is within the scope of the TBIVM. Not
> that 
> it would have immediately rung any alarm bells.
> 
> Do we have usage rules about manipulating OopStorage at a safepoint?
> If 
> some GC's can't cope with it then there should be some detection of
> that 
> in the OopStorage APIs.

In general, non-GC threads should not mess with GC roots (e.g.
OopStorage) during safepoints. I don't think that this plays well with
any GC. Not sure if it can be automatically detected, it sure would be
nice if that would fire an assert or so.

Roman