[External] : Re: Semantics of CollectedHeap::requires_barriers()

[Ron Pressler]

When frames are thawed from a chunk, then if and only if that chunk requires_barriers, then InstanceStackChunkKlass::barriers_for_oops_in_frame
invokes any store barriers on the thawed oops.

— Ron

> On 10 Feb 2021, at 19:08, Roman Kennke <rkennke at redhat.com> wrote:
> 
> 
>>> However, we might need barriers when copying frames out of the stack chunk (e.g. when thawing frames), because then we'd be loading from oop locations. Is this taken care of already somehow?
>> If you need barriers when thawing, then requires_barriers must return false, and then it’s taken care of.
> 
> Can you explain to me how that is supposed to work? If I understand it correctly, the frozen frame can contain any number of oops. When copying back the chunk into the stack, the GC must ensure:
> 1. That it copies back the correct copy of StackChunk (this might not actually be very relevant because the StackChunk is immutable, and thus both from-space and to-space copy should be equivalent).
> 2. More importantly, it must update/heal all oops in the chunk, otherwise it violates the strong to-space variant.
> 
> How and where is this achieved?
> 
> I am asking this, because I still see a failing Fuzz.java test:
> 
> http://cr.openjdk.java.net/~rkennke/hs_err_pid623629.log
> 
> and I suspect that thawing is not working correctly.
> 
> I would expect some sort of GC interface to achieve the above points, and conceptually it seems very similar to clone-barriers. But I don't see any of this, except NativeAccess<>::oop_load() calls, which definitely cannot achieve the desired effect. Or maybe no other GC has required such treatment yet, and therefore that's missing?
> 
> Roman
>