RFR: 8234508: VM_HeapWalkOperation::iterate_over_object reads non-strong fields with an on-strong load barrier

[Erik Österlund]

Hi Stefan,

Looks good.

Thanks,
/Erik

On 2019-12-04 11:15, Stefan Karlsson wrote:
> Hi all,
>
> Please review this fix to ensure we use the correct barrier when 
> resolving Refrence.referents in 
> VM_HeapWalkOperation::iterate_over_object.
>
> https://cr.openjdk.java.net/~stefank/8234508/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8234508
>
> Before this fix, the code would use an ON_STRONG barrier and 
> potentially reading a dying referent during the resurrection blocked 
> window. By using ON_UNKNOWN_OOP_REF we make sure that the correct 
> barrier is used on Reference.referents, and the load will return NULL 
> instead of a dead object.
>
> I'm also tagging the load as AS_NO_KEEPALIVE, since we likely don't 
> want to keep all referents unnecessarily alive. This relies on the fix 
> for '8235324: Dying objects are published from users of 
> CollectedHeap::object_iterate'.
>
> Thanks,
> StefanK