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

[Per Liden]

On 12/4/19 11:15 AM, 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/

Looks good.

/Per

> 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