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

[Stefan Karlsson]

Thanks, Per.

StefanK

On 2019-12-04 12:25, Per Liden wrote:
> 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