RFR: 8235324: Dying objects are published from users of CollectedHeap::object_iterate

[Stefan Karlsson]

Removed the unused oopDesc::keep_alive function:

https://cr.openjdk.java.net/~stefank/8235324/webrev.02.delta/
https://cr.openjdk.java.net/~stefank/8235324/webrev.02/

StefanK

On 2019-12-04 09:50, Stefan Karlsson wrote:
> Hi all,
>
> Please review this fix to make sure objects published from closures to 
> CollectedHeap::object_iterate are kept alive.
>
> https://cr.openjdk.java.net/~stefank/8235324/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8235324
>
> Caution needs to be taken when CollectedHeap::object_iterate is used. 
> It may:
> - Return non-reachable objects - for example, when G1 walks the prev 
> bitmap
> - Return the Reference.referent without an appropriate resurrection 
> barrier
> - Return an object loaded without a "strong"/"marking" barrier
>
> These objects should not escape the confines of the active safepoint 
> that calls object_iterate. If they do we run the risk of various 
> crashes when the objects get garbage collected.
>
> Unfortunately, there are a few closures that do this. The once we have 
> identified are:
> - HeapInspection::find_instances_at_safepoint
> - JVMTI object tagging
>
> There might be some merit to solving this by altering the 
> object_iterate API, but for now we'd like to fix the two problematic 
> areas listed above. The suggestion is to introduce a 
> CollectedHeap::keep_alive function that notifies the GC that the 
> object should be kept alive the current GC cycle (if active).
>
> This solution will also work with code that iterates over object 
> fields without using object_iterate. For example, with this fix we'll 
> be able to read the Reference.referent field in JDK-8234508, with 
> AS_NO_KEEPALIVE, and only keep the object alive if it actually gets 
> registered in the tag map. Without that, every single object reachable 
> through Reference.referent would be kept alive.
>
> The fix introduces a keep_alive() function for ZGC, G1, and 
> Shenandoah. It would be good to get reviews from maintainers of all 
> these GCs.
>
> Thanks,
> StefanK