RFR: 8235324: Dying objects are published from users of CollectedHeap::object_iterate
erik.osterlund at oracle.com
Wed Dec 4 14:23:52 UTC 2019
On 2019-12-04 11:34, Stefan Karlsson wrote:
> Removed the unused oopDesc::keep_alive function:
> 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.
>> Caution needs to be taken when CollectedHeap::object_iterate is used.
>> It may:
>> - Return non-reachable objects - for example, when G1 walks the prev
>> - Return the Reference.referent without an appropriate resurrection
>> - 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.
More information about the hotspot-gc-dev