RFR: 8235324: Dying objects are published from users of CollectedHeap::object_iterate
stefan.karlsson at oracle.com
Wed Dec 4 10:34:34 UTC 2019
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.
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