RFR: 8235324: Dying objects are published from users of CollectedHeap::object_iterate
stefan.karlsson at oracle.com
Wed Dec 4 08:50:37 UTC 2019
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
- 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
- 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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