On Thu, 14 Oct 2021 22:02:43 GMT, Coleen Phillimore <cole...@openjdk.org> wrote:
>> Ok, I see - grow the table. >> >> I'm not sure if you need to ask ik->is_loader_alive() or not, but I think >> you do. The InstanceKlass is removed from your table during class unloading >> but before that during concurrent class unloading, the class might not be >> alive while you look at it and concurrent class unloading hasn't gotten >> around to removing it yet. Since you save classes regardless of CLD, you >> have to check if it's alive. See classLoaderDataGraph.cpp >> ClassLoaderDataGraphIterator for example. The CLDG_lock only keeps the >> graph from not getting modified, but the classes in it might be dead. > > That said, I don't see where you return an InstanceKlass in the table, which > would need this check. Not in class_unloading_do because this follows the > _unloading list. So there is already support for concurrent class unloading today in JFR, and the protocol is built around the CLDG_lock. If JFR holds it, concurrent class unloading cannot run. If GC holds it, JFR cannot inspect classes. That's why the table inspection is guarded via the CLDG_lock, for mutual exclusion to avoid this problem. I.e. if concurrent class unloading is in progress, JFR will not inspect the table. ------------- PR: https://git.openjdk.java.net/jdk/pull/4731
