RFR: 8256811: Delayed/missed jdwp class unloading events [v17]

[Chris Plummer]

On Tue, 12 Jul 2022 22:02:59 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Currently, jdi only check and process class unloading event when it detects a new GC cycle.
>> 
>> After [JDK-8212879](https://bugs.openjdk.org/browse/JDK-8212879), posting class events can overlap with GC finish event, that results, sometimes, it only captures partial or even empty unloaded class list. The pending list usually can be flushed out at next GC cycle. But for the classes unloaded during the last GC cycle, the class unloading events may lost forever.
>> 
>> This patch checks and processes class unloading events unconditionally, suggested by @kbarrett, the last pending unloaded class list can be flushed by other events, such as `VM_DEATH`.
>> 
>> It also performs `commonRef_compact()` only when there are classes unloaded.
>> 
>> New test failed about 20% without patch, none with patch.
>> 
>> **Update**
>> There are significant changes from early patch. 
>> 
>> The new approach:
>> No longer removing dead objects and post events on VM thread. I believe it was implemented this way to workaround the following issues:
>> - JDI event handler uses JVMTI raw monitor, it requires thread in `_in_native` state
>> - The thread can not hold lock, which is needed to protect `JvmtiTagMap` while walking, when transition to `_in_native` state
>> 
>> The new solution breaks up into two steps:
>> - Collect all dead object tags with lock
>> - Transition to `_in_native` state and post object free events in one batch
>> 
>> This way, JDI event handler can process object free events upon arrivals without delay.
>> 
>> **Update 2**
>> There is a comment for ` JvmtiTagMap::check_hashmap()` that states `ObjectFree` events are posted before heap walks.
>> 
>> // This checks for posting and rehashing before operations that
>> // this tagmap table.  The calls from a JavaThread only rehash, posting is
>> // only done before heap walks.
>> void JvmtiTagMap::check_hashmap(bool post_events) {
>> 
>> Now, the events are actually posted after heap walks, but I don't think it makes any material material difference. 
>> Even the events are posted earlier in old code, but they are only processed after next GC cycle.
>
> Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix a bug

Ok. Everything is passing after your latest fix.

I think the test still has a synchronization bug, but probably not something important enough to worry about, and I'm guessing a lot of tests have this issue. The test disables events and then frees the raw monitor used in the callback. The problem is that JVMTI might already be in the process of calling the callback, so the callback will still attempt to use the monitor. The test shutdown code and the test callback code need to be synchronized to avoid this issue. However, there is a chicken and egg problem here, because you need a raw monitor for the synchronization. I think the real solution is to just not bother freeing up the raw monitor. I think we may have done this recently in some other test. Perhaps @sspitsyn remembers.

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PR: https://git.openjdk.org/jdk/pull/9168