RFR: 8257140: Crash in JvmtiTagMap::flush_object_free_events() [v2]

[Coleen Phillimore]

On 12/1/20 7:12 PM, Serguei Spitsyn wrote:
> On Tue, 1 Dec 2020 14:19:15 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:
>
>>> The JvmtiTagMap can be deleted while it is posting.  The JvmtiEnv is still on the list but the env is "disposed" of and the tag_map is deleted to save memory until the JvmtiEnv can be reclaimed at a safepoint and taken off the list.
>>>
>>> There's a check JvmtiEnvBase::check_for_periodic_clean_up() that determines whether the JvmtiEnv can be taken off the list and that check looks for whether threads are in the middle of JvmtiEnvIterator which sets Thread::jvmti_env_iteration_count.  So this part is safe for the JvmtiTagMap changes.
>>>
>>> The fix is to clear the JvmtiTagMapTable of the entries inside the table lock to save memory but leave JvmtiTagMap intact so that we can load a valid pointer when iterating through JvmtiEnvIterator.  We could also not do this at all and let the JvmtiTagMap destructor delete the table when the JvmtiEnv is also deleted, but we don't know what customer situation led to this code in the first place.
>> Coleen Phillimore has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
>>
>>   - Merge branch 'master' into more-jvmti-table
>>   - Rename JvmtiTagMap::clear() and fix JvmtiTagMapTable::clear() to null out deleted entries.
>>   - 8257140: Crash in JvmtiTagMap::flush_object_free_events()
> Hi Coleen,
>
> The JVMTI environment can be disposed with DisposeEnvironment:
>    https://docs.oracle.com/en/java/javase/15/docs/specs/jvmti.html#DisposeEnvironment
> It seems to me that this operation is better to wait until posting is finished. Otherwise, the information is going to be lost in the report. Of course, we could blame the agent to call the DisposeEnvironment too early but then the agent needs a way to check if posting is finished.

When the event is disposed, it calls set_event_callbacks(), which calls 
flush_object_free_events, which will flush the remaining object free 
events for this environment.  So the events aren't lost.  The bug was 
that after that, the JvmtiEnv is marked as DISPOSED but it's still on 
the list of JvmtiEnvIterator events that we walk until the next 
safepoint.  When the JvmtiEnv is disposed, it was deleting the tag_map 
but leaving it on the list, so later calls to flush_object_free_events 
was getting a bad tag_map pointer in the JvmtiEnv.

Coleen

>
> Thanks,
> Serguei
>
> -------------
>
> PR: https://git.openjdk.java.net/jdk/pull/1539