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

[Serguei Spitsyn]

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()

This is reasonable. Thank you for the explanation!
The fix looks good to me.

> When the event is disposed, it calls set_event_callbacks()...
You, probably meant "environment" is disposed, not "event".
This correction is for other readers.  :)

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Marked as reviewed by sspitsyn (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1539