Including serviceability-dev.
On 12/1/20 9:03 PM, Coleen Phillimore wrote:
On 12/1/20 7:12 PM, Serguei Spitsyn wrote:
On Tue, 1 Dec 2020 14:19:15 GMT, Coleen Phillimore
<cole...@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
