On Fri, 5 Nov 2021 16:43:04 GMT, Coleen Phillimore <cole...@openjdk.org> wrote:
>> The `ThreadsListHandle` protects `JavaThread` objects not `JvmtiThreadState`
>> objects.
>> `JvmtiThreadState::first()` returns the head of the global list of
>> `JvmtiThreadState`
>> objects for the system. Each `JvmtiThreadState` object contains a
>> `JavaThread*` and
>> we have to protect use of the `JavaThread*` which can happen in the
>> `recompute_thread_enabled(state)` call below.
>
> JvmtiThreadState objects point to JavaThread and vice versa, so I still don't
> see why you don't protect the first element.
I've written up a rather long analysis about how the use of
`JvmtiThreadState_lock`
in `JvmtiEventControllerPrivate::recompute_enabled()` means that we can safely
traverse the JvmtiThreadState list returned by `JvmtiThreadState::first()`
without
racing with an exiting JavaThread. I've sent it to you via direct email.
I could amend the comment above the ThreadsListHandle like this:
// If we have a JvmtiThreadState, then we've reached the point where
// threads can exist so create a ThreadsListHandle to protect them.
// The held JvmtiThreadState_lock prevents exiting JavaThreads from
// being removed from the JvmtiThreadState list we're about to walk
// so this ThreadsListHandle exists just to satisfy the lower level sanity
// checks that the target JavaThreads are protected.
ThreadsListHandle tlh;
-------------
PR: https://git.openjdk.java.net/jdk/pull/4677
