RFR: 8257831: Suspend with handshakes [v2]

[Robbin Ehn]

On Wed, 31 Mar 2021 10:48:11 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> Thanks @dcubed-ojdk - no it won't. The problem is that the signal can hit very late in a thread's termination process, after the JavaThread destructor has executed, so no query of JavaThread state is possible. So we needed something in the Thread state and the SR_lock was a good enough proxy for that. It may be possible to use a different Thread field (perhaps _ParkEvent but encapsulated in a Thread::has_terminated() helper method).
>
> SR_handler is used for OS-level suspend/resume (not to conflict with this change-set).
> This feature is only used by JFR (AFAIK), and JFR only samples threads on it's ThreadsList.
> This means the JavaThread can never be terminated, hence this code will always pass.
> 
> If someone else is using OS-level suspend/resume without a ThreadsList, the bug is there is no ThreadsList AFAICT.
> 
> Since ThreadLocalStorage::thread() is cleared last in ~Thread with Thread::clear_thread_current(); may be in the ~Thread destructor.
> So the argument is that would be safe to read stuff from Thread but not JavaThread?
> Since the compiler (and CPU) may reorder and optimize away code, so I argue reading from a half destroyed object is not a great idea.
> E.g. Monitor* _SR_lock; is not a volatile pointer; since reading from this memory is UB after destruction, compiler is free to remove or not publish the store to NULL.
> 
> So I suggest either to remove this check, since the only user is using a ThreadsList and any other should also be using that too.
> Or call Thread::clear_thread_current() before the JavaThread destructor is called, that way we can be certain that there is no UB.

I got some offline input from David, there seem to be an issue with signal being delivered after the ThreadsListHandle destructor. If that is the case a ThreadsList doesn't help here.

So I suggested we keep this out of this change-set and just take another suitable field to mirror what we have today.

`ParkEvent * _ParkEvent;` ?

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PR: https://git.openjdk.java.net/jdk/pull/3191