RFR: 8271348: Add stronger sanity check of thread state when polling for safepoint/handshakes [v2]

[Daniel D.Daugherty]

On Fri, 30 Jul 2021 00:02:30 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

>> I'll have to check to see who added that comment.
>
> It was added here:
> 
> commit bec8431683a36ad552a15cd7c4d5ca48058249a7
> Author: Robbin Ehn <rehn at openjdk.org>
> Date:   Fri Feb 15 14:15:10 2019 +0100
> 
>     8203469: Faster safepoints
>     
>     Reviewed-by: dcubed, pchilanomate, dholmes, acorn, coleenp, eosterlund

Here's the code block:

  uint64_t safepoint_id = SafepointSynchronize::safepoint_counter();
  // Check that we have a valid thread_state at this point
  switch(state) {
    case _thread_in_vm_trans:
    case _thread_in_Java:        // From compiled code
    case _thread_in_native_trans:
    case _thread_blocked_trans:
    case _thread_new_trans:

      // We have no idea where the VMThread is, it might even be at next safepoint.
      // So we can miss this poll, but stop at next.

      // Load dependent store, it must not pass loading of safepoint_id.
      thread->safepoint_state()->set_safepoint_id(safepoint_id); // Release store

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PR: https://git.openjdk.java.net/jdk/pull/4936