RFR: 8254263: Remove special_runtime_exit_condition() check from ~ThreadInVMForHandshake()

[Patricio Chilano Mateo]

On Sun, 11 Oct 2020 22:09:30 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Hi all,
>> 
>> Please review the following patch that removes the call to handle_special_runtime_exit_condition() in
>> ~ThreadInVMForHandshake(). This check was added by 8223572 to prevent a JavaThread that was suspended while trying to
>> process a handshake to transition back to java without noticing it was suspended. Since 8238761, a JavaThread executing
>> HandshakeState::process_by_self() will never become safe. It comes from an unsafe state and remains unsafe, so it
>> cannot be suspended during that time. Removing this check also removes one of the reasons
>> SafepointMechanism::process_if_requested() is recursive (the other one remains SafepointSynchronize::block()). Tested
>> in mach5, tiers 1-7.  Thanks, Patricio
>
> src/hotspot/share/runtime/interfaceSupport.inline.hpp line 146:
> 
>> 144:
>> 145:   ~ThreadInVMForHandshake() {
>> 146:     assert(_thread->thread_state() == _thread_in_vm, "should only call when leaving VM after handshake");
> 
> Can we also assert `!_thread->has_special_runtime_exit_condition()`? Or at least that the value of this has not changed
> between the TIVFH constructor and destructor.

has_special_runtime_exit_condition() checks for async exceptions, external_suspend and JFR suspend. The last two can
actually be set while we are in the handshake. It doesn't mean that the caller will consider the target suspended, but
the _suspend_flags might be different at construction and destruction of TIVFH. As for async exceptions, I realized we
have a similar issue as the one described in 8223572 when calling handle_polling_page_exception() on a return poll:

- T calls ThreadSafepointState::handle_polling_page_exception() because another thread called T.stop() (which we
  implement with a handshake)
- T calls SafepointMechanism::process_if_requested(thread())
- T calls HandshakeState::process_self_inner() and processes the handshake which calls set_pending_async_exception()
- T returns from SafepointMechanism::process_if_requested(thread()) and goes back to java without calling
  check_and_handle_async_exceptions() to throw the ThreadDeath() exception

I'm not sure if delaying the throw of ThreadDeath is allowed, so if we need to keep the current behaviour I can add a
call to check_and_handle_async_exceptions():

@@ -987,6 +987,7 @@ void ThreadSafepointState::handle_polling_page_exception() {
     // Process pending operation
     SafepointMechanism::process_if_requested(thread());
+   thread()->check_and_handle_async_exceptions();

     // restore oop result, if any

-------------

PR: https://git.openjdk.java.net/jdk/pull/577