RFR(s): 8252414: Redundant suspend check when determining if a java thread is safe

[Reingruber, Richard]

Hi Robbin,

your change looks correct to me. Thanks!

I see a related issue that allows a suspended thread to continue executing
java. Hope it's ok to ask about it here.

Assume a thread T is resumed and then taken off CPU in JavaThread::java_suspend_self_with_safepoint_check()
just before restoring its prior state [1].

In this situation T gets suspended again (by executing VM_ThreadSuspend)

Now T is put back on the CPU. There are code paths where T won't check its
suspend flags and return to executing java even though it is suspended.


Example: JavaCallWrapper

  Stack:

  JavaThread::java_suspend_self_with_safepoint_check() : void
  JavaThread::handle_special_runtime_exit_condition(bool) : void
  JavaCallWrapper::JavaCallWrapper(const methodHandle &, Handle, JavaValue *, Thread *)

Example: Polling page exception

  Stack:

  JavaThread::handle_special_runtime_exit_condition(bool) : void
  SafepointSynchronize::block(JavaThread *) : void
  SafepointMechanism::block_or_handshake(JavaThread *) : void
  SafepointMechanism::block_if_requested_slow(JavaThread *) : void
  SafepointMechanism::block_if_requested(JavaThread *) : void
  ThreadSafepointState::handle_polling_page_exception() : void

Would you agree or have I missed something?

Note that there is no problem when T is put back on CPU while a handshake is
executed on its behalf because after restoring its prior state T calls
SafepointMechanism::block_if_requested() in
JavaThread::java_suspend_self_with_safepoint_check().

Thanks, Richard.

[1] T is taken off CPU before restoring its prior state.
    https://github.com/openjdk/jdk/blob/30c2dbea95f817c93f84f0d8f7dc3b5c159b5e25/src/hotspot/share/runtime/thread.cpp#L2619

-----Original Message-----
From: hotspot-runtime-dev <hotspot-runtime-dev-retn at openjdk.java.net> On Behalf Of Robbin Ehn
Sent: Donnerstag, 27. August 2020 09:21
To: hotspot-runtime-dev at openjdk.java.net
Subject: RFR(s): 8252414: Redundant suspend check when determining if a java thread is safe

Hi all, please review.

In 8221207 - "Redo JDK-8218446 - SuspendAtExit hangs" we fixed so a
thread is always blocked when suspended.

And added this nice assert.
int JavaThread::java_suspend_self() {
   assert(thread_state() == _thread_blocked, "wrong state for 
java_suspend_self()");

When checking if a thread is safepoint/handshake safe there no need to 
look at ext suspend flag anymore, since the thread is blocked.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8252414
Code:
http://cr.openjdk.java.net/~rehn/8252414/webrev/index.html

Passes t1-5

Thanks, Robbin