!!! In construction now !!!!

HandshakeOperation* HandshakeState::get_op_for_self(bool allow_suspend, bool check_async_exception)

could just skip a HandshakeOperation if the _handshakee->is_vthread_transition_disabler() returns true, so the same suspension temporary disabling mechanism would be used as for _handshakee->is_disable_suspend().

The suggested fix is:

diff --git a/src/hotspot/share/runtime/handshake.cpp b/src/hotspot/share/runtime/handshake.cpp
index 89b02717a7a..360ef45bd07 100644
--- a/src/hotspot/share/runtime/handshake.cpp
+++ b/src/hotspot/share/runtime/handshake.cpp
@@ -521,7 +521,7 @@ HandshakeOperation* HandshakeState::get_op_for_self(bool allow_suspend, bool che
   assert(_lock.owned_by_self(), "Lock must be held");
   assert(allow_suspend || !check_async_exception, "invalid case");
 #if INCLUDE_JVMTI
- if (allow_suspend && _handshakee->is_disable_suspend()) {
+ if (allow_suspend && (_handshakee->is_disable_suspend() || _handshakee->is_vthread_transition_disabler())) {
     // filter out suspend operations while JavaThread is in disable_suspend mode
     allow_suspend = false;
   }