From 008e9ffd3002430174cbdaa4aac9355dd8b40c15 Mon Sep 17 00:00:00 2001
From: Fredrik Bredberg <fbredberg@openjdk.org>
Date: Mon, 8 Jan 2024 13:32:17 +0000
Subject: [PATCH] 8320317: ObjectMonitor NotRunnable is not really an
 optimization

Reviewed-by: eosterlund, dholmes, shade, dcubed

JBR-6819 Backport 8320317
---
 src/hotspot/share/runtime/objectMonitor.cpp | 68 ---------------------
 src/hotspot/share/runtime/objectMonitor.hpp |  1 -
 2 files changed, 69 deletions(-)

diff --git a/src/hotspot/share/runtime/objectMonitor.cpp b/src/hotspot/share/runtime/objectMonitor.cpp
index 374816fd355e..ba5c88b94daa 100644
--- a/src/hotspot/share/runtime/objectMonitor.cpp
+++ b/src/hotspot/share/runtime/objectMonitor.cpp
@@ -1877,10 +1877,6 @@ int ObjectMonitor::TrySpin(JavaThread* current) {
   ctr = _SpinDuration;
   if (ctr <= 0) return 0;
 
-  if (NotRunnable(current, (JavaThread*) owner_raw())) {
-    return 0;
-  }
-
   // We're good to spin ... spin ingress.
   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
   // when preparing to LD...CAS _owner, etc and the CAS is likely
@@ -1963,13 +1959,6 @@ int ObjectMonitor::TrySpin(JavaThread* current) {
     }
     prv = ox;
 
-    // Abort the spin if the owner is not executing.
-    // The owner must be executing in order to drop the lock.
-    // Spinning while the owner is OFFPROC is idiocy.
-    // Consider: ctr -= RunnablePenalty ;
-    if (NotRunnable(current, ox)) {
-      goto Abort;
-    }
     if (_succ == NULL) {
       _succ = current;
     }
@@ -2002,63 +1991,6 @@ int ObjectMonitor::TrySpin(JavaThread* current) {
   return 0;
 }
 
-// NotRunnable() -- informed spinning
-//
-// Don't bother spinning if the owner is not eligible to drop the lock.
-// Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
-// The thread must be runnable in order to drop the lock in timely fashion.
-// If the _owner is not runnable then spinning will not likely be
-// successful (profitable).
-//
-// Beware -- the thread referenced by _owner could have died
-// so a simply fetch from _owner->_thread_state might trap.
-// Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
-// Because of the lifecycle issues, the _thread_state values
-// observed by NotRunnable() might be garbage.  NotRunnable must
-// tolerate this and consider the observed _thread_state value
-// as advisory.
-//
-// Beware too, that _owner is sometimes a BasicLock address and sometimes
-// a thread pointer.
-// Alternately, we might tag the type (thread pointer vs basiclock pointer)
-// with the LSB of _owner.  Another option would be to probabilistically probe
-// the putative _owner->TypeTag value.
-//
-// Checking _thread_state isn't perfect.  Even if the thread is
-// in_java it might be blocked on a page-fault or have been preempted
-// and sitting on a ready/dispatch queue.
-//
-// The return value from NotRunnable() is *advisory* -- the
-// result is based on sampling and is not necessarily coherent.
-// The caller must tolerate false-negative and false-positive errors.
-// Spinning, in general, is probabilistic anyway.
-
-
-int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) {
-  // Check ox->TypeTag == 2BAD.
-  if (ox == NULL) return 0;
-
-  // Avoid transitive spinning ...
-  // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
-  // Immediately after T1 acquires L it's possible that T2, also
-  // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
-  // This occurs transiently after T1 acquired L but before
-  // T1 managed to clear T1.Stalled.  T2 does not need to abort
-  // its spin in this circumstance.
-  intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
-
-  if (BlockedOn == 1) return 1;
-  if (BlockedOn != 0) {
-    return BlockedOn != intptr_t(this) && owner_raw() == ox;
-  }
-
-  assert(sizeof(ox->_thread_state == sizeof(int)), "invariant");
-  int jst = SafeFetch32((int *) &ox->_thread_state, -1);;
-  // consider also: jst != _thread_in_Java -- but that's overspecific.
-  return jst == _thread_blocked || jst == _thread_in_native;
-}
-
-
 // -----------------------------------------------------------------------------
 // WaitSet management ...
 
diff --git a/src/hotspot/share/runtime/objectMonitor.hpp b/src/hotspot/share/runtime/objectMonitor.hpp
index da3a4b098279..693dd94f4082 100644
--- a/src/hotspot/share/runtime/objectMonitor.hpp
+++ b/src/hotspot/share/runtime/objectMonitor.hpp
@@ -340,7 +340,6 @@ class ObjectMonitor : public CHeapObj<mtObjectMonitor> {
   void      ReenterI(JavaThread* current, ObjectWaiter* current_node);
   void      UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* current_node);
   int       TryLock(JavaThread* current);
-  int       NotRunnable(JavaThread* current, JavaThread* Owner);
   int       TrySpin(JavaThread* current);
   void      ExitEpilog(JavaThread* current, ObjectWaiter* Wakee);