src/hotspot/share/runtime/objectMonitor.inline.hpp

rev 54415 : 8222295: more baseline cleanups from Async Monitor Deflation project
rev 54416 : Checkpoint latest preliminary review patches for full OpenJDK review; merge with 8222295.patch.
rev 54417 : imported patch dcubed.monitor_deflate_conc.v2.01

@@ -54,33 +54,45 @@
   void* owner = _owner;
   return owner != DEFLATER_MARKER ? owner : NULL;
 }
 
 inline void ObjectMonitor::clear() {
+  assert(_header != NULL, "must be non-NULL");
   assert(_contentions == 0, "must be 0: contentions=%d", _contentions);
   assert(_owner == NULL, "must be NULL: owner=" INTPTR_FORMAT, p2i(_owner));
 
+  _header = NULL;
+
   clear_using_JT();
 }
 
 inline void ObjectMonitor::clear_using_JT() {
-  // When clearing using a JavaThread, we leave _owner == DEFLATER_MARKER
-  // and _contentions < 0 to force any racing threads to retry. Unlike other
-  // *_using_JT() functions, we cannot assert AsyncDeflateIdleMonitors
-  // or Thread::current()->is_Java_thread() because clear() calls this
-  // function for the rest of its checks.
-
-  assert(_header != NULL, "must be non-NULL");
+  // Unlike other *_using_JT() functions, we cannot assert
+  // AsyncDeflateIdleMonitors or Thread::current()->is_Java_thread()
+  // because clear() calls this function for the rest of its checks.
+
+  if (AsyncDeflateIdleMonitors) {
+    // Async deflation protocol uses the _header, _contentions and _owner
+    // fields. While the ObjectMonitor being deflated is on the global free
+    // list, we leave those three fields alone; _owner == DEFLATER_MARKER
+    // and _contentions < 0 will force any racing threads to retry. The
+    // _header field is used by install_displaced_markword_in_object()
+    // in the last part of the deflation protocol so we cannot check
+    // its values here.
+    guarantee(_owner == NULL || _owner == DEFLATER_MARKER,
+              "must be NULL or DEFLATER_MARKER: owner=" INTPTR_FORMAT,
+              p2i(_owner));
+    guarantee(_contentions <= 0, "must be <= 0: contentions=%d", _contentions);
+  }
   assert(_waiters == 0, "must be 0: waiters=%d", _waiters);
   assert(_recursions == 0, "must be 0: recursions=" INTPTR_FORMAT, _recursions);
   assert(_object != NULL, "must be non-NULL");
   // Do not assert _ref_count == 0 here because a racing thread could
   // increment _ref_count, observe _owner == DEFLATER_MARKER and then
   // decrement _ref_count.
 
   set_allocation_state(Free);
-  _header = NULL;
   _object = NULL;
   // Do not clear _ref_count here because _ref_count is for indicating
   // that the ObjectMonitor* is in use which is orthogonal to whether
   // the ObjectMonitor itself is in use for a locking operation.
 }

@@ -146,34 +158,22 @@
 inline bool ObjectMonitor::is_new() const {
   return _allocation_state == New;
 }
 
 inline void ObjectMonitor::dec_ref_count() {
-  // The decrement needs to be MO_ACQ_REL. At the moment, the Atomic::dec
-  // backend on PPC does not yet conform to these requirements. Therefore
-  // the decrement is simulated with an Atomic::sub(1, &addr). Without
-  // this MO_ACQ_REL Atomic::dec simulation, AsyncDeflateIdleMonitors is
-  // not safe.
-  Atomic::sub((jint)1, &_ref_count);
+  // The decrement only needs to be MO_ACQ_REL since the reference
+  // counter is volatile.
+  Atomic::dec(&_ref_count);
   guarantee(_ref_count >= 0, "sanity check: ref_count=%d", _ref_count);
 }
 
 inline void ObjectMonitor::inc_ref_count() {
-  // The increment needs to be MO_SEQ_CST. At the moment, the Atomic::inc
-  // backend on PPC does not yet conform to these requirements. Therefore
-  // the increment is simulated with a load phi; cas phi + 1; loop.
-  // Without this MO_SEQ_CST Atomic::inc simulation, AsyncDeflateIdleMonitors
-  // is not safe.
-  for (;;) {
-    jint sample = OrderAccess::load_acquire(&_ref_count);
-    guarantee(sample >= 0, "sanity check: sample=%d", (int)sample);
-    if (Atomic::cmpxchg(sample + 1, &_ref_count, sample) == sample) {
-      // Incremented _ref_count without interference.
-      return;
-    }
-    // Implied else: Saw interference so loop and try again.
-  }
+  // The increment needs to be MO_SEQ_CST so that the reference
+  // counter update is seen as soon as possible in a race with the
+  // async deflation protocol.
+  Atomic::inc(&_ref_count);
+  guarantee(_ref_count > 0, "sanity check: ref_count=%d", _ref_count);
 }
 
 inline jint ObjectMonitor::ref_count() const {
   return OrderAccess::load_acquire(&_ref_count);
 }