8235795: replace monitor list mux{Acquire,Release}(&gListLock) with spin locks

Reviewed-by: dholmes, coleenp, rehn

Author: Daniel D. Daugherty

src/hotspot/share/runtime/objectMonitor.cpp

@@ -1987,7 +1987,7 @@ void ObjectMonitor::print_debug_style_on(outputStream* st) const {
   st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
   st->print_cr("  _header = " INTPTR_FORMAT, header().value());
   st->print_cr("  _object = " INTPTR_FORMAT, p2i(_object));
-  st->print_cr("  _next_om = " INTPTR_FORMAT, p2i(_next_om));
+  st->print_cr("  _next_om = " INTPTR_FORMAT, p2i(next_om()));
   st->print_cr("  _pad_buf0 = {");
   st->print_cr("    [0] = '\\0'");
   st->print_cr("    ...");

src/hotspot/share/runtime/objectMonitor.hpp

@@ -136,20 +136,22 @@ class ObjectMonitor {
   // Enforced by the assert() in header_addr().
   volatile markWord _header;        // displaced object header word - mark
   void* volatile _object;           // backward object pointer - strong root
- public:
-  ObjectMonitor* _next_om;          // Next ObjectMonitor* linkage
  private:
   // Separate _header and _owner on different cache lines since both can
   // have busy multi-threaded access. _header and _object are set at
   // initial inflation and _object doesn't change until deflation so
   // _object is a good choice to share the cache line with _header.
-  // _next_om shares _header's cache line for pre-monitor list historical
-  // reasons. _next_om only changes if the next ObjectMonitor is deflated.
   DEFINE_PAD_MINUS_SIZE(0, OM_CACHE_LINE_SIZE,
-                        sizeof(volatile markWord) + sizeof(void* volatile) +
-                        sizeof(ObjectMonitor *));
+                        sizeof(volatile markWord) + sizeof(void* volatile));
   void* volatile _owner;            // pointer to owning thread OR BasicLock
   volatile jlong _previous_owner_tid;  // thread id of the previous owner of the monitor
+  // Separate _owner and _next_om on different cache lines since
+  // both can have busy multi-threaded access. _previous_owner_tid is only
+  // changed by ObjectMonitor::exit() so it is a good choice to share the
+  // cache line with _owner.
+  DEFINE_PAD_MINUS_SIZE(1, OM_CACHE_LINE_SIZE, sizeof(void* volatile) +
+                        sizeof(volatile jlong));
+  ObjectMonitor* _next_om;          // Next ObjectMonitor* linkage
   volatile intx _recursions;        // recursion count, 0 for first entry
   ObjectWaiter* volatile _EntryList;  // Threads blocked on entry or reentry.
                                       // The list is actually composed of WaitNodes,
@@ -249,6 +251,14 @@ class ObjectMonitor {
   // _owner field. Returns the prior value of the _owner field.
   void*     try_set_owner_from(void* old_value, void* new_value);
 
+  ObjectMonitor* next_om() const;
+  // Simply set _next_om field to new_value.
+  void set_next_om(ObjectMonitor* new_value);
+  // Try to set _next_om field to new_value if the current value matches
+  // old_value, using Atomic::cmpxchg(). Otherwise, does not change the
+  // _next_om field. Returns the prior value of the _next_om field.
+  ObjectMonitor* try_set_next_om(ObjectMonitor* old_value, ObjectMonitor* new_value);
+
   jint      waiters() const;
 
   jint      contentions() const;

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

@@ -137,4 +137,24 @@ inline void* ObjectMonitor::try_set_owner_from(void* old_value, void* new_value)
   return prev;
 }
 
+// The _next_om field can be concurrently read and modified so we
+// use Atomic operations to disable compiler optimizations that
+// might try to elide loading and/or storing this field.
+
+inline ObjectMonitor* ObjectMonitor::next_om() const {
+  return Atomic::load(&_next_om);
+}
+
+// Simply set _next_om field to new_value.
+inline void ObjectMonitor::set_next_om(ObjectMonitor* new_value) {
+  Atomic::store(&_next_om, new_value);
+}
+
+// Try to set _next_om field to new_value if the current value matches
+// old_value. Otherwise, does not change the _next_om field. Returns
+// the prior value of the _next_om field.
+inline ObjectMonitor* ObjectMonitor::try_set_next_om(ObjectMonitor* old_value, ObjectMonitor* new_value) {
+  return Atomic::cmpxchg(&_next_om, old_value, new_value);
+}
+
 #endif // SHARE_RUNTIME_OBJECTMONITOR_INLINE_HPP