8331714: Make OopMapCache installation lock-free

Reviewed-by: zgu, coleenp

src/hotspot/share/oops/instanceKlass.cpp

@@ -1634,16 +1634,17 @@ void InstanceKlass::call_class_initializer(TRAPS) {
 
 void InstanceKlass::mask_for(const methodHandle& method, int bci,
   InterpreterOopMap* entry_for) {
-  // Lazily create the _oop_map_cache at first request
-  // Lock-free access requires load_acquire.
+  // Lazily create the _oop_map_cache at first request.
+  // Load_acquire is needed to safely get instance published with CAS by another thread.
   OopMapCache* oop_map_cache = Atomic::load_acquire(&_oop_map_cache);
   if (oop_map_cache == nullptr) {
-    MutexLocker x(OopMapCacheAlloc_lock);
-    // Check if _oop_map_cache was allocated while we were waiting for this lock
-    if ((oop_map_cache = _oop_map_cache) == nullptr) {
-      oop_map_cache = new OopMapCache();
-      // Ensure _oop_map_cache is stable, since it is examined without a lock
-      Atomic::release_store(&_oop_map_cache, oop_map_cache);
+    // Try to install new instance atomically.
+    oop_map_cache = new OopMapCache();
+    OopMapCache* other = Atomic::cmpxchg(&_oop_map_cache, (OopMapCache*)nullptr, oop_map_cache);
+    if (other != nullptr) {
+      // Someone else managed to install before us, ditch local copy and use the existing one.
+      delete oop_map_cache;
+      oop_map_cache = other;
     }
   }
   // _oop_map_cache is constant after init; lookup below does its own locking.

src/hotspot/share/runtime/mutexLocker.cpp

@@ -100,7 +100,6 @@ Mutex*   tty_lock                     = nullptr;
 Mutex*   RawMonitor_lock              = nullptr;
 Mutex*   PerfDataMemAlloc_lock        = nullptr;
 Mutex*   PerfDataManager_lock         = nullptr;
-Mutex*   OopMapCacheAlloc_lock        = nullptr;
 
 Mutex*   FreeList_lock                = nullptr;
 Mutex*   OldSets_lock                 = nullptr;
@@ -349,7 +348,6 @@ void mutex_init() {
     MUTEX_DEFL(PSOldGenExpand_lock          , PaddedMutex  , Heap_lock, true);
   }
 #endif
-  MUTEX_DEFL(OopMapCacheAlloc_lock          , PaddedMutex  ,  Threads_lock, true);
   MUTEX_DEFL(Module_lock                    , PaddedMutex  ,  ClassLoaderDataGraph_lock);
   MUTEX_DEFL(SystemDictionary_lock          , PaddedMonitor, Module_lock);
   MUTEX_DEFL(JNICritical_lock               , PaddedMonitor, AdapterHandlerLibrary_lock); // used for JNI critical regions

src/hotspot/share/runtime/mutexLocker.hpp

@@ -96,7 +96,6 @@ extern Mutex*   FullGCALot_lock;                 // a lock to make FullGCALot MT
 extern Mutex*   RawMonitor_lock;
 extern Mutex*   PerfDataMemAlloc_lock;           // a lock on the allocator for PerfData memory for performance data
 extern Mutex*   PerfDataManager_lock;            // a long on access to PerfDataManager resources
-extern Mutex*   OopMapCacheAlloc_lock;           // protects allocation of oop_map caches
 
 extern Mutex*   FreeList_lock;                   // protects the free region list during safepoints
 extern Mutex*   OldSets_lock;                    // protects the old region sets