Oilpan: disable conservative GCs during initial GC mixin construction.

Source/platform/heap/ThreadState.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 564 matching lines @@
     void shouldFlushHeapDoesNotContainCache() { m_shouldFlushHeapDoesNotContainCache = true; }
 
     void registerTraceDOMWrappers(v8::Isolate* isolate, void (*traceDOMWrappers)(v8::Isolate*, Visitor*))
     {
         m_isolate = isolate;
         m_traceDOMWrappers = traceDOMWrappers;
     }
 
     double collectionRate() const { return m_collectionRate; }
 
+    // By entering a gc-forbidden scope, conservative GCs will not
+    // be allowed while handling an out-of-line allocation request.
+    // Intended used when constructing subclasses of GC mixins, where
+    // the object being constructed cannot be safely traced & marked
+    // fully should a GC be allowed while its subclasses are being
+    // constructed.
+    void enterGCForbiddenScope()
+    {
+        m_gcForbiddenCount++;
+    }
+
 private:
     ThreadState();
     ~ThreadState();
 
     friend class SafePointBarrier;
     friend class SafePointAwareMutexLocker;
 
     void enterSafePoint(StackState, void*);
     NO_SANITIZE_ADDRESS void copyStackUntilSafePointScope();
     void clearSafePointScopeMarker()
@@ skipping 24 matching lines @@
     void cleanup();
     void cleanupPages();
 
     void unregisterPreFinalizerInternal(void*);
     void invokePreFinalizers(Visitor&);
 
 #if ENABLE(GC_PROFILING)
     void snapshotFreeList();
 #endif
 
-    // By entering a gc-forbidden scope, conservative GCs will not
-    // be allowed while handling an out-of-line allocation request.
-    // Intended used when constructing subclasses of GC mixins, where
-    // the object being constructed cannot be safely traced & marked
-    // fully should a GC be allowed while its subclasses are being
-    // constructed.
-    template<typename U, typename V> friend class AllocateObjectTrait;
-    void enterGCForbiddenScope() { m_gcForbiddenCount++; }
+    template<typename U> friend class GarbageCollectedMixinConstructorMarker;
     void leaveGCForbiddenScope()
     {
-        ASSERT(m_gcForbiddenCount > 0);
-        m_gcForbiddenCount--;
+        // FIXME: if a declared mixin object derives from another,
+        // its instantiation will leave this scope twice, which
+        // will lead to unbalanced accounting. If that happens
+        // nestedly as part of constructing another mixin, this
+        // might result in the forbidden GC scope being exited
+        // too early. Theoretically; no Blink mixin objects
+        // known to have such nested constructors and be at risk.
+        if (m_gcForbiddenCount)

[haraken, 2015/03/05 02:20:19]

Hmm, it is concerning that m_gcForbiddenCount can be unbalanced. It would be
safe as far as we decrease m_gcForbiddenCount too much, but it is dangerous if
we increase m_gcForbiddenCount too much because it will end up with suppressing
GCs for a long time. I don't think this will happen with this CL, but the fact
that the counter is not balanced looks not nice...

+            m_gcForbiddenCount--;
     }
 
     static WTF::ThreadSpecific<ThreadState*>* s_threadSpecific;
     static uintptr_t s_mainThreadStackStart;
     static uintptr_t s_mainThreadUnderestimatedStackSize;
     static SafePointBarrier* s_safePointBarrier;
 
     // We can't create a static member of type ThreadState here
     // because it will introduce global constructor and destructor.
     // We would like to manage lifetime of the ThreadState attached
@@ skipping 111 matching lines @@
         m_locked = false;
     }
 
     MutexBase& m_mutex;
     bool m_locked;
 };
 
 } // namespace blink
 
 #endif // ThreadState_h