[heap] Make AlwaysAllocateScope thread-safe.

src/heap/heap.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
 #include "src/allocation.h"
 #include "src/assert-scope.h"
+#include "src/atomic-utils.h"
 #include "src/globals.h"
 #include "src/heap/gc-idle-time-handler.h"
 #include "src/heap/incremental-marking.h"
 #include "src/heap/mark-compact.h"
 #include "src/heap/objects-visiting.h"
 #include "src/heap/spaces.h"
 #include "src/heap/store-buffer.h"
 #include "src/list.h"
 
 namespace v8 {
@@ skipping 746 matching lines @@
   static inline void MoveBlock(Address dst, Address src, int byte_size);
 
   // Notifies the heap that is ok to start marking or other activities that
   // should not happen during deserialization.
   void NotifyDeserializationComplete();
 
   intptr_t old_generation_allocation_limit() const {
     return old_generation_allocation_limit_;
   }
 
-  bool always_allocate() { return always_allocate_scope_depth_ != 0; }
-  Address always_allocate_scope_depth_address() {

[Michael Lippautz, 2015/09/03 13:10:37]

Don't even think about it...

-    return reinterpret_cast<Address>(&always_allocate_scope_depth_);
-  }
+  bool always_allocate() { return always_allocate_scope_count_.Value() != 0; }
 
   Address* NewSpaceAllocationTopAddress() {
     return new_space_.allocation_top_address();
   }
   Address* NewSpaceAllocationLimitAddress() {
     return new_space_.allocation_limit_address();
   }
 
   Address* OldSpaceAllocationTopAddress() {
     return old_space_->allocation_top_address();
@@ skipping 1334 matching lines @@
   intptr_t max_executable_size_;
   intptr_t maximum_committed_;
 
   // For keeping track of how much data has survived
   // scavenge since last new space expansion.
   int survived_since_last_expansion_;
 
   // ... and since the last scavenge.
   int survived_last_scavenge_;
 
-  int always_allocate_scope_depth_;
+  // This is not the depth of nested AlwaysAllocateScope's but rather a single
+  // count, as scopes can be acquired from multiple tasks (read: threads).
+  AtomicValue always_allocate_scope_count_;
 
   // For keeping track of context disposals.
   int contexts_disposed_;
 
   int global_ic_age_;
 
   int scan_on_scavenge_pages_;
 
   NewSpace new_space_;
   OldSpace* old_space_;
@@ skipping 594 matching lines @@
   DisallowHeapAllocation no_allocation;  // i.e. no gc allowed.
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(PathTracer);
 };
 #endif  // DEBUG
 }
 }  // namespace v8::internal
 
 #endif  // V8_HEAP_HEAP_H_