[heap] Make AlwaysAlloceScope thread-safe.

BUG=chromium:524425
LOG=N
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/1325173003

Cr-Commit-Position: refs/heads/master@{#30563}

src/heap/heap-inl.h

@@ -749,12 +749,12 @@ void Heap::SetSetterStubDeoptPCOffset(int pc_offset) {
 
 AlwaysAllocateScope::AlwaysAllocateScope(Isolate* isolate)
     : heap_(isolate->heap()), daf_(isolate) {
-  heap_->always_allocate_scope_depth_++;
+  heap_->always_allocate_scope_count_.Increment(1);
 }
 
 
 AlwaysAllocateScope::~AlwaysAllocateScope() {
-  heap_->always_allocate_scope_depth_--;
+  heap_->always_allocate_scope_count_.Increment(-1);
 }
 
 

src/heap/heap.cc

@@ -74,7 +74,7 @@ Heap::Heap()
       maximum_committed_(0),
       survived_since_last_expansion_(0),
       survived_last_scavenge_(0),
-      always_allocate_scope_depth_(0),
+      always_allocate_scope_count_(0),
       contexts_disposed_(0),
       global_ic_age_(0),
       scan_on_scavenge_pages_(0),

src/heap/heap.h

@@ -10,6 +10,7 @@
 
 #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"
@@ -776,10 +777,7 @@ class Heap {
     return old_generation_allocation_limit_;
   }
 
-  bool always_allocate() { return always_allocate_scope_depth_ != 0; }
-  Address always_allocate_scope_depth_address() {
-    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();
@@ -2134,7 +2132,9 @@ class Heap {
   // ... 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_;