[heap] Avoid concurrent accessors when releasing slot sets

Those sets are to be released on the main thread without concurrent
accesses. Making this explicit will give TSAN a chance to help us once
the surrounding code changes.

Bug: 
Change-Id: Ia73754caafbeec385d4c922fb8140e3e64f7378c
Reviewed-on: https://chromium-review.googlesource.com/541375Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46025}

src/heap/spaces.cc

@@ -1249,10 +1249,10 @@ template void MemoryChunk::ReleaseSlotSet<OLD_TO_OLD>();
 
 template <RememberedSetType type>
 void MemoryChunk::ReleaseSlotSet() {
-  SlotSet* slot_set = base::AsAtomicWord::Acquire_Load(&slot_set_[type]);
+  SlotSet* slot_set = slot_set_[type];
   if (slot_set) {
+    slot_set_[type] = nullptr;
     delete[] slot_set;
-    base::AsAtomicWord::Release_Store(&slot_set_[type], nullptr);
   }
 }
 
@@ -1277,11 +1277,10 @@ template void MemoryChunk::ReleaseTypedSlotSet<OLD_TO_OLD>();
 
 template <RememberedSetType type>
 void MemoryChunk::ReleaseTypedSlotSet() {
-  TypedSlotSet* typed_slot_set =
-      base::AsAtomicWord::Acquire_Load(&typed_slot_set_[type]);
+  TypedSlotSet* typed_slot_set = typed_slot_set_[type];
   if (typed_slot_set) {
+    typed_slot_set_[type] = nullptr;
     delete typed_slot_set;
-    base::AsAtomicWord::Release_Store(&typed_slot_set_[type], nullptr);
   }
 }
 

src/heap/spaces.h

@@ -461,19 +461,22 @@ class MemoryChunk {
     return typed_slot_set_[type];
   }
 
-  inline LocalArrayBufferTracker* local_tracker() { return local_tracker_; }
-  bool contains_array_buffers();
-
   template <RememberedSetType type>
   SlotSet* AllocateSlotSet();
+  // Not safe to be called concurrently.
   template <RememberedSetType type>
   void ReleaseSlotSet();
   template <RememberedSetType type>
   TypedSlotSet* AllocateTypedSlotSet();
+  // Not safe to be called concurrently.
   template <RememberedSetType type>
   void ReleaseTypedSlotSet();
+
   void AllocateLocalTracker();
   void ReleaseLocalTracker();
+  inline LocalArrayBufferTracker* local_tracker() { return local_tracker_; }
+  bool contains_array_buffers();
+
   void AllocateYoungGenerationBitmap();
   void ReleaseYoungGenerationBitmap();