Bug 1368922 - Set mIsDirty atomically. r=hiro

MozReview-Commit-ID: Ei3zCECVRFf

--HG--
extra : rebase_source : 0429db876e33329a250dced6e8bfcd965a5dfbc8

js/src/devtools/rootAnalysis/analyzeHeapWrites.js

@@ -35,6 +35,7 @@ function checkExternalFunction(entry)
         // Assume that atomic accesses are threadsafe.
         /^__atomic_fetch_/,
         /^__atomic_load_/,
+        /^__atomic_store_/,
         /^__atomic_thread_fence/,
     ];
     if (entry.matches(whitelist))
@@ -475,9 +476,6 @@ function ignoreContents(entry)
         // Unable to trace through dataflow, but straightforward if inspected.
         "Gecko_NewNoneTransform",
 
-        // Bug 1368922
-        "Gecko_UnsetDirtyStyleAttr",
-
         // Bug 1400438
         "Gecko_AppendMozBorderColors",
 

layout/style/DeclarationBlock.h

@@ -12,6 +12,7 @@
 #ifndef mozilla_DeclarationBlock_h
 #define mozilla_DeclarationBlock_h
 
+#include "mozilla/Atomics.h"
 #include "mozilla/ServoUtils.h"
 #include "mozilla/StyleBackendType.h"
 
@@ -33,8 +34,8 @@ class DeclarationBlock
 protected:
   explicit DeclarationBlock(StyleBackendType aType)
     : mImmutable(false)
-    , mIsDirty(false)
     , mType(aType)
+    , mIsDirty(false)
   {
     mContainer.mRaw = 0;
   }
@@ -154,10 +155,21 @@ private:
 
   // set when declaration put in the rule tree;
   bool mImmutable;
-  // True if this declaration has not been restyled after modified.
-  bool mIsDirty;
 
   const StyleBackendType mType;
+
+  // True if this declaration has not been restyled after modified.
+  //
+  // Since we can clear this flag from style worker threads, we use an Atomic.
+  //
+  // Note that although a single DeclarationBlock can be shared between
+  // different rule nodes (due to the style="" attribute cache), whenever a
+  // DeclarationBlock has its mIsDirty flag set to true, we always clone it to
+  // a unique object first. So when we clear this flag during Servo traversal,
+  // we know that we are clearing it on a DeclarationBlock that has a single
+  // reference, and there is no problem with another user of the same
+  // DeclarationBlock thinking that it is not dirty.
+  Atomic<bool, MemoryOrdering::Relaxed> mIsDirty;
 };
 
 } // namespace mozilla