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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/201590">201590</a></dd>
<dt>Author</dt> <dd>fpizlo@apple.com</dd>
<dt>Date</dt> <dd>2016-06-01 21:26:32 -0700 (Wed, 01 Jun 2016)</dd>
</dl>
<h3>Log Message</h3>
<pre>Structure::previousID() races with Structure::allocateRareData()
https://bugs.webkit.org/show_bug.cgi?id=158280
Reviewed by Mark Lam.
The problem is that previousID() would test hasRareData() and then either load the
previous Structure from the rare data, or load it directly. allocateRareData() would set
the hasRareData() bit separately from moving the Structure pointer into the rare data. So
we'd have a race that would cause previousID() to sometimes return the rarae data instead
of the previous Structure.
The fix is to get rid of the hasRareData bit. We can use the structureID of the
previousOrRareData cell to determine if it's the previousID or the RareData. This fixes the
race and it's probably not any slower.
* runtime/Structure.cpp:
(JSC::Structure::Structure):
(JSC::Structure::allocateRareData):
* runtime/Structure.h:</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceJavaScriptCoreChangeLog">trunk/Source/JavaScriptCore/ChangeLog</a></li>
<li><a href="#trunkSourceJavaScriptCoreruntimeStructurecpp">trunk/Source/JavaScriptCore/runtime/Structure.cpp</a></li>
<li><a href="#trunkSourceJavaScriptCoreruntimeStructureh">trunk/Source/JavaScriptCore/runtime/Structure.h</a></li>
</ul>
</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceJavaScriptCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/ChangeLog (201589 => 201590)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/ChangeLog 2016-06-02 04:07:14 UTC (rev 201589)
+++ trunk/Source/JavaScriptCore/ChangeLog 2016-06-02 04:26:32 UTC (rev 201590)
</span><span class="lines">@@ -1,3 +1,25 @@
</span><ins>+2016-06-01 Filip Pizlo <fpizlo@apple.com>
+
+ Structure::previousID() races with Structure::allocateRareData()
+ https://bugs.webkit.org/show_bug.cgi?id=158280
+
+ Reviewed by Mark Lam.
+
+ The problem is that previousID() would test hasRareData() and then either load the
+ previous Structure from the rare data, or load it directly. allocateRareData() would set
+ the hasRareData() bit separately from moving the Structure pointer into the rare data. So
+ we'd have a race that would cause previousID() to sometimes return the rarae data instead
+ of the previous Structure.
+
+ The fix is to get rid of the hasRareData bit. We can use the structureID of the
+ previousOrRareData cell to determine if it's the previousID or the RareData. This fixes the
+ race and it's probably not any slower.
+
+ * runtime/Structure.cpp:
+ (JSC::Structure::Structure):
+ (JSC::Structure::allocateRareData):
+ * runtime/Structure.h:
+
</ins><span class="cx"> 2016-06-01 Michael Saboff <msaboff@apple.com>
</span><span class="cx">
</span><span class="cx"> Runaway WebContent process CPU & memory @ foxnews.com
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoreruntimeStructurecpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/runtime/Structure.cpp (201589 => 201590)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/runtime/Structure.cpp 2016-06-02 04:07:14 UTC (rev 201589)
+++ trunk/Source/JavaScriptCore/runtime/Structure.cpp 2016-06-02 04:26:32 UTC (rev 201590)
</span><span class="lines">@@ -206,7 +206,6 @@
</span><span class="cx"> setDidPreventExtensions(false);
</span><span class="cx"> setDidTransition(false);
</span><span class="cx"> setStaticFunctionsReified(false);
</span><del>- setHasRareData(false);
</del><span class="cx"> setTransitionWatchpointIsLikelyToBeFired(false);
</span><span class="cx"> setHasBeenDictionary(false);
</span><span class="cx">
</span><span class="lines">@@ -238,7 +237,6 @@
</span><span class="cx"> setDidPreventExtensions(false);
</span><span class="cx"> setDidTransition(false);
</span><span class="cx"> setStaticFunctionsReified(false);
</span><del>- setHasRareData(false);
</del><span class="cx"> setTransitionWatchpointIsLikelyToBeFired(false);
</span><span class="cx"> setHasBeenDictionary(false);
</span><span class="cx">
</span><span class="lines">@@ -269,7 +267,6 @@
</span><span class="cx"> setDidPreventExtensions(previous->didPreventExtensions());
</span><span class="cx"> setDidTransition(true);
</span><span class="cx"> setStaticFunctionsReified(previous->staticFunctionsReified());
</span><del>- setHasRareData(false);
</del><span class="cx"> setHasBeenDictionary(previous->hasBeenDictionary());
</span><span class="cx">
</span><span class="cx"> TypeInfo typeInfo = previous->typeInfo();
</span><span class="lines">@@ -823,11 +820,9 @@
</span><span class="cx"> void Structure::allocateRareData(VM& vm)
</span><span class="cx"> {
</span><span class="cx"> ASSERT(!hasRareData());
</span><del>- StructureRareData* rareData = StructureRareData::create(vm, previous());
</del><ins>+ StructureRareData* rareData = StructureRareData::create(vm, previousID());
</ins><span class="cx"> WTF::storeStoreFence();
</span><span class="cx"> m_previousOrRareData.set(vm, this, rareData);
</span><del>- WTF::storeStoreFence();
- setHasRareData(true);
</del><span class="cx"> ASSERT(hasRareData());
</span><span class="cx"> }
</span><span class="cx">
</span></span></pre></div>
<a id="trunkSourceJavaScriptCoreruntimeStructureh"></a>
<div class="modfile"><h4>Modified: trunk/Source/JavaScriptCore/runtime/Structure.h (201589 => 201590)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/JavaScriptCore/runtime/Structure.h 2016-06-02 04:07:14 UTC (rev 201589)
+++ trunk/Source/JavaScriptCore/runtime/Structure.h 2016-06-02 04:26:32 UTC (rev 201590)
</span><span class="lines">@@ -274,13 +274,21 @@
</span><span class="cx">
</span><span class="cx"> // Will just the prototype chain intercept this property access?
</span><span class="cx"> JS_EXPORT_PRIVATE bool prototypeChainMayInterceptStoreTo(VM&, PropertyName);
</span><del>-
</del><ins>+
+ bool hasRareData() const
+ {
+ return isRareData(m_previousOrRareData.get());
+ }
+
</ins><span class="cx"> Structure* previousID() const
</span><span class="cx"> {
</span><span class="cx"> ASSERT(structure()->classInfo() == info());
</span><del>- if (hasRareData())
- return rareData()->previousID();
- return previous();
</del><ins>+ // This is so written because it's used concurrently. We only load from m_previousOrRareData
+ // once, and this load is guaranteed atomic.
+ JSCell* cell = m_previousOrRareData.get();
+ if (isRareData(cell))
+ return static_cast<StructureRareData*>(cell)->previousID();
+ return static_cast<Structure*>(cell);
</ins><span class="cx"> }
</span><span class="cx"> bool transitivelyTransitionedFrom(Structure* structureToFind);
</span><span class="cx">
</span><span class="lines">@@ -602,12 +610,11 @@
</span><span class="cx"> DEFINE_BITFIELD(bool, didPreventExtensions, DidPreventExtensions, 1, 20);
</span><span class="cx"> DEFINE_BITFIELD(bool, didTransition, DidTransition, 1, 21);
</span><span class="cx"> DEFINE_BITFIELD(bool, staticFunctionsReified, StaticFunctionsReified, 1, 22);
</span><del>- DEFINE_BITFIELD(bool, hasRareData, HasRareData, 1, 23);
- DEFINE_BITFIELD(bool, hasBeenFlattenedBefore, HasBeenFlattenedBefore, 1, 24);
- DEFINE_BITFIELD(bool, hasCustomGetterSetterProperties, HasCustomGetterSetterProperties, 1, 25);
- DEFINE_BITFIELD(bool, didWatchInternalProperties, DidWatchInternalProperties, 1, 26);
- DEFINE_BITFIELD(bool, transitionWatchpointIsLikelyToBeFired, TransitionWatchpointIsLikelyToBeFired, 1, 27);
- DEFINE_BITFIELD(bool, hasBeenDictionary, HasBeenDictionary, 1, 28);
</del><ins>+ DEFINE_BITFIELD(bool, hasBeenFlattenedBefore, HasBeenFlattenedBefore, 1, 23);
+ DEFINE_BITFIELD(bool, hasCustomGetterSetterProperties, HasCustomGetterSetterProperties, 1, 24);
+ DEFINE_BITFIELD(bool, didWatchInternalProperties, DidWatchInternalProperties, 1, 25);
+ DEFINE_BITFIELD(bool, transitionWatchpointIsLikelyToBeFired, TransitionWatchpointIsLikelyToBeFired, 1, 26);
+ DEFINE_BITFIELD(bool, hasBeenDictionary, HasBeenDictionary, 1, 27);
</ins><span class="cx">
</span><span class="cx"> private:
</span><span class="cx"> friend class LLIntOffsetsExtractor;
</span><span class="lines">@@ -693,11 +700,10 @@
</span><span class="cx"> bool isValid(ExecState*, StructureChain* cachedPrototypeChain) const;
</span><span class="cx">
</span><span class="cx"> void pin();
</span><del>-
- Structure* previous() const
</del><ins>+
+ bool isRareData(JSCell* cell) const
</ins><span class="cx"> {
</span><del>- ASSERT(!hasRareData());
- return static_cast<Structure*>(m_previousOrRareData.get());
</del><ins>+ return cell && cell->structureID() != structureID();
</ins><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> StructureRareData* rareData() const
</span></span></pre>
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