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From 00352dd86bfa102b6e4b792120e3ef3498a27d1e Mon Sep 17 00:00:00 2001
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From: Russell Epstein <repstein@apple.com>
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Date: Fri, 17 Nov 2023 15:48:32 -0800
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Subject: [PATCH] Cherry-pick b0a755e34426.
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https://bugs.webkit.org/show_bug.cgi?id=265067
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Race condition between JSObject::getDirectConcurrently users and Structure::flattenDictionaryStructure
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https://bugs.webkit.org/show_bug.cgi?id=265067
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rdar://118548733
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Reviewed by Justin Michaud and Mark Lam.
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Like Array shift/unshift, flattenDictionaryStructure is the other code which can shrink butterfly for named properties (no other code does it).
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Compiler threads rely on the fact that normally named property storage never shrunk. And we should catch this exceptional case by taking a cellLock
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in the compiler thread. But flattenDictionaryStructure is not taking cellLock correctly.
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This patch computes afterOutOfLineCapacity first to detect that whether this flattening will shrink the butterfly.
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And if it is, then we take a cellLock. We do not need to take it if we do not shrink the butterfly.
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* Source/JavaScriptCore/runtime/Structure.cpp:
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(JSC::Structure::flattenDictionaryStructure):
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Canonical link: https://commits.webkit.org/267815.577@safari-7617-branch
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Canonical link: https://commits.webkit.org/265870.632@safari-7616.2.9.10-branch
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---
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Source/JavaScriptCore/runtime/Structure.cpp | 28 +++++++++++++++------
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1 file changed, 21 insertions(+), 7 deletions(-)
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diff --git a/Source/JavaScriptCore/runtime/Structure.cpp b/Source/JavaScriptCore/runtime/Structure.cpp
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index 2922e2478794c..9d094e2c8adc8 100644
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--- a/Source/JavaScriptCore/runtime/Structure.cpp
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+++ b/Source/JavaScriptCore/runtime/Structure.cpp
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@@ -913,17 +913,31 @@ Structure* Structure::flattenDictionaryStructure(VM& vm, JSObject* object)
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checkOffsetConsistency();
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ASSERT(isDictionary());
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ASSERT(object->structure() == this);
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-
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- GCSafeConcurrentJSLocker locker(m_lock, vm);
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-
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- object->setStructureIDDirectly(id().nuke());
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- WTF::storeStoreFence();
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+ Locker<JSCellLock> cellLocker(NoLockingNecessary);
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+
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+ PropertyTable* table = nullptr;
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size_t beforeOutOfLineCapacity = this->outOfLineCapacity();
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+ size_t afterOutOfLineCapacity = beforeOutOfLineCapacity;
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if (isUncacheableDictionary()) {
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- PropertyTable* table = propertyTableOrNull();
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+ table = propertyTableOrNull();
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ASSERT(table);
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+ PropertyOffset maxOffset = invalidOffset;
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+ if (unsigned propertyCount = table->size())
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+ maxOffset = offsetForPropertyNumber(propertyCount - 1, m_inlineCapacity);
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+ afterOutOfLineCapacity = outOfLineCapacity(maxOffset);
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+ }
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+ // This is the only case we shrink butterfly in this function. We should take a cell lock to protect against concurrent access to the butterfly.
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+ if (beforeOutOfLineCapacity != afterOutOfLineCapacity)
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+ cellLocker = Locker { object->cellLock() };
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+
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+ GCSafeConcurrentJSLocker locker(m_lock, vm);
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+
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+ object->setStructureIDDirectly(id().nuke());
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+ WTF::storeStoreFence();
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+
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+ if (isUncacheableDictionary()) {
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size_t propertyCount = table->size();
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// Holds our values compacted by insertion order. This is OK since GC is deferred.
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@@ -955,7 +969,7 @@ Structure* Structure::flattenDictionaryStructure(VM& vm, JSObject* object)
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setDictionaryKind(NoneDictionaryKind);
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setHasBeenFlattenedBefore(true);
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- size_t afterOutOfLineCapacity = this->outOfLineCapacity();
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+ ASSERT(this->outOfLineCapacity() == afterOutOfLineCapacity);
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if (object->butterfly() && beforeOutOfLineCapacity != afterOutOfLineCapacity) {
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ASSERT(beforeOutOfLineCapacity > afterOutOfLineCapacity);
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eabdullin
Andrew Lukoshko