Bug 83839 – GC in the middle of JSObject::allocatePropertyStorage can cause badness

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[Mark Hahnenberg]

Currently, when we're in the middle of JSObject::allocatePropertyStorage, our Structure has inconsistent information. The Structure has just had a property added to it and it thinks it is bigger in both size and capacity, but our actual property backing store is still the same size as before. This can lead the garbage collector to do bad things based on this inaccurate information. We should instead check to see if we need to allocate more space first and do so if necessary, and then add the new property to our structure.

[Mark Hahnenberg]

Created attachment 137007 [details]
Patch

[Build Bot]

Comment on attachment 137007 [details]
Patch

Attachment 137007 [details] did not pass win-ews (win):
Output: http://queues.webkit.org/results/12391710

[Mark Hahnenberg]

Created attachment 137023 [details]
Patch

[Geoffrey Garen]

Comment on attachment 137023 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=137023&action=review

I think you want an interface like this:

// Add property with structure transition
newStorage = reallocPropertyStorage(m_propertyStorage.get(), ...); // Returns new storage, does not assign.
newStructure = Structure::addPropertyTransition(...); // Returns new structure, does not assign.
setPropertyStorage(newStorage, newStructure); // Assigns structure and storage "atomically".

// Add property without structure transition
newStorage = reallocPropertyStorage(m_propertyStorage.get(), ...); // Returns new storage, does not assign.
setPropertyStorage(newStorage, m_structure.get()); // Assigns structure and storage "atomically".

Since storage and structure need to be in sync, we should never set one without setting the other.

> Source/JavaScriptCore/runtime/JSObject.h:661
> +        if (structure()->shouldResizePropertyStorage())

Let's call this "shouldGrowPropertyStorage". "Resize" is imprecise, because we don't shrink.

> Source/JavaScriptCore/runtime/JSObject.h:662
> +            allocatePropertyStorage(globalData, structure()->propertyStorageCapacity(), structure()->suggestedPropertyStorageResizeAmount());

Let's call this "growPropertyStorage" or "reallocPropertyStorage" to help clarify that we're allocating for a second time.

> Source/JavaScriptCore/runtime/JSObject.h:727
> +    Structure* structure = Structure::addPropertyTransition(globalData, this->structure(), propertyName, attributes, specificFunction, offset);

This addPropertyTransition will allocate a new structure, which can cause a GC. During that GC, our Structure's information about our backing store will be out of date (specifically, it will underestimate the size / capacity of our backing store). I'm not sure if that will definitely cause a problem, but it seems not so good. We never want our information about our backing store to be out of date.

[Mark Hahnenberg]

Created attachment 137104 [details]
Patch

[Build Bot]

Comment on attachment 137104 [details]
Patch

Attachment 137104 [details] did not pass win-ews (win):
Output: http://queues.webkit.org/results/12400123

[Mark Hahnenberg]

Created attachment 137112 [details]
Patch

[Build Bot]

Comment on attachment 137112 [details]
Patch

Attachment 137112 [details] did not pass win-ews (win):
Output: http://queues.webkit.org/results/12403044

[Mark Hahnenberg]

Created attachment 137121 [details]
Patch

[Geoffrey Garen]

Comment on attachment 137121 [details]
Patch

r=me

Did the test cases I mentioned pan out?

[WebKit Review Bot]

Comment on attachment 137121 [details]
Patch

Clearing flags on attachment: 137121

Committed r114255: <http://trac.webkit.org/changeset/114255>

[WebKit Review Bot]

All reviewed patches have been landed.  Closing bug.

[Mark Hahnenberg]

<rdar://problem/10884040>

[Gavin Barraclough]

Reverted in r116494, this causes many failures if COLLECT_ON_EVERY_ALLOCATION is enabled.

[Mark Hahnenberg]

Rolling this change back in since the underlying bug that it revealed should have been fixed in http://trac.webkit.org/changeset/116565.

[Mark Hahnenberg]

Created attachment 142313 [details]
Patch

[Mark Hahnenberg]

Resubmitting patch to make sure everything still builds.

[Mark Hahnenberg]

Committed r117343: <http://trac.webkit.org/changeset/117343>