Bug 196392 – [JSC] JSWrapperMap should not use Objective-C Weak map (NSMapTable with NSPointerFunctionsWeakMemory) for m_cachedObjCWrappers

• Component: JavaScriptCore
• Status: REOPENED

[Yusuke Suzuki]

We do not use Objective-C weak semantics here. Objective-C JSValue has a dealloc function, and JSValue is tightly coupled to JSContext (it has _context field).
So JSValue is only stored in one wrapper map held by JSContext (JSWrapperMap). So, we can manage lifetime just unregistering itself from JSWrapperMap in [JSValue dealloc].
Why we would like to avoid Objective-C weak semantics is that it allocates much memory. In very simple hello_world case, we create 4 JSValue wrappers, and it consumes 10KB for weak semantics.

[Yusuke Suzuki]

Created attachment 366254 [details]
Patch

WIP

[Yusuke Suzuki]

Created attachment 366320 [details]
Patch

[Yusuke Suzuki]

Comment on attachment 366320 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=366320&action=review

> Source/JavaScriptCore/API/JSContext.mm:374
> +    JSC::JSLockHolder locker(toJS(m_context));

It is not necessary since we just drop Hash table entry, which contains a pointer, and do nothing on this pointer. I'll remove this lock.

[Saam Barati]

Comment on attachment 366320 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=366320&action=review

> Source/JavaScriptCore/API/JSWrapperMap.mm:604
> +    void* m_wrapper { nullptr };

As you mentioned, let's use __unsafe_unretained

[Yusuke Suzuki]

Comment on attachment 366320 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=366320&action=review

Thanks!

>> Source/JavaScriptCore/API/JSWrapperMap.mm:604
>> +    void* m_wrapper { nullptr };
> 
> As you mentioned, let's use __unsafe_unretained

Fixed.

[Yusuke Suzuki]

Committed r243672: <https://trac.webkit.org/changeset/243672>

[Radar WebKit Bug Importer]

<rdar://problem/49443020>

[WebKit Commit Bot]

Re-opened since this is blocked by bug 196952

[Yusuke Suzuki]

I rolled out this patch for now. The problem was this patch does not account that JSC.framework is thread-safe. JSC VM can be used multiple threads, and JSC framework takes appropriate lock internally. And [JSValue release] can be called in one thread while the other thread uses it. So the problem happens when,

1. Thread A calls [JSValue release].
2. Thread A is now in [JSValue dealloc], and about to unregister JSValue from wrapper map.
3. Thread B gets the JSValue from the wrapper map.
4. Thread A unregisters the JSValue, and destroy it.
5. Thread B uses the destroyed JSValue.

Appropriate fix in this patch's direction would be like, defining custom [JSValue release] and unregister it from wrapper map and destroy it in an atomic manner.
But since this introduces a lot of complexity, for now, I've just reverted this patch.