Bug 209437 – Assertion triggered on iOS 13: Multiple locks on web thread

• Status: RESOLVED
• Resolution: CONFIGURATION CHANGED
• Component: WebCore Misc.

[Darren Mo]

Created attachment 394302 [details]
Stack trace on (iPhone9,3 13.3.1 17D50)

Starting in iOS 13 and still occurring in iOS 13.4, an assertion in `_WebThreadLock` is being hit: `Multiple locks on web thread not allowed! Please file a bug. Crashing now...`.

The attached stack trace points to the following `WebCore` code:
```
                     __ZL14_WebThreadLockv.cold.1:        // _WebThreadLock() [clone .cold.1]
0000000189ab5708         sub        sp, sp, #0x20                               ; CODE XREF=__ZL14_WebThreadLockv+336
0000000189ab570c         stp        x29, x30, [sp, #0x10]
0000000189ab5710         add        x29, sp, #0x10
0000000189ab5714         bl         __ZL20CurrentThreadContextv                 ; CurrentThreadContext()
0000000189ab5718         adrp       x8, #0x189b51000                            ; 0x189b51340@PAGE
0000000189ab571c         add        x8, x8, #0x340                              ; 0x189b51340@PAGEOFF, "void _WebThreadLock()"
0000000189ab5720         stp        x8, x0, [sp]
0000000189ab5724         adrp       x0, #0x1bf154000                            ; 0x1bf154850@PAGE
0000000189ab5728         add        x0, x0, #0x850                              ; 0x1bf154850@PAGEOFF, @"%s, %p: Multiple locks on web thread not allowed! Please file a bug. Crashing now..."
0000000189ab572c         bl         imp___stubs___ZNK2sh14TConstantUniongtERKS0_ ; sh::TConstantUnion::operator>(sh::TConstantUnion const&) const
0000000189ab5730         brk        #0x0
0000000189ab5734         brk        #0x1
```

We do not have information about what was happening in the app prior to the crash aside from the attached stack trace.

[Alexey Proskuryakov]

Would you be able to characterize customer impact, and whether this app can move to modern WebKit API?

[Radar WebKit Bug Importer]

<rdar://problem/60858737>

[Darren Mo]

(In reply to Alexey Proskuryakov from comment #1)
> Would you be able to characterize customer impact, and whether this app can
> move to modern WebKit API?

We’ve been seeing this crash for years. ~340k crash reports since 2016.

We are using iOS’s `UIWebView`. By “modern WebKit API”, do you mean `WKWebView`?

[Alexey Proskuryakov]

> We are using iOS’s `UIWebView`. By “modern WebKit API”, do you mean `WKWebView`?

Correct.

[Darren Mo]

(In reply to Alexey Proskuryakov from comment #4)
> > We are using iOS’s `UIWebView`. By “modern WebKit API”, do you mean `WKWebView`?
> 
> Correct.

We are in the process of migrating now. To be clear, this crash will not occur when using `WKWebView`, right? Is this a known issue with `UIWebView`?

[Alexey Proskuryakov]

WebThread isn't used with WKWebView (it uses a separate process for web content), so this issue definitely won't happen.

I don't know if it's necessarily a legacy WebKit bug - searching the Web for "Multiple locks on web thread not allowed", I see many hits explaining programming model violations in client code that can lead to this. But legacy WebKit certainly has architectural threading issues of its own too.

[Darren Mo]

Great, thanks! I will mark this bug as resolved then.