[webkit-dev] DropAllLocks assertion on iOS (threading issue?)

Ian Ragsdale ian.ragsdale at gmail.com
Thu Dec 11 19:25:25 PST 2014

Hi Geoff, thanks for the quick response!

I don't believe I'm making any use of SPI, aside from whatever underlying use there is in UIWebView - I'm pretty sure that's not exposed to normal iOS apps (at least not that I'm aware of).

As predicted, setting JSC_slowPathAllocsBetweenGCs does appear to make it easier to trigger the crash. I did that and then did an Instruments sample using the iOS simulator, and got the crash to happen pretty quickly. However, I'm not really sure what to look for, since I'm a newbie to WebKit internals. Any suggestions for what to look for in the call tree?

Thanks again,

> On Dec 11, 2014, at 8:11 PM, Geoffrey Garen <ggaren at apple.com> wrote:
> Hi Ian.
>> From looking at the source, it tries to drop all locks from the current javascript VM before calling the delegate, and when it does that it asserts if the VM is busy garbage collecting.
> That’s right.
>> I'm guessing there needs to be some sort of guard there to make sure the VM isn't doing GC before dropping the locks?
> In JavaScriptCore, garbage collection is an atomic operation that must run to completion before the next allocation.
> The reason this is an assertion — and can’t be a guard — is that, if we called out to a delegate in the middle of garbage collection, we would definitely corrupt the heap. So, there’s nothing you can guard against: the game is already lost.
> The bug here happened earlier: Somehow, the collector was left thinking that it was in the busy state, even though — as we can see from the backtrace — it wasn’t actually doing anything.
>> I'm pretty positive I'm not calling into the UIWebView from any thread other than the main thread, and I don't think I have any control over the WebThread or the GC threads, so I'm not sure if there's anything I can do, but I do have a fairly reliable repro, so if there's something it makes sense for me to test, I can do so.
> Does you app make any use of WebKit SPI? If it does, the SPI might not be thread-safe even if called from the main thread, and so you might be corrupting the heap.
> One technique that might make this bug more reproducible is to set the environment variable JSC_slowPathAllocsBetweenGCs to a low number (as low as possible without making your app unusable) — that will trigger collection more frequently.
> Another thing you can try is to record an Instruments time profile of the app as you go through the steps to make the app crash. That will give us an overview of what the app was doing leading up to the crash, whether it used UIWebView from a secondary thread or invoked SPI, and so on.
> Geoff

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