[webkit-dev] DropAllLocks assertion on iOS (threading issue?)
ggaren at apple.com
Thu Dec 11 18:11:07 PST 2014
> 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?
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.
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