parExtendDenseArray is using HeapSlot::init, which fires post-barriers. The store buffer is not thread-safe so this results in intermittent failures on all parallel tests. There are probably more of these. Is there a way to assert that we are running on the main thread so that I can find anyone else calling the store buffer off the main thread?

This adds !InParallelSection assertions. This did turn up the problems I fixed earlier when that patch was not applied. There are no remaining failures with that patch applied.

Comment on attachment 761818 [details] [diff] [review] v1 Review of attachment 761818 [details] [diff] [review]: ----------------------------------------------------------------- ::: js/src/gc/StoreBuffer.h @@ +498,5 @@ > void releaseVerificationData(); > bool containsEdgeAt(void *loc) const; > > + /* We cannot call InParallelSection directly because of a circular dependency. */ > + bool inParallelSection() const; You could resolve this with a -inl.h file, I suppose. But this is probably easier.

Yes, -inl.h is a horrid hack that I'd like to avoid if at all possible. Thankfully, there is no problem with out-of-lining here, since it is debug-only. https://hg.mozilla.org/integration/mozilla-inbound/rev/75575b5f073b