Hi Simon and Peter, I just wanted to draw your attention to !337 [1], where I try to fix #15994. While it's possible I have missed something, it looks to me like there has been a bug lurking here for quite some time. In particular on a weak memory model machine we do not place a write barrier between the construction of the result of a thunk evaluation and making the results visible to other cores via the indirection's indirectee field. This essentially means that a thread looking at the indirectee may essentially see uninitialized memory. Specifically, updateWithIndirection is currently: # Evaluation result p2 initialized by caller ... ((StgInd *)p1)->indirectee = p2; write_barrier(); SET_INFO(p1, &stg_BLACKHOLE_info); ... whereas I think this should rather be # Evaluation result p2 initialized by caller ... write_barrier(); ((StgInd *)p1)->indirectee = p2; SET_INFO(p1, &stg_BLACKHOLE_info); ... I describe the reasoning for this in more detail in the Note added in !337. All-in-all, I'm rather surprised to find this bug. While we won't see this issue manifest on x86_64 due to this architecture's memory model, we have long supported ARM, PowerPC, and SPARC, all of which have weakly-ordered execution modes. I can't find any relevant open tickets from these platforms. The fact that this wasn't noticed on these platforms makes me wonder whether I am just missing something. Let me know what you think. Cheers, - Ben [1] https://gitlab.haskell.org/ghc/ghc/merge_requests/337