#15449: Nondeterministic Failure on aarch64 with -jn, n > 1 -------------------------------------+------------------------------------- Reporter: tmobile | Owner: tmobile Type: bug | Status: new Priority: normal | Milestone: 8.10.1 Component: Compiler | Version: 8.4.3 Resolution: | Keywords: Operating System: Linux | Architecture: aarch64 Type of failure: Compile-time | Test Case: crash or panic | Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by bgamari): Hmm, indeed let's think through this. There are two cases that `updateWithIndirection` needs to safely handle. In both case it is turning some sort of closure into an indirection: * updating a blackhole (e.g. as called from the `stg_upd_frame` entry code) * updating a thunk (e.g. from `Threads.c:updateThunk`) Note that the free variable fields of a thunk and the indirectee field of a thunk do not overlap (see `StgThunkHeader` for how this is so). Consequently it is safe to write to the indirectee field before writing to the info table pointer. In the event that we see this interleaving: {{{ Thread A Thread B --------------------------- --------------------------- X->indirectee = Y enter X X->info = stg_BLACKHOLE_info }}} we will, at worst, duplicate the evaluation of `X`. There is no chance to stumble into unsoundness here. The important thing, as mentioned in comment:22, is that the newly-created result is fully visible before the `updatee` field is written. This can be ensured by placing a barrier somewhere after the construction of the result but before the write to `updatee`. Consequently, I think my patch, wherein `updateWithIndirection` is {{{ #define updateWithIndirection(p1, p2, and_then) \ W_ bd; \ \ OVERWRITING_CLOSURE(p1); \ StgInd_indirectee(p1) = p2; \ prim_write_barrier; \ SET_INFO(p1, stg_BLACKHOLE_info); \ . . . }}} ought to be correct. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15449#comment:24 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler