Hi Edward, Thanks very much for this reply, it answers a lot of questions I'd had. I'd hoped that ordering would be preserved through C--, but c'est la vie. Optimizing compilers are ever the bane of concurrent algorithms!
stg/SMP.h does define a loadLoadBarrier, which is exposed in Ryan Newton's atomic-primops package. From the docs, I think that's a general read barrier, and should do what I want. Assuming it works properly, of course. If I'm lucky it might even be optimized out. Thanks, John On Mon, Dec 30, 2013 at 6:04 AM, Edward Z. Yang <ezy...@mit.edu> wrote: > Hello John, > > Here are some prior discussions (which I will attempt to summarize > below): > > http://www.haskell.org/pipermail/haskell-cafe/2011-May/091878.html > http://www.haskell.org/pipermail/haskell-prime/2006-April/001237.html > http://www.haskell.org/pipermail/haskell-prime/2006-March/001079.html > > The guarantees that Haskell and GHC give in this area are hand-wavy at > best; at the moment, I don't think Haskell or GHC have a formal memory > model—this seems to be an open research problem. (Unfortunately, AFAICT > all the researchers working on relaxed memory models have their hands > full with things like C++ :-) > > If you want to go ahead and build something that /just/ works for a > /specific version/ of GHC, you will need to answer this question > separately for every phase of the compiler. For Core and STG, monads > will preserve ordering, so there is no trouble. However, for C--, we > will almost certainly apply optimizations which reorder reads (look at > CmmSink.hs). To properly support your algorithm, you will have to add > some new read barrier mach-ops, and teach the optimizer to respect them. > (This could be fiendishly subtle; it might be better to give C-- a > memory model first.) These mach-ops would then translate into > appropriate arch-specific assembly or LLVM instructions, preserving > the guarantees further. > > This is not related to your original question, but the situation is a > bit better with regards to reordering stores: we have a WriteBarrier > MachOp, which in principle, prevents store reordering. In practice, we > don't seem to actually have any C-- optimizations that reorder stores. > So, at least you can assume these will work OK! > > Hope this helps (and is not too inaccurate), > Edward > > Excerpts from John Lato's message of 2013-12-20 09:36:11 +0800: > > Hello, > > > > I'm working on a lock-free algorithm that's meant to be used in a > > concurrent setting, and I've run into a possible issue. > > > > The crux of the matter is that a particular function needs to perform the > > following: > > > > > x <- MVector.read vec ix > > > position <- readIORef posRef > > > > and the algorithm is only safe if these two reads are not reordered (both > > the vector and IORef are written to by other threads). > > > > My concern is, according to standard Haskell semantics this should be > safe, > > as IO sequencing should guarantee that the reads happen in-order. Of > > course this also relies upon the architecture's memory model, but x86 > also > > guarantees that reads happen in order. However doubts remain; I do not > > have confidence that the code generator will handle this properly. In > > particular, LLVM may freely re-order loads of NotAtomic and Unordered > > values. > > > > The one hope I have is that ghc will preserve IO semantics through the > > entire pipeline. This seems like it would be necessary for proper > handling > > of exceptions, for example. So, can anyone tell me if my worries are > > unfounded, or if there's any way to ensure the behavior I want? I could > > change the readIORef to an atomicModifyIORef, which should issue an > mfence, > > but that seems a bit heavy-handed as just a read fence would be > sufficient > > (although even that seems more than necessary). > > > > Thanks, > > John L. >
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