Re: memory ordering

Hey John, so you're wanting atomic reads and writes? I'm pretty sure that you want to use atomic memory operations for this. I believe Ryan Newton has some tooling you can use right now for that. On Fri, Dec 20, 2013 at 3:57 AM, Christian Höner zu Siederdissen < choener@tbi.univie.ac.at> wrote:

Hi John,

I guess you probably want to "pseq x". See below for an example. Since your 2nd action does not depend on your 1st.

Gruss, Christian

import Debug.Trace import GHC.Conc

main = do x <- return (traceShow "1" $ 1::Int) -- x `pseq` print (2::Int) print (2::Int) print x

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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

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following:

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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

* John Lato [20.12.2013 02:36]: the properly. In

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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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