woops, i mean cmpxchgq
On Sat, Feb 1, 2014 at 1:36 AM, Carter Schonwald <[email protected] > wrote: > ok, i can confirm that on my 64bit mac, both clang and gcc use cmpxchgl > rather than cmpxchg > i'll whip up a strawman patch on head that can be cherrypicked / tested > out by ryan et al > > > On Sat, Feb 1, 2014 at 1:12 AM, Carter Schonwald < > [email protected]> wrote: > >> Hey Ryan, >> looking at this closely >> Why isn't CAS using CMPXCHG8B on 64bit architectures? Could that be the >> culprit? >> >> Could the issue be that we've not had a good stress test that would >> create values that are equal on the 32bit range, but differ on the 64bit >> range, and you're hitting that? >> >> Could you try seeing if doing that change fixes things up? >> (I may be completely wrong, but just throwing this out as a naive >> "obvious" guess) >> >> >> On Sat, Feb 1, 2014 at 12:58 AM, Ryan Newton <[email protected]> wrote: >> >>> Then again... I'm having trouble seeing how the spec on page 3-149 of >>> the Intel manual would allow the behavior I'm seeing: >>> >>> >>> http://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-manual-325462.pdf >>> >>> Nevertheless, this is exactly the behavior we're seeing with the current >>> Haskell primops. Two threads simultaneously performing the same CAS(p,a,b) >>> can both think that they succeeded. >>> >>> >>> >>> >>> >>> On Sat, Feb 1, 2014 at 12:33 AM, Ryan Newton <[email protected]> wrote: >>> >>>> I commented on the commit here: >>>> >>>> >>>> https://github.com/ghc/ghc/commit/521b792553bacbdb0eec138b150ab0626ea6f36b >>>> >>>> The problem is that our "cas" routine in SMP.h is similar to the C >>>> compiler intrinsic __sync_val_compare_and_swap, in that it returns the old >>>> value. But it seems we cannot use a comparison against that old value to >>>> determine whether or not the CAS succeeded. (I believe the CAS may fail >>>> due to contention, but the old value may happen to look like our old >>>> value.) >>>> >>>> Unfortunately, this didn't occur to me until it started causing bugs >>>> [1] [2]. Fixing casMutVar# fixes these bugs. However, the way I'm >>>> currently fixing CAS in the "atomic-primops" package is by using >>>> __sync_bool_compare_and_swap: >>>> >>>> >>>> https://github.com/rrnewton/haskell-lockfree/commit/f9716ddd94d5eff7420256de22cbf38c02322d7a#diff-be3304b3ecdd8e1f9ed316cd844d711aR200 >>>> >>>> What is the best fix for GHC itself? Would it be ok for GHC to >>>> include a C compiler intrinsic like __sync_val_compare_and_swap? Otherwise >>>> we need another big ifdbef'd function like "cas" in SMP.h that has the >>>> architecture-specific inline asm across all architectures. I can write the >>>> x86 one, but I'm not eager to try the others. >>>> >>>> Best, >>>> -Ryan >>>> >>>> [1] https://github.com/iu-parfunc/lvars/issues/70 >>>> [2] https://github.com/rrnewton/haskell-lockfree/issues/15 >>>> >>>> >>> >>> _______________________________________________ >>> ghc-devs mailing list >>> [email protected] >>> http://www.haskell.org/mailman/listinfo/ghc-devs >>> >>> >> >
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