https://ghc.haskell.org/trac/ghc/ticket/8724#ticket is the ticket
when i'm more awake i'll experiment some more On Sat, Feb 1, 2014 at 2:33 AM, Carter Schonwald <[email protected] > wrote: > i have a ticket for tracking this, though i'm thinking my initial attempt > at a patch generates the same object code as it did before. > > @ryan, what CPU variant are you testing this on? is this on a NUMA machine > or something? > > > On Sat, Feb 1, 2014 at 1:58 AM, Carter Schonwald < > [email protected]> wrote: > >> 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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