Bad news: apparent bug in casMutVar going back to 7.2

[Carter Schonwald]

woops, i mean cmpxchgq


On Sat, Feb 1, 2014 at 1:36 AM, Carter Schonwald <carter.schonwald at gmail.com
> 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 <
> carter.schonwald at gmail.com> 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 <rrnewton at gmail.com> 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 <rrnewton at gmail.com> 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
>>>>
>>>>
>>>
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>>>
>>
>
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