Bad news: apparent bug in casMutVar going back to 7.2

[Carter Schonwald]

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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