Bad news: apparent bug in casMutVar going back to 7.2

Carter Schonwald carter.schonwald at gmail.com
Sat Feb 1 07:33:24 UTC 2014


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 <carter.schonwald at gmail.com
> wrote:

> 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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