Re: [Sbcl-help] SBCL and Common Lisp memory model

[Stas B. <sta...@gm...>]

james anderson <jam...@se...> writes:

> good evening;
>
> On 18 Feb 2014, at 8:19 AM, Stas Boukarev wrote:
>
>> Siebe de Vos <Si...@de...> writes:
>> 
>>> Hi,
>>> ...
>>> This would be a simple test:
>>> 
>>> (let ((shared (cons t nil)))
>>>   (sb-thread:make-thread (lambda ()
>>>                            (loop
>>>                               (let ((cons (cons nil nil)))
>>>                                 (setf (car cons) t
>>>                                       shared cons)))))
>>>   (sb-thread:make-thread (lambda ()
>>>                            (loop
>>>                               (assert (consp shared)
>>>                                       () "#'cons needs a barrier!")
>>>                               (assert (car shared)
>>>                                       () "(setf car) needs a barrier!")))))
>>> 
>>> Can it fail on SBCL?
>> It does fail on PPC.
>
> the second assertion or the first?
> if the second, because the car value was nil?
> if so, is it correct to infer that the ppc memory is sufficiently
> incoherent, that the reading thread sees the write to the 'shared'
> binding before the write to the car?
> if so, does the behavior change if the mutation is wrapped in a function?
>
> (i would endeavor to answer the questions myself, but have been unable
> to build a threaded sbcl on my available ppc host...)
The second one, allocation was fixed recently to insert SYNC.

-- 
With best regards, Stas.