[Vm-dev] Clarification needed for sqCompareAndSwapRes

Tue Apr 22 22:21:29 UTC 2014

On Tue, Apr 22, 2014 at 3:18 PM, Nicolas Cellier <
nicolas.cellier.aka.nice at gmail.com> wrote:

>
>
> 2014-04-23 0:12 GMT+02:00 Eliot Miranda <eliot.miranda at gmail.com>:
>
>>
>> Hi Esteban,
>>
>>
>> On Tue, Apr 22, 2014 at 2:40 PM, Esteban Lorenzano <estebanlm at gmail.com>wrote:
>>
>>>
>>>
>>> On 22 Apr 2014, at 23:34, Nicolas Cellier <
>>> nicolas.cellier.aka.nice at gmail.com> wrote:
>>>
>>>
>>> 2014-04-22 12:47 GMT+02:00 Esteban Lorenzano <estebanlm at gmail.com>:
>>>
>>>>
>>>> thanks Nicolas for your words.
>>>> now… let’s go back to our common passion: to help this community on
>>>> having the best vm possible :)
>>>>
>>>> Esteban
>>>>
>>>>
>>> Nah, concerning the best vm I feel disqualified, I let Eliot care of it
>>> ;)
>>>
>>> We just want a better VM with a few hacks, corrections and support for
>>> some specific Pharo libraries/extensions.
>>> Eliot, do you agree that the sqCompareAndSwapRes provided #ifdefTARGET_OS_IS_IPHONE is buggish (see below)?
>>>
>>>
>> To be honest I don't know.  I don't have an iPhone development library
>> handy.  John McIntosh write the code and he's more than competent.  I'd
>> rather leave this to others with experience of the platform to check.  I
>> /think/ the desired semantics are clear enough.  If they're not I can fix
>> that.  But I want to leave writing the correct code for the iPhone to those
>> that are actually using the platform.  Forgive me.
>>
>>
>
> OK, thanks Eliot, then I'll trust what the comments say.
> If John is able to confirm that'll be better, but I think I get a good
> picture of the problem now.
> I cannot build either, but once Esteban can test it and if it works OK,
> then we'll propose a fix for integration in your svn branch, sounds right?
>

perfect, thanks !

>
>  Do you agree with my proposed fix, or have a better idea?
>>>
>>>
>>> I will test it on iPad as soon as I can (busy days)… if that helps :)
>>>  can you propose a pull request?
>>>
>>> cheers,
>>> Esteban
>>>
>>>
>>> Thanks
>>>
>>>
>>>> On 22 Apr 2014, at 11:14, Nicolas Cellier <
>>>> nicolas.cellier.aka.nice at gmail.com> wrote:
>>>>
>>>> I want to publicly apologize for sounding harsh toward Esteban.
>>>> I think he found a great bug and should be thanked and rewarded for
>>>> that, rather than bashed for a maybe incomplete fix.
>>>> It was miss-communcation from my part, and I must tell that I wish we
>>>> had more Esteban for doing the hard work on pharo-vm branch.
>>>>
>>>> Nicolas
>>>>
>>>> 2014-04-22 1:01 GMT+02:00 Nicolas Cellier <
>>>> nicolas.cellier.aka.nice at gmail.com>:
>>>>
>>>>>
>>>>> 2014-04-22 1:00 GMT+02:00 Nicolas Cellier <
>>>>> nicolas.cellier.aka.nice at gmail.com>:
>>>>>
>>>>> 2014-04-22 0:48 GMT+02:00 Nicolas Cellier <
>>>>>> nicolas.cellier.aka.nice at gmail.com>:
>>>>>>
>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> 2014-04-22 0:40 GMT+02:00 Nicolas Cellier <
>>>>>>> nicolas.cellier.aka.nice at gmail.com>:
>>>>>>>
>>>>>>> In platforms/Cross/vm/sqAtomicOps.h a comment tells
>>>>>>>>
>>>>>>>> * sqCompareAndSwapRes(var,old,new,res) arranges atomically that if
>>>>>>>> var's value
>>>>>>>> * is equal to old, then var's value is set to new, and that in any
>>>>>>>> case, res
>>>>>>>> * is set to the previous value of var.
>>>>>>>>
>>>>>>>> But for some implementation, if var==old, res is first set to var
>>>>>>>> (old) then to new which is the new value of var:
>>>>>>>>
>>>>>>>> #if defined(__GNUC__) && (defined(i386) || defined(__i386) || defined(__i386__) || defined(_X86_))
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>  #ifdef TARGET_OS_IS_IPHONE
>>>>>>>> # define sqCompareAndSwap(var,old,new) OSAtomicCompareAndSwap32(old, new, &var)
>>>>>>>> /* N.B.  This is not atomic in fetching var's old value :( */
>>>>>>>> # define sqCompareAndSwapRes(var,old,new,res) do { res = var; if (OSAtomicCompareAndSwap32(old, new, &var)) res = new; } while (0)
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Indeed, comparedAndSwap32 will answer true if var==old as explained
>>>>>>>> here
>>>>>>>> https://www.mikeash.com/pyblog/friday-qa-2011-03-04-a-tour-of-osatomic.html
>>>>>>>>
>>>>>>>> That is going to be a problem in lockSignalQueue in
>>>>>>>> ./platforms/Cross/vm/sqExternalSemaphores.c
>>>>>>>> because variable old will then be 1 in all cases and the loop won't
>>>>>>>> exit once the lock acquired:
>>>>>>>>
>>>>>>>> static inline void lockSignalQueue()
>>>>>>>> {
>>>>>>>>     volatile int old;
>>>>>>>>     /* spin to obtain a lock */
>>>>>>>>
>>>>>>>>     do {
>>>>>>>>         sqLowLevelMFence();
>>>>>>>>         sqCompareAndSwapRes(sigLock, 0, 1, old );
>>>>>>>>     } while (old != 0);
>>>>>>>>
>>>>>>>> }
>>>>>>>>
>>>>>>>> Knowing we are on iphone, I would just write it:
>>>>>>>>
>>>>>>>> static inline void lockSignalQueue()
>>>>>>>> {
>>>>>>>>     /* spin to obtain a lock */
>>>>>>>>     do {
>>>>>>>>         sqLowLevelMFence();
>>>>>>>>     } while (! OSAtomicCompareInt32(sigLock, 0, 1));
>>>>>>>> }
>>>>>>>>
>>>>>>> ahem, of course, the arguments are not in the same order:
>>>>>>>      while (! OSAtomicCompareInt32( 0, 1, &sigLock))
>>>>>>>
>>>>>>>
>>>>>>>> Esteban "fixed" that by swapping the arguments in the macrosqCompareAndSwapRes
>>>>>>>>
>>>>>>>
>>>>>> Here is a ref to the "fix":
>>>>>>
>>>>>>
>>>>>> https://github.com/nicolas-cellier-aka-nice/pharo-vm/commit/7f3f991283344d4cf0aa6a50b8615416062ec381
>>>>>>
>>>>>>
>>>>>
>>>>> argh,
>>>>> https://github.com/pharo-project/pharo-vm/commit/7f3f991283344d4cf0aa6a50b8615416062ec381
>>>>> those forks are driving me nuts
>>>>>
>>>>>
>>>>>>   But IMO, this is wrong, as I understand it, it completely
>>>>>>>> desactivates the lock...
>>>>>>>>
>>>>>>>> I did not know about the problem, but found the diff with Eliot's
>>>>>>>> branch very suspicious so I opened
>>>>>>>>
>>>>>>>> https://pharo.fogbugz.com/f/cases/13199/Stinky-argument-inversion-of-atomic-swap-MACRO-for-iphone-gcc-branch
>>>>>>>>
>>>>>>>> and pulled a request
>>>>>>>> https://github.com/pharo-project/pharo-vm/pull/38 for restoring
>>>>>>>> (the supposedly wrong) Eliot's version.
>>>>>>>>
>>>>>>>> Now I need some guru advice, because this kind of code is
>>>>>>>> brainfucking (especially when old in sender is not old in sendee).
>>>>>>>>
>>>>>>>> Do you agree that TARGET_OS_IS_IPHONE branch is wrong?
>>>>>>>> Do you agree that Esteban's fix is wrong?
>>>>>>>> What do you suggest for a fix?
>>>>>>>>
>>>>>>>>
>>>>>>> maybe
>>>>>>>
>>>>>>> # define sqCompareAndSwapRes(var,old,new,res) do { res = var; if (OSAtomicCompareAndSwap32(old, new, &var)) res = old; } while (0)
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>> Thanks
>>>>>>>>
>>>>>>>> Nicolas
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>>
>>>
>>
>>
>> --
>> best,
>> Eliot
>>
>>
>
>

-- 
best,
Eliot
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/vm-dev/attachments/20140422/5304fbd6/attachment-0001.htm