[Vm-dev] Clarification needed for sqCompareAndSwapRes

Tue Apr 22 22:18:21 UTC 2014

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?

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
>
>
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