[Vm-dev] Clarification needed for sqCompareAndSwapRes

[Nicolas Cellier]

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)?
Do you agree with my proposed fix, or have a better idea?

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/38for 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
>>>>>
>>>>>
>>>>
>>>
>>
>
>
>
>
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