[Vm-dev] Re: Clarification needed for sqCompareAndSwapRes

[Nicolas Cellier]

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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