[Vm-dev] Re: Clarification needed for sqCompareAndSwapRes

Nicolas Cellier nicolas.cellier.aka.nice at gmail.com
Mon Apr 21 23:00:03 UTC 2014


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


> 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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