[Vm-dev] Re: Clarification needed for sqCompareAndSwapRes

[Nicolas Cellier]

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
> 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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