Re: Singletons with ARC
Re: Singletons with ARC
- Subject: Re: Singletons with ARC
- From: Jean-Daniel Dupas <email@hidden>
- Date: Fri, 09 Dec 2011 10:45:21 +0100
Le 9 déc. 2011 à 10:36, Jean-Daniel Dupas a écrit :
>
> Le 9 déc. 2011 à 08:47, Ken Thomases a écrit :
>
>> On Dec 9, 2011, at 1:11 AM, Uli Kusterer wrote:
>>
>>> On 09.12.2011, at 07:55, Ken Thomases wrote:
>>>>
>>>> Double-checked locking is broken. It is an anti-pattern in many languages, including the C family under most common implementations. Don't use it. Google it if you want to confirm. One reference: <http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html>.
>>>
>>> Is this an issue that just exists when you want to write against ANSI C, or is this an actual, practical concern when writing on the Mac, for MacOS or iOS?
>>
>> It is an actual, practical concern. The compilers can re-order memory accesses. The CPU can, too.
>>
>> The second and subsequent threads through the double-checked lock can see the singleton pointer as non-nil, but the pointed-to object may not be valid. Its content may not have been written to memory, yet.
>>
>
> It was a real concern on PPC, but on Intel platform AFAIK.
I mean but NOT on Intel plateform.
> Memory barriers are noop on x86, as it does not perform out of order execution of memory operations.
>
> http://locklessinc.com/articles/singleton_pattern/
>
> That said,avoiding this pattern in the future may be a good thing.
>
> -- Jean-Daniel
>
>
>
>
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-- Jean-Daniel
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