draft proposal for ref counting in D

[Rainer Schuetze]

On 13.10.2013 19:05, Sönke Ludwig wrote:
> Am 13.10.2013 17:15, schrieb Sean Kelly:
>> On Sunday, 13 October 2013 at 07:03:39 UTC, Rainer Schuetze wrote:
>>>
>>> According to the "Handbook of Garbage Collection" by Richard Jones
>>> eager lock-free reference counting can only be done with a cas2
>>> operation modifying two seperate locations atomically (algorithm 18.2
>>> "Eager reference counting with CompareAndSwap is broken"). This might
>>> be the quoted paper:
>>> http://scholr.ly/paper/2199608/lock-free-reference-counting
>>>
>>> Unfortunately the CAS2 operation does not exist in most processors.
>>
>> I suppose it's worth noting that Boost (and now standard C++) has a
>> shared_ptr that works across threads and the implementation I've seen
>> doesn't use a mutex.  In fact, I think the Boost one doesn't even use
>> CAS on x86, though it's been quite a few years so my memory could be
>> wrong on that last detail.
>
> I didn't read the paper, but I'd suspect that the paper refers to the
> case where both, the reference count _and_ the reference is thread-safe,
> since the boost/c++ shared_ptr only has a thread-safe reference count
> after all.

I haven't read it either, but AFAICT the cas2 operation is used two 
modify the pointer and the reference count at the same time atomically.

I just checked boost::shared_ptr, it uses cas operations on the 
reference counts. It has the same problem as described in my example, 
see the read/write example 3 here: 
http://www.boost.org/doc/libs/1_54_0/libs/smart_ptr/shared_ptr.htm#ThreadSafety

boost::shared_ptr is also unsafe with respect to calling member 
functions through "->" as it doesn't increment the reference count.