pass-by-ref semantics for structs (was Deque impl.)

[Rainer Schuetze]

On 31.01.2013 22:37, Steven Schveighoffer wrote:
> On Thu, 31 Jan 2013 15:55:11 -0500, Rainer Schuetze <r.sagitario at gmx.de>
> wrote:
>>
>>>
>>>>
>>>> - how do you do reference counting when accessing shared containers in
>>>> multiple threads? Consider clearing the last reference to a shared
>>>> reference counted object while another thread reads this reference.
>>>> With usual atomic operations on the reference count only:
>>>>
>>>>    1. the reading thread reads the payload pointer
>>>>    2. the writing thread reads the payload pointer, decrements the
>>>> reference count and frees the array
>>>>    3. the reading thread increments the reference count, but accesses
>>>> the deallocated array.
>>>
>>> I would be concerned if a thread can get ahold of a reference counted
>>> pointer without having the counter incremented already on its behalf.
>>>
>>
>> You have to read the pointer to increment the counter to get hold of
>> the reference.
>
> Right, but where do you get the original pointer from?  Wouldn't that
> reference have incremented the count?
>

Any reference that is accessible from multiple threads, e.g. a global 
shared variable.

shared(Container) allObjects;

Without any thread having a reference to it, the payload's reference 
count is 1. The reading thread above increments the counter when making 
a local copy, but the writing thread decrements the counter when 
assigning Container.init. If both run concurrently, with the sequence 1 
to 3 above, trouble is inevitable.