Assignment of shared values

[Andrei Alexandrescu via Digitalmars-d]

On 8/7/14, 11:22 AM, Peter Alexander wrote:
> On Thursday, 7 August 2014 at 16:44:37 UTC, Andrei Alexandrescu wrote:
>> Yah. The basic intent behind "shared" is to disallow unwitting racy
>> code, and allow users to define robust shared abstractions.
>>
>> Consider this code at top level:
>>
>> struct S {
>>   private long a, b, c;
>>   ...
>> }
>> shared S g_theS;
>>
>> All threads will "see" the same g_theS. The intent of "shared" is to
>> disallow racy/wrong use of g_theS across threads. However, assigning
>> g_theS is by default allowed and does something definitely racy.
>>
>> So I think we should do the following:
>>
>> * if S.sizeof <= size_t.sizeof, generate atomic assignment
>> automatically for shared values.
>>
>> * if S.sizeof == 2 * size_t.sizeof, generate special 128-bit atomic
>> assignment for shared values on 64-bit systems, and 64-bit atomic
>> assignment on 32-bit systems.
>>
>> * In all other cases, reject code during compilation and require an
>> opAssign for shared values.
>>
>> This will break code. It may even break correct code (benign races or
>> code that uses external locking). On the other hand, it seems like the
>> right thing to do by the intended semantics of "shared".
>
> Even with that change, you'll still be able to do:
>
> S otherS = ...;
> g_theS.a = otherS.a;
> g_theS.b = otherS.b;
> g_theS.c = otherS.c;
>
> Right? This violates the same invariants as:

Not if they're private. Access to non-encapsulated data is not supposed 
to be regulated by the language.

> g_theS = otherS;
>
> Can you explain the practical benefits of enforcing atomicity of shared
> objects only at the individual statement level? I must be missing
> something.

The whole idea behind shared is disallowing implicit races, a common 
source of bugs. All shared data is assumed to be visible to several 
threads at once, and as such is subject to additional restrictions 
compared to non-shared data, all of which is considered thread-local. 
That's why e.g. recently RMW operations on shared integrals were disallowed.


Andrei