Something needs to happen with shared, and soon.

[luka8088]

On Tuesday, 13 November 2012 at 09:11:15 UTC, luka8088 wrote:
> On 12.11.2012 3:30, Walter Bright wrote:
>> On 11/11/2012 10:46 AM, Alex Rønne Petersen wrote:
>>> It's starting to get outright embarrassing to talk to 
>>> newcomers about D's
>>> concurrency support because the most fundamental part of it 
>>> -- the
>>> shared type
>>> qualifier -- does not have well-defined semantics at all.
>>
>> I think a couple things are clear:
>>
>> 1. Slapping shared on a type is never going to make algorithms 
>> on that
>> type work in a concurrent context, regardless of what is done 
>> with
>> memory barriers. Memory barriers ensure sequential 
>> consistency, they do
>> nothing for race conditions that are sequentially consistent. 
>> Remember,
>> single core CPUs are all sequentially consistent, and still 
>> have major
>> concurrency problems. This also means that having templates 
>> accept
>> shared(T) as arguments and have them magically generate correct
>> concurrent code is a pipe dream.
>>
>> 2. The idea of shared adding memory barriers for access is not 
>> going to
>> ever work. Adding barriers has to be done by someone who knows 
>> what
>> they're doing for that particular use case, and the compiler 
>> inserting
>> them is not going to substitute.
>>
>>
>> However, and this is a big however, having shared as 
>> compiler-enforced
>> self-documentation is immensely useful. It flags where and 
>> when data is
>> being shared. So, your algorithm won't compile when you pass 
>> it a shared
>> type? That is because it is NEVER GOING TO WORK with a shared 
>> type. At
>> least you get a compile time indication of this, rather than 
>> random
>> runtime corruption.
>>
>> To make a shared type work in an algorithm, you have to:
>>
>> 1. ensure single threaded access by aquiring a mutex
>> 2. cast away shared
>> 3. operate on the data
>> 4. cast back to shared
>> 5. release the mutex
>>
>> Also, all op= need to be disabled for shared types.
>
>
> This clarifies a lot, but still a lot of people get confused 
> with:
> http://dlang.org/faq.html#shared_memory_barriers
> is it a faq error ?
>
> and also with http://dlang.org/faq.html#shared_guarantees said, 
> I come to think that the fact that the following code compiles 
> is either lack of implementation, a compiler bug or a faq error 
> ?
>
> //////////
>
> import core.thread;
>
> void main () {
>   shared int i;
>   (new Thread({ i++; })).start();
> }

Um, sorry, the following code:

//////////

import core.thread;

void main () {
   int i;
   (new Thread({ i++; })).start();
}