[dmd-concurrency] draft 7

[Fawzi Mohamed]

On 1-feb-10, at 23:10, Andrei Alexandrescu wrote:

> Fawzi Mohamed wrote:
>> I gave a quick reading about the fact that reading needs a lock  
>> because otherwise it might not be updated almost forever, this is  
>> (as far as I know) wrong.
>> Yes the view of one thread might be offset with respect with the  
>> one of another, but not indefinitely so.
>> The main reason to put the sync is to ensure that one sees a  
>> consistent view of the value.
>> If a value is always updated in an atomic way then the sync is not  
>> needed.
>
> It's a classic that if you read (without handshake) a value in a  
> loop thinking you're doing spinning, various compiler and processor  
> optimizations will cache the value and spin forever. The  
> synchronization in there is needed for the handshake, and can be  
> optimized by the compiler if a simple barrier is needed.

you don't need a barrier, you need a volatile statement to avoid  
compiler optimizations

>
> But no handshake == incorrect code.
>
>
> Andrei
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