Double Checked Locking

[Andrei Alexandrescu]

On 12/17/11 1:56 AM, Andrew Wiley wrote:
> On Sat, Dec 17, 2011 at 1:47 AM, Andrew Wiley<wiley.andrew.j at gmail.com>  wrote:
>> I was looking through Jonathan Davis's pull request to remove static
>> constructors from std.datetime, and I realized that I don't know
>> whether Double Checked Locking is legal under D's memory model, and
>> what the requirements for it to work would be.
>> (if you're not familiar with the term, check out
>> http://en.wikipedia.org/wiki/Double-checked_locking - it's a useful
>> but problematic programming pattern that can cause subtle concurrency
>> bugs)
>> It seems like it should be legal as long as the variable tested and
>> initialized is flagged as shared so that the compiler enforces proper
>> fences, but is this actually true?
>
> This entry in the FAQ makes me suspicious:
> ```
> What does shared have to do with memory barriers?
>
> Reading/writing shared data emits memory barriers to ensure sequential
> consistency (not implemented).
> ```
>
> So DCL should be alright with data flagged as shared, but it's not
> implemented in the compiler?

That is correct.

Andrei