Low-Lock Singletons In D

[Andrei Alexandrescu]

On 5/7/13 12:30 PM, deadalnix wrote:
> On Tuesday, 7 May 2013 at 16:14:50 UTC, Steven Schveighoffer wrote:
>> Not really. Whether it is entered or not is dictated by the vtable.
>> Even classic double-check locking doesn't need an acquire outside the
>> lock. Even if your CPU's view of the variable is outdated, the check
>> after the memory barrier inside the lock only occurs once. After that,
>> steady state is achieved. All subsequent reads need no memory
>> barriers, because the singleton object will never change after that.
>>
>> The only thing we need to guard against is non-atomic writes, and out
>> of order writes of the static variable (fixed with a memory barrier).
>> Instruction ordering OUTSIDE the lock is irrelevant, because if we
>> don't get the "steady state" value (not null), then we go into the
>> lock to perform the careful initialization with barriers.
>>
>> I think aligned native word writes are atomic, so we don't have to
>> worry about that.
>>
>
> That is incorrect as the thread not going into the lock can see a
> partially initialized object.

Correct.

Andrei