Low-Lock Singletons In D

[Andrei Alexandrescu]

On 5/7/13 12:46 PM, Steven Schveighoffer wrote:
> On Tue, 07 May 2013 12:30:05 -0400, deadalnix <deadalnix at gmail.com> 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 Even if your CPU's view of the variable is outdated, the check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.
>
> The memory barrier prevents that. You don't store the variable until the
> object is initialized. That is the whole point.

A memory barrier is not a one-way thing, i.e. not only the writer must 
do it. Any operation on shared memory is a handshake between the writer 
and the reader. If the reader doesn't do its bit, it can see the writes 
out of order no matter what the writer does.

Andrei