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variable is outdated, the check after the memory barrier inside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.
