On 5/7/13 12:46 PM, Steven Schveighoffer wrote:
On Tue, 07 May 2013 12:30:05 -0400, deadalnix <[email protected]> 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 Even if your CPU's view of the variable is outdated, the check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
