Re: Simple and fast Singleton pattern for ya.

Tech-Archive recommends: Fix windows errors by optimizing your registry

From: Jon Skeet [C# MVP] (skeet_at_pobox.com)
Date: 02/10/04 

Date: Tue, 10 Feb 2004 06:12:07 -0000

William Stacey <staceywREMOVE@mvps.org> wrote:
> Not exactly what happens here. This is not the same as the double checked
> issue. DCL uses an unsynchronized testing of the object reference.

But that's not the problem with DCL - it then uses a synchronized
testing of the object reference if it was null.

The problem with the DCL is that write-ordering isn't guaranteed unless
you've got a memory barrier, so the reference itself can be written
before object has finished initializing. I don't see anything in your
code to stop a second thread correctly seeing that a first thread is
initializing the object, and then waiting until that initialization is
finished.

> So the
> ref could be assigned by thread1 and then context switch, then thread2 could
> get that ref and try to use it before construction. This does not happen
> here because construction is completed before the interlock releases the
> lock. The actual creation is protected by a memory barrier using the
> "lock()". The assignment also creates a "write" barrier on the ref from
> doco I have seen.

Yes, you've got a write memory barrier. You don't have a read one. The
write memory barrier makes sure that everything is written before the
barrier. It doesn't guarantee what order those writes become visible
in.

> Only then is the flag set atomically. If we want to be
> really sure, we could also add a Thread.MemoryBarrier() instruction before
> we open the lock like so:
> ...
> Thread.MemoryBarrier();
> Interlocked.Exchange(ref created, 1);
> ...

You'd need a memory barrier on read as well though - that's what would
make it thread-safe.

To me, this is looking a lot more complicated - and, as we've seen,
debatable in terms of thread-safety - than any of the more common
patterns. 

-- 
Jon Skeet - <skeet@pobox.com>
http://www.pobox.com/~skeet
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